Bridged and Unbridged Nickel-Nickel Bonds Supported by Cyclopentadienyl and Phosphine Ligand Sets

Article abstract of DOI:10.1021/acs.organomet.0c00708

A series of Ni complexes [Ni(Cl)2(PR3)2] with R = Me (1-Me), nPr (1-nPr), and nBu (1-nBu) and nickelocenes [Ni(η5-C5H4R′)2] with R′ = H (2-H), Me (2-Me), and SiMe3 (2-SiMe3) were synthesized and characterized. From these complexes, the synthesis of the Ni complexes [NiCl(PR3)(η5-C5H4R′)] R = Me, R′ = H (3-Me), R= nBu, R′ = H (3-nBu), R = nPr, R′ = H (3-nPr), R = Et, R′ = Me (4), and R = Et, R′ = SiMe3 (5) was achieved. All complexes were fully characterized, including single crystal X-ray crystallography. Complexes 3-R, 4, and 5 were then used to obtain homobimetallic Ni complexes with rare examples of unbridged Ni-Ni bonds [{Ni(η5-C5H5)(PR3)}2], with R = Me (7-Me) and R = nPr (7-nPr) being structurally characterized by single crystal X-ray diffraction. In order to probe the effect of bridging ligands on the Ni-Ni bond, the bridged complex [{Ni(μ:η5-C5H4CH2CH2P(tBu)2)}2] (8) was synthesized from the monomeric precursor [Ni(Cl)(κ1-η5-C5H4CH2CH2PtBu2)] (6). The Ni-Ni distances in 7-Me, 7-nPr, and 8 were found to be 2.407(1), 2.3931(6), and 2.6027(17) ?, respectively, the latter seemingly lengthened compared to the other two due to the tethered nature of the bridging ligand. DFT calculations confirm the presence of unbridged σ-bonds between the Ni atoms in 7-Me and 7-nPr and show that the bridging ligand in 8 has a minimal effect on the character of the Ni-Ni bond.

Full text of DOI:10.1021/acs.organomet.0c00708

pubs.acs.org/Organometallics
Article
Bridged and Unbridged Nickel−Nickel Bonds Supported by
Cyclopentadienyl and Phosphine Ligand Sets
Peter A. Cleaves, Alexander J. Ayres, Lisa Vondung, John C. Stewart, Philip J. Cobb, Ashley J. Wooles,
and Stephen T. Liddle*
Cite This: https://dx.doi.org/10.1021/acs.organomet.0c00708
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: A series of Ni complexes [Ni(Cl)2(PR3)2] with R = Me (1-
n
n
Me), Pr (1-nPr), and Bu (1-nBu) and nickelocenes [Ni(η5-C5H4R′)2]
with R′ = H (2-H), Me (2-Me), and SiMe3 (2-SiMe3) were synthesized
and characterized. From these complexes, the synthesis of the Ni
complexes [NiCl(PR3)(η5-C5H4R′)] R = Me, R′ = H (3-Me), R= nBu, R′
= H (3-nBu), R = nPr, R′ = H (3-nPr), R = Et, R′ = Me (4), and R = Et,
R′ = SiMe3 (5) was achieved. All complexes were fully characterized,
including single crystal X-ray crystallography. Complexes 3-R, 4, and 5
were then used to obtain homobimetallic Ni complexes with rare
examples of unbridged Ni−Ni bonds [{Ni(η5-C5H5)(PR3)}2], with R =
Me (7-Me) and R = nPr (7-nPr) being structurally characterized by single
crystal X-ray diffraction. In order to probe the effect of bridging ligands on
the Ni−Ni bond, the bridged complex [{Ni(μ:η5-C5H4CH2CH2P(tBu)2)}2] (8) was synthesized from the monomeric precursor
[Ni(Cl)(κ1-η5-C5H4CH2CH2PtBu2)] (6). The Ni−Ni distances in 7-Me, 7-nPr, and 8 were found to be 2.407(1), 2.3931(6), and
2.6027(17) Å, respectively, the latter seemingly lengthened compared to the other two due to the tethered nature of the bridging
ligand. DFT calculations confirm the presence of unbridged σ-bonds between the Ni atoms in 7-Me and 7-nPr and show that the
bridging ligand in 8 has a minimal effect on the character of the Ni−Ni bond.
and late transition metals allows for the facile activation of
polar bonds. The synthesis of such complexes requires
unbridged Ni-Ni complexes as starting materials. Ideally,
they should be accessible by facile synthetic routes and allow
for easy variation of ligands. However, whilst unbridged
homobimetallic complexes are known for many transition
metals,1,3 the number of examples for Ni is limited. A vast
majority of Ni-Ni complexes contain bridging ligands or larger
π-systems which enable a positive bonding interaction between
the monomers.15,16
Prior to this work, only three clear-cut examples of Ni−Ni
bonds without bridging ligands had been reported (Figure 1).
A few more seemingly unbridged Ni-Ni complexes are known,
but closer inspection reveals ligand−ligand bonding inter-
actions.15 The first example of an unbridged Ni-Ni complex
was K4[Ni2(CN)6], which has a short Ni−Ni bond length of
2.32 Å.17 Two organometallic complexes were subsequently
reported, both supported by cyclopentadienyl and phosphines
INTRODUCTION
■
Bimetallic complexes with direct metal−metal (M−M) bonds
have drawn intense interest since the 1960s, since they provide
interesting examples for extending bonding concepts.1 Such
M−M units can be viewed as intermediate between the
smallest unit of mononuclear metal complexes to clusters, and
ultimately bulk metals.2 Accordingly, they often possess unique
properties and so are of interest across various fields, such as
mimicking and understanding metal surface catalysis reactions,
as structural subunits of metal−organic frameworks, photo-
sensitizers, in bio-inorganic chemistry, or materials chem-
istry.1,3,4 In the area of homogeneous catalysis, bimetallic
complexes with direct M−M interactions are increasingly
receiving attention.5,6 This interest is driven by the unusual
synergic reactivity of two metals in close proximity that often
cannot be achieved with monometallic congeners.7−10
Due to rising costs and sustainability concerns, many efforts
have been made in recent years to move away from Pd and Pt
to more abundant 3d metals.11 As the lightest group 10
homologue, Ni is thus increasingly targeted as an alternative to
Pd- and Pt-based catalysts, and Ni-Ni complexes have shown
promising activities.10,12,13 Besides the burgeoning application
of Ni-Ni complexes as catalysts, they are promising starting
materials for the synthesis of heterobimetallic complexes,
which are of increasing interest.14 The combination of early
Received: November 5, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00708
© XXXX American Chemical Society
Organometallics XXXX, XXX, XXX−XXX
A

Organometallics
pubs.acs.org/Organometallics
Article
systematically in light of their general paucity. Our aim was to
establish a modular synthetic route to allow for easy variation
of substituents, and the ready steric and electronic tunability of
phosphines and cyclopentadienyls led us to use them in this
study. Furthermore, we investigated a tethered cyclopenta-
dienyl-phosphine system for comparison. Here, we report the
results of this study which has yielded a range of Ni precursors
and three new Ni-Ni complexes, where two are unbridged and
one is bridged.
RESULTS AND DISCUSSION
■
Synthetic Approach. Attempting to systematically probe
the steric and electronic effects of substituents, we prepared a
range of [NiCl(PR3)(η5-C5H4R′)] complexes (Scheme 1).
The complexes [NiCl2(PR3)2] (1-R, R = Et, nBu) have
previously been reported in the literature, but the R = Me, nPr
variants have surprisingly not previously been described.20
Nickelocene 2-H, complex 3-Et, and the Ni-Ni complex 7-Et
have been reported previously, but are included here for
completeness.18,20−22
Additionally, we utilized 6 with a tethered cyclopentadienyl
phosphine ligand as a precursor to a bridged Ni-Ni complex 8
(Scheme 2) in order to provide comparisons between bridged
and unbridged Ni-Ni complexes. The methodology is
essentially the same for both schemes, with the principal
change being whether the cyclopentadienyl-phosphine combi-
nation is tethered or not.
Synthesis of Ni Complexes 1-R. The complexes
[Ni(Cl)2(PR3)2] (R = Me, 1-Me; R = nPr, 1-nPr) were
obtained as red crystalline solids in good yields (75% and 58%,
respectively) by the reaction of NiCl2 with the appropriate
phosphine (Scheme 1). An initial color change from orange to
dark blue was observed upon the addition of PMe3 to NiCl2,
which is attributed to the formation of [Ni(Cl)2(PMe3)3], as
reported by Sun and co-workers.24 [Ni(Cl)2(PMe3)3] was
Figure 1. Known complexes with an unbridged Ni−Ni bond.15−19
or bulky isocyanide ligands.18,19 Figueroa’s complex was
obtained by reaction of nickelocene with an aryl isocyanide
ligand. Notably, reduction of the steric bulk of the aryl group
yields a dimeric complex with bridging isocyanides. Wilke’s
complex was obtained by a different route, reducing [(η5-
Cp)Ni(PEt3)Cl] with activated Mg. However, bulkier PPh3
and C5Me5 analogues proved elusive. These examples show
that ligand bulkiness and the reducing agent must be carefully
chosen to obtain unbridged Ni-Ni complexes.
With the increasing interest in Ni-Ni complexes generally
and our interest in preparing M−M bonds where one metal is
an f-element, we set out to explore dimeric Ni complexes more
a
Scheme 1. General Synthetic Route to Ni Complexes
a
Complexes 1-Et, 1-nBu, 2-H, 3-Et, and 7-Et have been reported previously.18,20−22
B
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Scheme 2. Synthesis of 6 As Taken from ref 23 and Reduction to the Bridged Ni-Ni Complex 8
noted to be thermally unstable, resulting in the dissociation of
a PMe3 ligand to afford 1-Me upon heating. The solid state
molecular structures of 1-Me and 1-nPr were determined to
confirm their formulations (Figure 2).
which is in line with two equivalent phosphorus atoms in each
complex. The observed downfield shifts compared to the free
PR3 ligands (δ(PMe3) = −62 ppm, δ(PnPr3) = −33 ppm) are
in line with the expected decrease of electron density at the
phosphorus atom upon coordination of PR3 to the nickel atom.
Synthesis and X-ray Diffraction Analysis of Nickel-
ocene Derivatives 2-R. The nickelocene derivatives [Ni(η5-
C5H4R′)2] (R′ = Me, 2-Me; R′ = SiMe3, 2-SiMe3) were
prepared via the reaction of [Ni(NH3)6]Cl2 with 2 equivalent
of either [Na(C5H4Me)] or [K(C5H4SiMe3)]. Following
workup, 2-Me was purified via sublimation to afford dark
green crystals (23% yield), and 2-SiMe3 was crystallized from
iso-hexane to afford a dark green microcrystalline solid (31%
yield). Unfortunately, despite repeated attempts, crystals of 2-
SiMe3 suitable for single crystal X-ray diffraction studies could
not be prepared; however, the solid state structure of 2-Me
could be determined and is shown in Figure 3. The methyl
Figure 3. Solid state structure of complex 2-Me at 90 K with selective
labeling. Thermal ellipsoids are set at 50% probability, and hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and angles
[deg]: Ni1−Cp(c) 1.8120(7), Ni1−C1 2.1836(13), Ni1−C2
2.1736(13), Ni1−C3 2.1639(13), Ni1−C4 2.1710(13), Ni1−C5
2.1949(13), Cp(c)1−Ni1−Cp(c)2 179.991(1).
Figure 2. Solid state structures of complexes (a) 1-Me and (b) 1-nPr
at 90 K with selective labeling. Thermal ellipsoids are set at 50%
probability, and hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and angles [deg]: 1-Me: Ni1−P1 2.2262(6), Ni1−Cl1
2.1596(5), P1−Ni1−P1a 180.00; 1-nPr Ni1−P1 2.232(6), Ni1−P2
2.2260(5), Ni1−Cl1 2.1658(5), Ni1−Cl2 2.1578(5), P1−Ni1−P2
178.63(2).
groups in 2-Me are sited anti to each other with respect to the
(η5-C5H4Me)···Ni···(η5-C5H4Me) vector, and the Ni−cyclo-
pentadienyl centroid distance is 1.8120(7) Å. The mean Ni−C
distance in 2-Me is 2.1774(13) Å, which corresponds well with
the analogous Ni−C bonds in [Ni(η5-C5H5)] (mean: 2.178(4)
Å), despite the variation in the cyclopentadienyl ligand.26
Synthesis of Ni Chlorotrialkylphosphine Cyclopenta-
dienyl Complexes 3-R, 4, and 5. With a range of Ni-
phosphine and nickelocene complexes in hand, we attempted
the synthesis of different Ni(II) chlorotrialkylphosphine
cyclopentadienyl complexes as precursors for the Ni-Ni
dimeric complexes. Only one such Ni complex, [Ni(Cl)(η5-
C5H5)(PEt3)] (3-Et), had been previously reported, and thus
X-ray Diffraction and NMR Analysis of Ni Complexes
1-R. The 16-electron Ni(II) complexes 1-Me and 1-nPr adopt
the anticipated square planar coordination geometries (sum of
bond angles around Ni ∼ 360°), with the phosphine ligands
trans to each other in each complex. The average Ni−P and
Ni−Cl distances in 1-Me (2.2263(6) and 2.1596(5) Å) and
1-nPr (2.2293(6) and 2.1618(5) Å) compare well to the
previously reported compound [Ni(Cl)2(PEt3)2], 1-Et
(2.2329(5) and 2.1628(5) Å).25 For 1-nBu, only spectroscopic
data and no solid state structure has been reported.20
Single resonances were observed in the 31P{1H} NMR
spectra of both 1-Me (−23.1 ppm) and 1-nPr (−3.2 ppm),
C
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Figure 4. Solid state structures of complexes (a) 3-Me at 90 K, (b) 3-nPr at 150 K, and (c) 3-nBu at 90 K with selective labeling. Thermal ellipsoids
are set at 50% probability, and hydrogen atoms are omitted for clarity.
Figure 5. Solid state structures of complexes (a) 4, (b) 5, and (c) 6 all at 90 K with selective labeling. Thermal ellipsoids are set at 50% probability,
and hydrogen atoms are omitted for clarity.
a
Table 1. Selected Mean Bond Lengths (Å) and Angles (deg) for Complexes 3-Me, 3-Et, 3-nPr, 3-nBu, 4, 5, and 6
3-Me
3-Et22
3-nPr
3-nBu
4
5
6
Ni−P
Ni−Cl
Ni−Cp(c)
P−Ni−Cp(c)
Cl−Ni−Cp(c)
P−Ni−Cl
2.1612(10)
2.1834(10)
1.7476(16)
132.83(7)
131.55(7)
95.56(4)
2.1505(3)
2.1871(3)
1.753(8)
137.5(3)
130.4(3)
92.06(5)
2.1529(4)
2.1826(5)
1.7560(9)
133.45(4)
131.84(4)
94.42(2)
2.1412(5)
2.1973(5)
1.7490(10)
134.07(4)
133.14(4)
92.68(2)
2.1446(8)
2.1930(8)
1.7461(14)
133.12(5)
132.31(5)
94.57(3)
2.1441(10)
2.1921(10)
1.7444(16)
133.93(6)
132.98(6)
92.74(3)
2.1600(8)
2.2299(7)
1.7477(13)
122.71(5)
133.42(5)
103.40(3)
a
Cp(c) = centroid at cyclopentadienyl.
we sought to extend this to a range of different analogues to
probe their reactivity in the formation of dimeric Ni
complexes.22 The reaction of nickelocene with the bis(trialkyl-
phosphine) Ni-dichlorides 1-Me, 1-nPr, or the literature
known analogue 1-nBu20 in refluxing THF afforded the
corresponding Ni(II) chlorotrialkylphosphine cyclopentadienyl
yield (54% and 30% yield, respectively). The solid state
structures of 4 and 5 are illustrated in Figure 5.
X-ray Diffraction Analysis of Ni Chlorotrialkylphos-
phine Cyclopentadienyl Complexes 3-R, 4, and 5. The
metal atoms of 3-Me, 3-nPr, 3-nBu, 4, and 5 (Figures 4 and 5)
are coordinated in a two-legged piano-stool coordination
mode, and each lies in a plane formed by the Cp ring centroid,
PR3 and chloride (sum of bond angles between the Cp ring
centroid, PR3, and chloride ∼ 360°), which is in agreement
with the previously reported 3-Et.22 The coordination mode of
the cyclopentadienyl ligands is independent of any substituents
present, with each being bound to the Ni center in an η5-mode.
There is little variation between the Ni−centroid, Ni−P, and
Ni−Cl bond distances in each of 3, 4, and 5, despite the
variation in ligand environment, and these are in agreement
with the previously reported 3-Et (bond lengths and angles are
outlined in Table 1).
n
complexes [Ni(Cl)(η5-C5H5)(PR3)] (R = Me, 3-Me; R = Pr,
n
3-nPr; R = Bu, 3-nBu) as red crystalline solids (21, 81, 58%
yield, respectively). The solid state structures of 3-Me, 3-nPr,
and 3-nBu are illustrated in Figure 4.
In order to probe the effect of substitution on the
cyclopentadienyl ligand on the reactivity, the Ni-triethylphos-
phine complex [Ni(Cl)2(PEt3)2], 1-Et, was reacted with the
methyl and SiMe3 substituted nickelocene derivatives 2-Me
and 2-SiMe3 via the same route as before in refluxing THF.
The PEt3 substituted precursors were chosen in order to
facilitate better comparison with the literature example 3-Et.
[Ni(Cl)(η5-C5H4Me)(PEt3)], 4, and [Ni(Cl)(η5-C5H4SiMe3)-
(PEt3)], 5, were obtained as red crystalline solids in moderate
Spectroscopic Characterization of Ni Chlorotrialkyl-
phosphine Cyclopentadienyl Complexes 3-R, 4, and 5.
D
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
In the 31P{1H} NMR spectra, a downfield shift of the PR3
resonances compared to complexes 1-R was observed (δ(3-
Me) = −11.5 ppm, δ(3-nPr3) = 16.0 ppm, δ(3-nBu3) = 17.3
ppm). This is due to the change in the coordination
environment around the Ni atom, which brings the centroid
of the cyclopentadienyl ligand at an angle of 131−134° to the
phosphine ligand, meaning that there is no ligand directly trans
to the phosphine. Thus, the phosphine can donate more
strongly compared to complexes 1-R, which leads to less
electron density around the phosphorus atom and thus a
downfield shift. The resonances in the 31P{1H} NMR spectra
for 4 and 5 are 27.6 and 24.9 ppm, respectively. These values
are quite similar, which is as expected with both bearing ethyl
groups on phosphorus and comparable Ni−P bond lengths.
The shifts for 4 and 5 are also in the same range as the
literature reported shift for 3-Et (32 ppm).27
The UV−vis spectra were collected in toluene for complexes
3-Me, 3-nPr, 3-nBu, 4, and 5, and all have a maximum
extinction coefficient of ∼700 dm−3 mol−1 cm−1 at wavelengths
of between 496 and 502 nm. These absorptions are both more
intense and at slightly longer wavelengths than the
corresponding square planar precursors 1-Me and 1-nPr (ε =
∼300 dm−3 mol−1 cm−1, λ = 472−488 nm), which can be
attributed to the change in coordination geometry from square
planar to two-legged piano-stool, as well as the introduction of
the cyclopentadienyl ligands.
Synthesis and X-ray Diffraction Analysis of the
Tethered Ni Chlorotrialkylphosphine Cyclopentadienyl
Complex 6. In order to provide for comparison with a
bridged Ni-Ni dimer complex, we additionally prepared
[Ni(Cl)(κ1-η5-C5H4CH2CH2PtBu2)] (6) following the re-
ported procedure as shown in Scheme 2.23 Since the structure
in the solid state had not been determined before, we
undertook single crystal X-ray diffraction studies on crystals of
6 grown from a saturated toluene solution. The solid state
structure of 6 is shown in Figure 5c. Complex 6 crystallizes as a
discrete mononuclear chelated species, adopting a two-legged
piano-stool geometry around the Ni atom (sum of bond angles
between PR3, chloride, and Cp centroid ∼ 360°). The bond
distances are comparable to those in complexes 3-Me, 3-Et,
3-nPr, 3-nBu, 4, and 5 (see Table 1); however, as a result of the
strained nature of the chelating ligand, the bond angles do
differ, with the P−Ni−Cp(c) angle narrowing by ∼10°
(122.71(5)° compared to 132.83(7)−137.5(3)°), along with
the opening of the corresponding P−Ni−Cl angle (103.40(3)°
compared to 92.06(5)−95.56(4)°). The Cl−Ni−Cp(c) angle
is mostly unchanged, however, and lies just above the standard
uncertainty of the range of angles for the other complexes
(133.42(5)° for 6 vs the range 130.4(3)−133.14(4)°). The
ethylene tether adopts a staggered conformation, and the
cyclopentadienyl carbon attached to this tether (C5) also lies
closer to the Ni center than the other carbons in the ring
(Ni1−C5 2.022(3) Å, Ni1−C1/2/3/4av 2.167(3) Å); this was
also observed in analogous compounds described by
Mg yields the formal Ni(I) complexes. Each of the monomeric
complexes 3-Me, 3-nPr, 3-nBu, 4, 5, and 6 was reacted with
activated Mg in attempts to prepare formal Ni(I)-Ni(I) species
of the general form [{Ni(η5-C5H4R′)(PR3)}2]. In all reactions,
a slow color change to dark green was observed. In the cases of
3-Me, 3-nPr, and 6, we were able to successfully isolate the Ni-
Ni dimers [{Ni(η5-C5H5)(PMe3)}2], 7-Me (53%), [{Ni(η5-
C5 H5 )(Pn Pr3 )}2 ], 7-n Pr (80%), and [{Ni(μ:η5 -
C5H4CH2CH2P(tBu)2)}2], 8 (<1%), respectively. Unfortu-
nately, from the reactions of 3-nBu, 4, or 5 with activated Mg,
only intractable mixtures were obtained, and attempts to
crystallize any reaction products led to further decomposition.
The successful isolation of the dimeric complexes with both
phosphine and cyclopentadienyl-based ligands thus seems to
be much more sensitive to the substitution patterns of the
cyclopentadienyl ligands than that of the phosphines. This is in
contrast to the observation by Figueroa and co-workers of the
steric bulk of the L-donor isocyanide ligand determining the
structure.19 The steric demands of the phosphine ligands do
not seem to play a major role in the successful preparation of
the dimeric species, contrasting the assumptions made in the
work by Wilke and co-workers.18 Inspection of the Tolman
cone angles reveals that, depending on how exactly the values
are calculated (original Tolman/for tetrahedral Ni complex),
PEt3, PnPr3, PPh3, and PnBu3 all have a very similar steric
demands: PEt3 (132°/169°), PnPr3 (132°/169°), PPh3 (145°/
166°), and PnBu3 (132°/172°).28 Thus, the unsuccessful
isolation of 7-nBu and 7-Ph cannot be attributed to steric
effects alone. This demonstrates how crucial the choice of alkyl
substituent on the phosphine ligand is for the formation of Ni-
Ni complexes.
NMR Spectroscopic Characterization of Dimeric Ni
Complexes 7-R. The 31P{1H} NMR resonances for the PR3
substituents in 7-R are very close to those observed for the
monomeric precursor complexes 3-R, with −10.1 ppm for 7-
Me and 18.0 ppm for 7-nPr. This signifies an upfield shift of 1.4
and 1.3 ppm, respectively. The 31P{1H} NMR resonance for 7-
Et was reported to be at 26.9 ppm, which is a 3.3 ppm shift
upfield compared to the monomeric 3-Et.18,22 The resonances
for 7-Me, 7-Et, and 7-nPr follow the trend expected from the
free alkylphosphine ligands with 7-Me being the furthest
upfield, followed by 7-nPr approximately 30 ppm downfield
and 7-Et shifted another 9 ppm downfield. For reduction of
the Ni atoms from +II to +I, a larger change in chemical shift
might be reasonably anticipated, as the P atom might be
donating less electron density to the Ni ion in the reduced
complexes. However, the trend towards more upfield
resonances corresponds well to the increased electron density
on P. The smaller change in chemical shift compared to the
differences between 1-R and 3-R can be rationalized by the
smaller changes in the coordination geometry around the Ni
atom with the P−Ni−Cp(c) angle opening up by only 10°
when going from 3-R to 7-R, as opposed to 40° from 1-R to 3-
R.
1
5
t
̈
Butenschon (e.g., [Ni(Me)(κ -η -C5H4CH2CH2P Bu2)],
Ni1−C1 2.099(11) Å, Ni1−C2/3/4/5av 2.125(11) Å).23
Synthesis of the Dimeric Ni Cyclopentadienyl
Phosphino Complexes 7-R and 8. The synthesis of the
dimeric formally Ni(I) complexes was achieved by reduction of
the monomeric Ni(II) complexes. The identity of the reducing
agent is vitally important, as most, more common, reducing
agents, such as alkali metals or KC8 are too reducing, resulting
in over-reduction and intractable products. However, activated
X-ray Diffraction Analysis of Dimeric Ni Complexes 7-
R and 8. The solid state structures of 7-nPr and 8 are
illustrated in Figure 6. The solid state structure of 7-Me is
shown in the Supporting Information (Figure S1). The X-ray
data obtained for 7-Me were of poor quality, so reliable
analysis of the metrical parameters of the complex is not
possible but the connectivity is clear and confirms the
formulation of 7-Me to be analogous to that of 7-nPr.
Complexes 7-Me and 7-nPr adopt geometries akin to that of 7-
E
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
n
explained by the higher steric demand of the Pr substituents
on the phosphine ligands compared to the ethyl groups in 7-
Et.
In both unbridged Ni-Ni complexes 7-Me and 7-nPr, the
substituents are in a cis arrangement, as previously observed for
the ethyl substituted analogue reported by Wilke.18 The
bridging ligand in 8 changes this to a trans arrangement. A
comparison of the Ni−Ni interactions reveals that the tethered
environment has imposed a not inconsiderable lengthening of
∼0.2 Å to the Ni−Ni bond in 8 compared to 7-Et and 7-nPr
(see Table 2). The unbridged Ni−Ni bond in 7-nPr is also
slightly shorter than the reported distance of 2.407(1) Å for 7-
Et.18 The elongation of the bond in 8 with otherwise similar
substituents is most likely due to the bridging ligand leading to
steric hindrance.
The structure of 8 retains a two-legged piano-stool
coordination geometry around the Ni centers (sum of bond
angles between PR3, other Ni, and Cp centroid ∼ 360°). The
cyclopentadienyl centroid bond distances are slightly longer in
the dinuclear structure than in the precursor Ni(II) complex 6,
resulting from the slightly larger size of the formal Ni(I) ion.
The coordination geometry is altered from that of the
untethered analogue as a result of the ethylene linkage
imposing steric constraints upon the relative positions of the
coordinated fragments of the ligand. Accordingly, the P−Ni−
Ni angle is enlarged in comparison to that of 7-Et and 7-nPr, as
is the torsion angle P−Ni−Ni−P.
Trends in the Experimental Characterization Data.
Table 3 summarizes the key spectroscopic and crystallographic
data for the synthesized complexes. Several trends can be
observed in the spectroscopic and crystallographic data. The
observed resonances in the 31P{1H} NMR are shifted
increasingly downfield with each complex along the reaction
sequence, starting from the Ni(II) dichloro diphosphine
complexes [Ni(Cl)2(PR3)2] 1-R, followed by the Ni(II)
chlorotrialkylphosphine cyclopentadienyl complexes [Ni(Cl)-
(η5-C5H4R′)(PR3)] 3-R/4/5, going to the resulting formal
Ni(I) cyclopentadienyl phosphino dimers [{Ni(η5-C5H5)-
(PR3)}2] 7-R. This observation can be explained by the
increasing donation of electron density by the phosphine
ligand to the nickel atom. Simultaneously, the Ni−P bond
lengths decrease throughout the sequence, also indicating a
strengthening of the Ni−P bond. The largest changes in both
31P{1H} NMR resonances as well as Ni−P bond lengths are
observed when introducing the cyclopentadiene ligands, which
can be attributed to the change in coordination environment
around the nickel atoms. Similarly, the maximum absorption in
the UV−vis spectra changes significantly between the nickel-
ocene complexes 2-R and the Ni(II) chlorotrialkylphosphine
Figure 6. Solid state structures of complexes (a) 7-nPr and (b) 8 at
150 K with selective labeling. Thermal ellipsoids are set at 50%
probability, and hydrogen atoms are omitted for clarity.
Et,22 with the cyclopentadienyl rings adopting an η5 binding
mode. This contrasts to [{Pd(Cp)(PMe3)}2], reported by
Werner and co-workers, in which the cyclopentadienyl ligands
behave like allylic fragments, adopting a bridging conforma-
tion.29,30 The difference in binding mode can be attributed to
the relative size of the metals (Ni 3d 1.10 Å vs Pd 4d 1.20 Å).31
The Ni centers in 7-nPr are coordinated in two-legged piano-
stool fashion, though the bond distances and angles vary to
that reported for 7-Et, with slightly shorter Ni−Ni and Ni-
Cp(c) distances and smaller P−Ni−Ni and Cp(c)−Ni−Ni
angles. These are detailed in Table 2. The torsion angles P−
Ni−Ni−P are quite similar to 100.84(8)° for 7-Et and
103.66(4)° for 7-nPr. The slightly larger angle for 7-nPr can be
Table 2. Selected Mean Bond Lengths (Å), Angles (deg), and Torsion Angles (deg) for the Dimeric Complexes 7-Et,18 7-nPr,
and 8
7-Et18
2.407(1)
7-nPr
2.3931(6)
8
Ni−Ni
Ni−P
2.6027(17)
2.1649(19)
1.790(4)
2.116(7)−2.191(7)
142.00(14)
101.22(7)
2.122(1)
2.1228(9)
Ni−Cp(c)
Ni−C
P−Ni−Cp(c)
P−Ni−Ni
Cp(c)−Ni−Ni
P−Ni−Ni−P
1.7935(18)
2.105(3)−2.166(4)
140.68(11)
92.89(7)
126.25(7)
100.84(8)
1.7814(17)
2.101(3)−2.171(4)
139.81(8)
91.99(3)
126.72(7)
116.38(13)
121.25(8)
103.66(4)
F
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Table 3. Selected Spectroscopic and Crystallographic Data for All New Complexes
31P{1H} NMR
UV−vis
bond lengths (Å)
compound
δ (ppm)
λmax (nm)
Ni−P
Ni−Cp(c)
Ni−Ni
1-Me
1-nPr
2-Me
2-SiMe3
3-Me
3-nPr
3-nBu
4
−23.1
−3.2
472
488
679
697
497
502
502
496
501
2.2263(6)
2.2293(6)
1.8120(7)
−11.5
16
17.3
27.6
24.9
18
2.1612(10)
2.1529 (4)
2.1412(5)
2.1446(8)
2.1441(10)
2.1228(9)
1.7476(16)
1.7560(9)
1.7490(10)
1.7461(14)
1.7444(16)
1.7814(17)
5
7-nPr
7-Me
8
2.3931(6)
−10.1
2.1649(19)
1.790(4)
2.6027(17)
cyclopentadienyl complexes 3-R, 4, and 5. In contrast, the
dimerization only leads to small shifts in the observed 31P{1H}
NMR resonances and bond metrics, due to the coordination
environment around nickel not changing much. Looking at the
same types of complexes (for example, 3-R) with different alkyl
groups on the phosphine ligand, a clear trend can be observed
for the resonances in the 31P{1H} NMR spectra. The
resonances are shifted downfield when going from methyl to
n-propyl to n-butyl. This trend is also observed for the free PR3
ligands and rationalized by the increasing negative inductive
effect of alkyl groups on phosphorus. The second influence on
the observed 31P{1H} NMR resonances stems from the
coordination to Ni and can be observed in the Ni−P bond
lengths. These decrease slightly going from 3-Me (2.1612(10)
Å) to 3-nPr (2.1529(4) Å) and 3-nBu (2.1412(5) Å), showing
the strengthening of the Ni−P bonds associated with
increasing donation of electron density from the phosphine
ligands to Ni.
Density Functional Theory Calculations on Dimeric
Ni Complexes 7-R and 8. In order to gain more insight into
the Ni−Ni bonding situation, we performed DFT calculations
on compounds 7-Me, 7-nPr, and 8. The bond lengths and
angles of the optimized geometries match well with the
structures in the solid state with experimental solid state vs
computed gas-phase deviations in the range of 0.03 Å and 0.4°,
respectively. The Ni−Ni bond length in 7-Me (2.4193 Å) is
calculated to be very similar to that of 7-nPr (2.4257 Å). The
same holds true for the Ni−P and P−C bond lengths, enabling
the assumption that the calculated structures are qualitatively
representative of the experimental solid state structures. In
addition to the dimers 7-Me and 7-nPr with the phosphine
ligands in a cis arrangement, the trans isomers were optimized
for comparison. For 7-nPr, a minimum structure with a trans
configuration of the ligands could be found, albeit 69 kJ/mol
higher in energy. This large energy difference explains the sole
observation of the cis isomer in the experiments. The
optimization of trans-7-Me leads to cis-7-Me, which can be
attributed to less steric hindrance between the ligands in the cis
configuration compared to a trans arrangement. Consequently,
the Ni−Ni bond length is shorter in the cis complexes,
enabling better orbital overlap and stabilization of the Ni−Ni
bond. The same reasoning can be applied to 7-nPr, where the
Ni−Ni bond length in the trans isomer is elongated to 2.7075
Å (cis: 2.4257 Å) and the complex is destabilized by 69 kJ/mol.
Charges for the Ni atoms were calculated with different
methods and are given in Table 4.32−34 Due to the different
Table 4. Computed Charges for the Nickel Atoms and
Nalewajski−Mrozek Bond Orders for the Nickel−Nickel
Bonds in 7-Me, 7-nPr, and 8
charge on Ni
Nalewajski−Mrozek bond order for
NBO Hirshfeld MDC-q
Ni−Ni
7-Me
7-nPr
8
0.37
0.38
0.43
0.15
0.15
0.16
−0.03
0.05
0.02
0.45
0.45
0.44
approaches in defining the atoms in a molecule and
calculations of the charge, the values differ, but most
importantly produce a consistent overall trend. The calculated
charges on the Ni atoms are as expected rather small with little
deviation between the complexes. For the Ni atom, NBO
charges are in the range of 0.37−0.43, Hirshfeld charges are
0.15−0.16, and MDC-q charges are between −0.03 and 0.05.
Thus, the Ni atom in these complexes can be described with
the formal low oxidation state of +I. For further character-
ization of the nature of the Ni−Ni bond, Nalewajski−Mrozek
bond orders were calculated. These are basis set independent
and have been shown to correlate well with experimental data
for transition metal complexes.35 The calculated bond orders
of 0.44 for 8 and 0.45 for 7-Me and 7-nPr support the
description of the Ni−Ni bonds as single bonds with
significant ionic contribution, where a charge-shift nature
may be operating (see below). The slightly lower bond order
for the bridged complex was expected due to the elongated
Ni−Ni bond length, which is implied by the sterics of the
bridging ligand.
A topological analysis of the electron density was performed
with the Quantum Theory of Atoms in Molecules
(QTAIM).36 The QTAIM analysis found bond paths and
bond critical points (BCPs) between the nickel atoms in all
three complexes. At the BCP, several parameters are
informative of the nature of the bond: The electron density
ρ(r) shows a minimum at the BCP. The Laplacian ∇2ρ(r) is
negative for covalent bonds, but can be positive, for example,
for closed-shell and ionic interactions. Another insightful value
is the electronic energy density H(r), which is the sum of the
kinetic and potential energy: H(r) = G(r) + V(r). The energy
density H(r) is negative for covalent bonds and positive for
ionic bonds, H bonds, and van der Waals bonds.37 A summary
of the results obtained from a QTAIM analysis of complexes 7-
Me, 7-nPr, and 8 can be found in Table 5. The Laplacians of
G
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
reduction of these dimers would be expected to result in
annihilation of the Ni−Ni bonds and formation of monomeric
[Ni(Cp)(PR3)]1− 18-electron anions that are analogous to the
[Fe(Cp)(CO)2]1− anion.
Table 5. Values for the Electron Density ρ(r), the Laplacian
∇2ρ(r), and Electronic Energy Density H(r) at the Ni−Ni
BCPs for 7-Me, 7-nPr, and 8
ρ(r)
∇2ρ(r)
H(r)
ε
7-Me
7-nPr
8
0.051
0.051
0.039
0.069
0.051
0.035
−0.012
−0.012
−0.009
0.027
0.017
0.023
SUMMARY AND CONCLUSIONS
■
A modular approach for the preparation of new Ni-Ni
complexes was investigated. We were able to synthesize a
library of Ni complexes with different phosphine and
substituted cyclopentadienyl ligands. In the first step, Ni
the electron density are positive, but close to zero and in the
range of mainly ionic bonds for all three complexes (0.07 for 7-
Me, 0.05 for 7-nPr, and 0.04 for 8).38−41 The electronic energy
density H(r) was found to be −0.012 for 7-Me and 7-nPr and
−0.009 for 8, suggesting a small amount of covalent bonding.
These values indicating an ambiguity between ionic and
covalent bonding are very typical for metal−metal bonds and
have recently been shown to be possibly best described as
charge-shift bonds.42 The bond ellipticity ε at the bond critical
points of 0.02−0.03 is close to zero in all three complexes,
which corresponds well with a bond of mainly σ-character
between the nickel atoms. The lower values for ρ(r), ∇2ρ(r),
and H(r) for 8 compared to 7-Me and 7-nPr confirm the lower
Ni−Ni bond order and an overall weaker Ni−Ni bond in 8.
This is in line with the observed elongated Ni−Ni bond length,
which is caused by the steric strain of the bridging ligand.
Inspection of the molecular orbitals shows that the main
contribution to the nickel−nickel bond for all three complexes
is a σ-bonding orbital which is mainly formed by metal d-
orbitals (Figure 7). In 7-Me, both Ni atoms contribute
similarly to the MO (47.3% and 45.9%). The main
contributions come from the dx −y orbitals with 20.7% and
15.7% for the two Ni ions, respectively, and the Ni1 dz
(13.0%) and Ni2 dxy orbitals (13.9%). Similar contributions
to the σ-bonding MO are obtained for 7-nPr with 46.4% and
45.6% contribution for the two Ni atoms. The main
contributions are from dxz (29.9%) and dz (11.1%) orbitals
for Ni1 and from dyz (28.1%) and dz (11.3%) for Ni2. As
expected from inspection of the molecular orbitals, the
contributions to the HOMO-8 are again similar for 8: Ni1
contributes 47.1%, where 14.9% are from the dxy orbital, 12.3%
complexes [Ni(Cl)2(PR3)2] (1-Me, R = Me; 1-nPr, R = Pr;
n
1-nBu, R = nBu) and nickelocene complexes [Ni(η5-C5H4R′)2]
with R′ = H (2-H), Me (2-Me), and SiMe3 (2-SiMe3) were
synthesized. From these, Ni chloro cyclopentadienyl phos-
phine complexes [NiCl(PR3)(η5-C5H4R′)] (3-Me, R = Me, R′
= H; 3-nBu, R= nBu, R′ = H; 3-nPr, R = nPr, R′ = H; 4, R = Et,
R′ = Me; 5, R = Et, R′ = SiMe3) were synthesized via a ligand
exchange route. Reduction of these monomeric Ni complexes
with activated Mg resulted in dimerization and formation of
Ni-Ni complexes. Complexes [{Ni(η5-C5H5)(PR3)}2] (7-Me
and 7-nPr) could be isolated and characterized spectroscopi-
cally and by single crystal X-ray crystallography. These two
dimeric species contain very rare unbridged Ni−Ni bonds,
adding two more examples to only three previously known
ones. So far, only two other examples with cyclopentadienyl
and phosphine or isocyanide ligands had been known, whereas
the other known Ni-Ni species contained solely cyanide
ligands and is therefore difficult to adapt for further
applications. The systematic variation of the substituents on
the phosphine and cyclopentadienyl ligands allowed for
insights into the influence of the ligands on the dimer
formation and the Ni−Ni bond properties to be probed. It was
shown that the previously assumed steric effects (as also
observed in isocyanide complexes) seem to be rather negligible
for the phosphines, as 7-nPr could be isolated successfully, but
the sterically very similar 7-nBu could not be isolated. DFT
calculations show the Ni−Ni bonding interaction in the
dimeric complexes to be of σ-type, and possibly exhibiting
charge-shift character. No interactions between the ligands
were found, which confirms the unbridged nature of the Ni−
Ni bonds. Additionally, the bridged Ni-Ni complex [{Ni(μ:η5-
C5H4CH2CH2P(tBu)2)}2] (8) was synthesized from the
monomeric precursor [Ni(Cl)(κ1-η5-C5H4CH2CH2PtBu2)]
(6) to allow for direct comparison of the Ni−Ni bonds
between the bridged dimer 8 and the unbridged dimers 7-Me
and 7-nPr. The bridging ligand in 8 leads to an elongated Ni−
2
2
2
2
2
2
2
from the dx −y and 11.4% from the dyz orbital. Ni2 contributes
2
2
2
44.5%, with 23.2% from the dx −y and 9.2% from the dz . No
interaction was found between the ligands of the two Ni
fragments and the Ni−Ni bonds in 7-Me and 7-nPr,
confirming the unbridged nature of those Ni−Ni bonds. In
each case, the LUMO of these complexes represents a Ni-Ni
σ* MO, which nicely accounts for the fact that 2-electron
Figure 7. HOMO-8 of (a) 7-Me, (b) 7-nPr, and (c) 8 showing the Ni−Ni σ-bonds. H atoms are omitted for clarity. Contour level set to 0.05.
H
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
C12H14Ni: C, 66.44; H, 6.50; N, 0.00%. Found: C, 66.21; H, 6.35; N,
0.00%. H NMR (C6D6, 295 K): δ (ppm) 0.42 (s, CH3). No signal
Ni bond distance, as well as an enlarged P−Ni−Ni−P torsion
angle compared to 7-Me and 7-nPr. Bonding analysis of the
Ni−Ni bond in 8 reveals the bond to be slightly weaker than in
7-Me and 7-nPr, which can be accredited to the steric strain
imposed by the tethering ligand. The Ni-Ni complexes
disclosed here could be employed as starting materials for
hetero-bimetallic complexes or larger cluster molecules.
1
could be observed for the C5H4 protons. Due to poor solubility, no
13C{1H} NMR could be obtained. FTIR (Nujol): v (cm−1) 768 (w),
̃
776 (m), 929 (w), 1402 (w), 2729 (w). UV/vis (Toluene, 295 K):
λmax (679 nm), ε = 64 dm−3 mol−1 cm−1.
Preparation of [Ni(η5-C5H4SiMe3)2] (2-SiMe3). [Ni(NH3)6]Cl2
(3.46 g, 14.92 mmol) was added to a solution of [K(η5-C5H4SiMe3)]
(5.28 g, 29.93 mmol) in THF (75 mL), and the reaction mixture was
gently heated to 60 °C for 5 h. After this time, volatiles were removed
in vacuo and the product was extracted into hexane (20 mL). Volatiles
were removed in vacuo from the mother liquor, yielding 2-SiMe3 as a
dark green oil. Yield: 1.56 g, 31 %. Anal. Calcd for C16H26NiSi2: C,
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out
using Schlenk techniques, or in an MBraun Unilab glove box, under
an atmosphere of dry and oxygen-free dinitrogen. Tetrahydrofuran
(THF), diethyl ether, iso-hexane, pentane, and toluene were dried by
passage through activated alumina and degassed before use.
Methylcyclohexane was distilled under dinitrogen from K. All solvents
were stored over K-mirrors (with the exception of THF, which was
stored over activated 4 Å molecular sieves). Deuterated benzene was
distilled from K, degassed by three freeze-pump-thaw cycles and
stored under dinitrogen. The compounds [Ni(Cl)2(PEt3)2],20 [Ni-
(Cl)2(PnBu3)2],20 [Ni(η5-C5H5)2],21 [Ni(NH3)6]Cl2,21 [Na(η5-
C5H4Me)],43 [K(η5-C5H4TMS)],44 [Ni(Cl)(η5-C5H5)(PEt3)],18
spiro[4.2]hepta-1,3-diene,45 [Ni(Cl)(κ1:{(η5-C5H4)(CH2)2}
P(tBu)2],23 and activated Mg46 were prepared according to literature
procedures. All other reagents were used as received.
1
57.67; H, 7.86; N, 0.00%. Found: C, 57.83; H, 7.78; N, 0.00%. H
NMR (C6D6, 295 K): δ (ppm) 0.37 (9H, s, Si(CH3)3), 1.19(2H, m,
C5H4), 1.52 (2H, m, C5H4). Due to poor solubility, no 13C{1H} NMR
could be obtained. FTIR (Nujol): ṽ
(cm−1) 774 (m), 1005 (m), 1542
(m), 2342 (w). UV/vis (Toluene, 295 K): λmax (697 nm), ε = 94
dm−3 mol−1 cm−1.
Preparation of [Ni(Cl)(η5-C5H5)(PMe3)] (3-Me). THF (50 mL)
was added to a stirring mixture of 1-Me (2.18 g, 7.74 mmol) and 2-H
(1.48 g, 7.84 mmol). The reaction mixture was refluxed at 90 °C for
30 h. After this time, the resulting dark red solution was allowed to
cool to room temperature, filtered, and volatiles were removed in
vacuo. The resulting red solid was extracted into toluene (20 mL), and
the solution was cooled to −30 °C for 24 h to afford 3-Me as red
blocks. Yield: 0.75 g, 21%. Anal. Calcd for C8H14ClNiP: C, 40.83; H,
1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on a Bruker
400 spectrometer operating at 400.1, 100.1, and 162.0 MHz,
respectively; chemical shifts are quoted in ppm and are relative to
Me4Si or external 85% H3PO4. FTIR spectra were recorded on a
Bruker tensor 27 spectrometer in Nujol, and absorptions are reported
in inverse centimeters as w (weak), m (medium), or s (strong). UV/
vis spectra were recorded in a 10 mm quartz cell on a Perkin Elmer
Lambda 750 UV/VIS/NIR Spectrometer. Elemental microanalyses
were carried out by Mr. Stephen Boyer (London Metropolitan
University), or Mr. Mark Jennings (University of Manchester).
Preparation of [Ni(Cl)2(PMe3)2] (1-Me). Trimethylphosphine
(20 mL, 20.00 mmol, 1.0 M in THF) was added to a stirring slurry of
anhydrous NiCl2 (1.35 g, 10.42 mmol) in THF (50 mL) at room
temperature. The resulting blue mixture was refluxed at 90 °C for 20
h. After this time, the resulting red solution was allowed to cool to
room temperature, volatiles were removed in vacuo, and the resulting
oily red residue was extracted into pentane (20 mL). The solution was
stored at −30 °C for 24 h to afford 1-Me as red blocks. Yield: 2.20 g,
75%. Anal. Calcd for C6H18Cl2NiP2: C, 25.58; H, 6.44; N, 0.00%.
1
6.00; N, 0.00 %. Found: C, 40.98; H, 6.11; N, 0.00 %. H NMR
(C6D6, 295 K): δ (ppm) 0.95 (9H, d, 2JHP = 10.8 Hz, CH3), 5.12 (5H,
s, C5H5). 13C{1H} NMR (C6D6, 295 K): δ (ppm) 16.06 (d, JCP = 30.2
Hz, CH3), 91.95 (d, JCP = 2.0 Hz, C5H5). 31P{1H} NMR (C6D6, 295
K): δ (ppm) −11.5. FTIR (Nujol): ṽ
(cm−1) 676 (w), 853 (w), 953
(m), 1284 (w). UV/vis (Toluene, 295 K): λmax (497 nm), ε = 750
dm−3 mol−1 cm−1.
Preparation of [Ni(Cl)(η5-C5H5)(PnPr3)] (3-nPr). THF (50 mL)
was added to a stirring mixture of 1-nPr (1.96 g, 4.35 mmol) and 2-H
(0.89 g, 4.71 mmol). The reaction mixture was refluxed at 90 °C for
20 h. After this time, the dark red solution was allowed to cool to
room temperature, filtered, and volatiles were removed in vacuo. The
resulting red solid was extracted into hexane (20 mL), and the
solution was stored at room temperature for 24 h to afford 3-nPr as
red plates. Yield: 2.24 g, 81%. Anal. Calcd for C14H26ClNiP: C, 52.63;
1
H, 8.20; N, 0.00%. Found: C, 52.59; H, 8.30; N, 0.00%. H NMR
(C6D6, 295 K): δ (ppm) 0.99 (9H, t, 3JHH = 8.0 Hz, CH3), 1.36 (6H,
m, CH2), 1.58 (6H, m, CH2), 5.17 (5H, s, C5H5). 13C{1H} NMR
(C6D6, 295 K): δ (ppm) 15.78 (d, JCP = 14.1 Hz, CH2), 17.93 (s,
CH3), 26.73 (d, 2JCP = 27.0 Hz, CH2), 91.91 (d, JCP = 2.0 Hz, C5H5).
1
Found: C, 25.70; H, 6.55; N, 0.00%. H NMR (C6D6, 295 K): δ
(ppm) 1.04 (18H, s, CH3). 13C{1H} NMR (C6D6, 295 K): δ (ppm)
11.51 (s, CH3). 31P{1H} NMR (C6D6, 295 K): δ (ppm) −23.1. FTIR
31P{1H} NMR (C6D6, 295 K): δ (ppm) 16.0. FTIR (Nujol): v (cm−1)
̃
(Nujol): ṽ
(cm−1) 672 (w), 704 (w), 945 (w), 1409 (m), 1626 (m).
UV/vis (Toluene, 295 K): λmax (472 nm), ε = 268 dm−3 mol−1 cm−1.
Preparation of [Ni(Cl)2(PnPr3)2] (1-nPr). Tripropylphosphine (3
mL, 15.00 mmol) was added to a stirring slurry of anhydrous NiCl2
(0.98 g, 7.56 mmol) in THF (50 mL) at room temperature, and the
reaction mixture was refluxed at 90 °C for 20 h. After this time, the
resulting red solution was allowed to cool to room temperature,
volatiles were removed in vacuo, and the resulting oily red residue was
extracted into hexane (20 mL). The solution was stored at −30 °C for
24 h to afford 1-nPr as red blocks. Yield: 1.96 g, 58%. Anal. Calcd for
C18H42Cl2NiP2: C, 48.04; H, 9.41; N, 0.00%. Found: C, 47.85; H,
9.30; N, 0.00%. 1H NMR (C6D6, 295 K): δ (ppm) 1.15 (18H, t, 3JHH
= 8.0 Hz, CH3), 1.66 (12H, m, CH2), 1.85 (12H, m, CH2). 13C{1H}
NMR (C6D6, 295 K): δ (ppm) 16.07 (s, CH3), 17.94 (s, CH2), 23.63
(s, CH2). 31P{1H} NMR (C6D6 295 K): δ (ppm) −3.2. FTIR
775 (w), 1218 (w), 1402 (m), 1946 (w), 2029 (w). UV/vis (Toluene,
295 K): λmax (502 nm), ε = 737 dm−3 mol−1 cm−1.
Preparation of [Ni(Cl)(η5-C5H5)(PnBu3)] (3-nBu). THF (50 mL)
was added to a stirring mixture of 1-nBu (1.20 g, 2.25 mmol) and 2-H
(0.44 g, 2.33 mmol). The reaction mixture was refluxed at 90 °C for
21 h. After this time, volatiles were removed in vacuo and the resulting
red oil was extracted into hexane (20 mL). The solution was cooled to
−30 °C for 24 h to afford 3-nBu as red plates. Yield: 1.27 g, 58 %.
Anal. Calcd for C17H32ClNiP: C, 56.47; H, 8.92; N, 0.00%. Found: C,
1
56.36; H, 9.05; N, 0.00%. H NMR (C6D6, 295 K): δ (ppm) 0.98
(9H, t, 3JHH = 8.0 Hz, CH3), 1.46 (12H, m, 3JHH = 8.0 Hz, CH2), 1.62
(6H, m, CH2), 5.21 (5H, s, C5H5). 13C{1H} NMR (C6D6, 295 K): δ
(ppm) 13.68 (d, 2JCP = 14.1 Hz, CH2), 24.14 (s, CH3), 24.29 (d, JCP
=
27.0 Hz, CH2), 91.94 (d, JCP = 2.0 Hz, C5H5). 31P{1H} NMR (C6D6,
(Nujol): v
̃
(cm−1) 660 (w), 1076 (s), 1414 (w). UV/vis (Toluene,
295 K): δ (ppm) 17.3. FTIR (Nujol): v
̃
(cm−1) 609 (w), 890 (w),
295 K): λmax (488 nm), ε = 381 dm−3 mol−1 cm−1.
905 (w), 1403 (m), 3131 (w, br). UV/vis (Toluene, 295 K): λmax
Preparation of [Ni(η5-C5H4Me)2] (2-Me). [Ni(NH3)6]Cl2 (2.80
g, 12.08 mmol) was added to a solution of [Na(η5-C5H4Me)] (2.48 g,
24.31 mmol) in THF (75 mL), and the reaction mixture was stirred
for 18 h at room temperature. After this time, volatiles were removed
in vacuo, giving a brown solid which was purified by sublimation,
yielding green blocks of 2-Me. Yield: 0.61 g, 23%. Anal. Calcd for
(502 nm), ε = 740 dm−3 mol−1 cm−1.
Preparation of [Ni(Cl)(η5-C5H4Me)(PEt3)] (4). THF (50 mL)
was added to a stirring mixture of 1-Et (0.69 g, 1.89 mmol) and 2-Me
(0.42 g, 1.94 mmol). The reaction mixture was refluxed at 90 °C for
20 h. After this time, the resulting dark red solution was allowed to
cool to room temperature, filtered, and volatiles were removed in
I
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
vacuo. The resulting red solid was extracted into toluene (10 mL).
The solution was cooled to −30 °C for 24 h to afford 4 as red blocks.
Yield: 0.57 g, 54%. Anal. Calcd for C12H22ClNiP: C, 49.46; H, 7.61;
N, 0.00%. Found: C, 49.61; H, 7.68; N, 0.00%. 1H NMR (C6D6, 295
K): δ (ppm) 1.02 (9H, m, CH3), 1.31 (6H, m, CH2), 1.71 (3H, d,
4JHH = 3.6 Hz, C5H4Me), 4.26 (6H, q, 3JHH = 2.0 Hz, CH2), 5.57 (4H,
m, br, C5H4Me). 13C{1H} NMR (C6D6, 295 K): δ (ppm) 7.96 (s,
CH3), 13.46 (s, CH3), 16.76 (d, JCP = 17.1 Hz, CH2), 80.72
(C5H4Me), 98.98 (s, C5H4Me), 113.58 (s, Cq-CH3). 31P{1H} NMR
Crystallographic details and data, structure of 7-Me in
the solid state, NMR spectra, and DFT computational
details (PDF)
Coordinates of optimized structures (XYZ)
Accession Codes
CCDC 1940201−1940212 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(C6D6, 295 K): δ (ppm) 27.6 (m, br). FTIR (Nujol): ṽ
(cm−1) 768
(m), 917 (w), 1351 (m), 1416 (w), 2341 (w), 2360 (w). UV/vis
(Toluene, 295 K): λmax (496 nm), ε = 1018 dm−3 mol−1 cm−1.
Preparation of [Ni(Cl)(η5-C5H4TMS)(PEt3)] (5). THF (50 mL)
was added to a stirring mixture of 1-Et (1.46 g, 3.99 mmol) and 2-
SiMe3 (1.16 g, 3.48 mmol). The reaction mixture was refluxed at 90
°C for 20 h. After this time, the resulting dark red solution was
allowed to cool to room temperature, filtered, and volatiles were
removed in vacuo. The resulting red solid was extracted into
methylcyclohexane (10 mL). The solution was cooled to 9 °C for
24 h, affording 5 as red needles. Yield: 0.62 g, 30%. 1H NMR (C6D6,
295 K): δ (ppm) 0.56 (9H, s, Si(CH3)3), 1.02 (9H, m, br, CH3), 1.28
(6H, m, br, CH2), 5.12 (4H, m, C5H4Me). 13C{1H} NMR (C6D6, 295
K): δ (ppm) −0.17 (s, Si-(CH3)3), 7.97 (s, CH3), 16.50 (d, JCP = 14.1
Hz, CH2), 91.37 (s, C5H4), 91.78 (s, Cq), 95.8 (d, JCP = 5.1 Hz,
C5H4). 31P{1H} NMR (C6D6, 295 K): δ (ppm) 24.9 (br). UV/vis
(Toluene, 295 K): λmax (501 nm), ε = 610 dm−3 mol−1 cm−1.
Preparation of [{Ni(η5-C5H5)(PMe3)}2] (7-Me). A cold (0 °C)
solution of 3-Me (0.60 g, 2.55 mmol) in THF (50 mL) was added to
activated Mg (30 mg, 1.23 mmol). The reaction mixture was stirred
for 5 h at 0 °C, during which a slow color change from red to green
occurred. The resulting green solution was allowed to warm to room
temperature, and the volatiles were removed in vacuo. The dark green
residue was extracted into pentane (10 mL), and the solution was
cooled to 9 °C for 48 h, affording 7-Me as dark green needles. Yield:
0.26 g, 53 %. Anal. Calcd for C16H28Ni2P21: C, 48.08; H, 7.06; N, 0.00
%. Found: C, 47.89; H, 6.89; N, 0.00 %. H NMR (C6D6, 295 K): δ
(ppm) 1.21 (18H, m, CH3), 5.38 (10H, s, C5H5). 13C{1H} NMR
(C6D6, 295 K): δ (ppm) 24.00 (CH3), 86.64 (C5H5). 31P{1H} NMR
(C6D6, 295 K): δ (ppm) −10.1.
AUTHOR INFORMATION
Corresponding Author
■
Stephen T. Liddle − Department of Chemistry, The University
of Manchester, Manchester M13 9PL, United Kingdom;
orcid.org/0000-0001-9911-8778; Email: steve.liddle@
manchester.ac.uk
Authors
Peter A. Cleaves − Department of Chemistry, The University
of Manchester, Manchester M13 9PL, United Kingdom
Alexander J. Ayres − School of Chemistry, University of
Nottingham, Nottingham NG7 2RD, United Kingdom
Lisa Vondung − Department of Chemistry, The University of
Manchester, Manchester M13 9PL, United Kingdom
John C. Stewart − School of Chemistry, University of
Nottingham, Nottingham NG7 2RD, United Kingdom
Philip J. Cobb − Department of Chemistry, The University of
Manchester, Manchester M13 9PL, United Kingdom
Ashley J. Wooles − Department of Chemistry, The University
of Manchester, Manchester M13 9PL, United Kingdom;
orcid.org/0000-0001-7411-9627
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00708
Preparation of [{Ni(η5-C5H5)(PnPr3)}2] (7-nPr). A cold (0 °C)
solution of 3-nPr (1.55 g, 4.86 mmol) in THF (50 mL) was added to
activated Mg (59 mg, 2.43 mmol). The reaction mixture was stirred
for 5 h at 0 °C, during which a slow color change from red to green
occurred. The resulting green solution was allowed to warm to room
temperature, and the volatiles were removed in vacuo. The dark green
residue was extracted into pentane (10 mL), and the solution was
cooled to 9 °C for 24 h, affording 7-nPr as dark green blocks. Yield:
1.10 g, 80 %. Anal. Calcd for C28H52Ni2P21: C, 59.20; H, 9.23; N, 0.00
%. Found: C, 59.02; H, 9.51; N, 0.00 %. H NMR (C6D6, 295 K): δ
(ppm) 0.97 (18H, m, CH3), 1.33 (12H, m, CH2), 1.69 (12H, m,
PCH2) 5.37 (10H, s, C5H5). Due to poor solubility, no 13C{1H}
NMR data could be obtained. 31P{1H} NMR (C6D6, 295 K): δ (ppm)
18.0.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the EPSRC (grants EP/G051763/1 and EP/
M027015/1), ERC (grants 239621 and 612724), Royal
Society (UF110005), Leopoldina Fellowship Programme,
German National Academy of Sciences Leopoldina (grant
LPDS 2018-08), the Universities of Nottingham and Man-
chester, and the University of Manchester Computational
Shared Facility for computational resources.
Preparation of [{Ni(μ:η5-C5H4CH2CH2P(tBu)2)}2] (8). A solution
of 6 (4.53 g, 13.68 mmol) in THF (30 mL) was added dropwise to
cold (−78 °C) activated Mg (0.393 g, 16.17 mmol). The reaction
mixture was allowed to warm to 0 °C and stirred at this temperature
for 4 h. The solvent was removed in vacuo to afford a complex mixture
of products as a sticky dark brown/green solid. Recrystallization of a
portion from toluene afforded a small crop of dark green crystals of 8
suitable for X-ray crystallographic analysis. Yield < 3%. Insufficient
material for full analysis.
REFERENCES
■
(1) Multiple Bonds Between Metal Atoms, 3rd ed.; Cotton, F. A.,
Murillo, C. A., Walton, R. A., Eds.; Springer Science and Business
Media, 2005.
(2) Astruc, D. Organometallic Chemistry and Catalysis; Springer
Berlin Heidelberg: Berlin, 2007; Vol. 98. DOI: 10.1007/978-3-540-
46129-6.
(3) Molecular Metal-Metal Bonds; Liddle, S. T., Ed.; Wiley-VCH
Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015; Vol. 16.
DOI: 10.1002/9783527673353.
(4) Berry, J. F.; Lu, C. C. Metal−Metal Bonds: From Fundamentals
to Applications. Inorg. Chem. 2017, 56 (14), 7577−7581.
(5) Kornecki, K. P.; Berry, J. F. Metal−Metal Bond-Containing
Complexes as Catalysts for C−H Functionalization. In Progress in
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00708.
J
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Inorganic Chemistry, Vol. 58; Wiley, 2014; pp 225−302.
DOI: 10.1002/9781118792797.ch04.
(25) Schranz, I.; Lief, G. R.; Carrow, C. J.; Haagenson, D. C.;
Grocholl, L.; Stahl, L.; Staples, R. J.; Boomishankar, R.; Steiner, A.
Reversal of Polarization in Amidophosphines: Neutral- and Anionic-
KP Coordination vs. Anionic-KP,N Coordination and the Formation
of Nickelaazaphosphiranes. Dalton Trans. 2005, No. 20, 3307.
(26) Seiler, P.; Dunitz, J. D. The Structure of Nickelocene at Room
Temperature and at 101 K. Acta Crystallogr., Sect. B: Struct. Crystallogr.
Cryst. Chem. 1980, 36 (10), 2255−2260.
(27) Black, S. I.; Young, G. B. Synthesis and Spectroscopic
Characteristics of 2-Methyl-2-Phenylpropyl- and Dimethyl(Phenyl)-
Silylmethylnickel(II) Complexes. Polyhedron 1989, 8 (5), 585−596.
(28) Jover, J.; Cirera, J. Computational Assessment on the Tolman
Cone Angles for P-Ligands. Dalton Trans. 2019, 48 (40), 15036−
15048.
(6) Powers, I. G.; Uyeda, C. Metal−Metal Bonds in Catalysis. ACS
Catal. 2017, 7 (2), 936−958.
(7) Peters, R. Cooperative Catalysis; Peters, R., Ed.; Wiley-VCH
Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015.
DOI: 10.1002/9783527681020.
(8) Pal, S.; Uyeda, C. Evaluating the Effect of Catalyst Nuclearity in
Ni-Catalyzed Alkyne Cyclotrimerizations. J. Am. Chem. Soc. 2015, 137
(25), 8042−8045.
(9) Kwon, D.-H.; Proctor, M.; Mendoza, S.; Uyeda, C.; Ess, D. H.
Catalytic Dinuclear Nickel Spin Crossover Mechanism and Selectivity
for Alkyne Cyclotrimerization. ACS Catal. 2017, 7 (7), 4796−4804.
(10) Powers, I. G.; Andjaba, J. M.; Luo, X.; Mei, J.; Uyeda, C.
Catalytic Azoarene Synthesis from Aryl Azides Enabled by a
Dinuclear Ni Complex. J. Am. Chem. Soc. 2018, 140 (11), 4110−
4118.
̈
(11) Gandeepan, P.; Muller, T.; Zell, D.; Cera, G.; Warratz, S.;
Ackermann, L. 3d Transition Metals for C−H Activation. Chem. Rev.
2019, 119 (4), 2192−2452.
(12) Steiman, T. J.; Uyeda, C. Reversible Substrate Activation and
Catalysis at an Intact Metal−Metal Bond Using a Redox-Active
Supporting Ligand. J. Am. Chem. Soc. 2015, 137 (18), 6104−6110.
(13) Ghorai, D.; Finger, L. H.; Zanoni, G.; Ackermann, L. Bimetallic
Nickel Complexes for Aniline C−H Alkylations. ACS Catal. 2018, 8
(12), 11657−11662.
(29) Werner, H.; Kraus, H.-J. Di-μ-(Cyclopentadienyl)-Bis(Tri-
Isopropylphosphine)Dipalladium(I). J. Chem. Soc., Chem. Commun.
1979, 67 (18), 814−815.
(30) Werner, H. Novel Types of Metal-Metal Bonded Complexes
Containing Allyl and Cyclopentadienyl Bridging Ligands. Adv.
Organomet. Chem. 1981, 19 (C), 155−182.
̈
(31) Pyykko, P. Additive Covalent Radii for Single-, Double-, and
Triple-Bonded Molecules and Tetrahedrally Bonded Crystals: A
Summary. J. Phys. Chem. A 2015, 119 (11), 2326−2337.
(32) Hirshfeld, F. L. Bonded-Atom Fragments for Describing
Molecular Charge Densities. Theor. Chim. Acta 1977, 44 (2), 129−
138.
(14) Cooper, B. G.; Napoline, J. W.; Thomas, C. M. Catalytic
Applications of Early/Late Heterobimetallic Complexes. Catal. Rev.:
Sci. Eng. 2012, 54 (1), 1−40.
(15) Peng, S.-M.; Goedken, V. L. Cofacial Dimer of a
Dihydrooctaaza[14]Annulene Complex Containing a Nickel-Nickel
Bond and Related Monomeric Complexes. J. Am. Chem. Soc. 1976, 98
(26), 8500−8510.
(33) Swart, M.; van Duijnen, P. T.; Snijders, J. G. A Charge Analysis
Derived from an Atomic Multipole Expansion. J. Comput. Chem.
2001, 22 (1), 79−88.
(34) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural Population
Analysis. J. Chem. Phys. 1985, 83 (2), 735−746.
(35) Michalak, A.; DeKock, R. L.; Ziegler, T. Bond Multiplicity in
Transition-Metal Complexes: Applications of Two-Electron Valence
Indices. J. Phys. Chem. A 2008, 112 (31), 7256−7263.
(36) Bader, R. F. W. Atoms in Molecules. Acc. Chem. Res. 1985, 18
(1), 9−15.
(37) Cremer, D.; Kraka, E. Chemical Bonds without Bonding
Electron Density ? Does the Difference Electron-Density Analysis
Suffice for a Description of the Chemical Bond? Angew. Chem., Int. Ed.
Engl. 1984, 23 (8), 627−628.
(38) Gardner, B. M.; Patel, D.; Cornish, A. D.; McMaster, J.; Lewis,
W.; Blake, A. J.; Liddle, S. T. The Nature of Unsupported Uranium-
Ruthenium Bonds: A Combined Experimental and Theoretical Study.
Chem. - Eur. J. 2011, 17 (40), 11266−11273.
(39) Van der Maelen, J. F.; Cabeza, J. A. A Topological Analysis of
the Bonding in [M2(CO)10] and [M3(μ-H)3(CO)12] Complexes (M
= Mn, Tc, Re). Theor. Chem. Acc. 2016, 135 (3), 64.
(40) Ayala, R.; Galindo, A. A QTAIM and DFT Study of the Dizinc
Bond in Non-Symmetric [CpZn2Ln] Complexes. J. Organomet. Chem.
2019, 898, 120878.
(41) Gervasio, G.; Bianchi, R.; Marabello, D. About the Topological
Classification of the Metal-Metal Bond. Chem. Phys. Lett. 2004, 387
(4−6), 481−484.
(42) Joy, J.; Danovich, D.; Kaupp, M.; Shaik, S. Covalent vs Charge-
Shift Nature of the Metal−Metal Bond in Transition Metal
Complexes: A Unified Understanding. J. Am. Chem. Soc. 2020, 142
(28), 12277−12287.
(16) Peng, S. M.; Ibers, J. A.; Millar, M.; Holm, R. H. Crystal and
Molecular Structure of Bis[(Tetramethyl[14]Hexaenatotetraaza)-
Nickel] Bis(Trifluoromethanesulfonate): Conversion of a Metal-
Stabilized π-Radical to a Diamagnetic Dimer with a Long Metal-Metal
Bond and Eclipsed Macrocyclic Ligands upon Crysta. J. Am. Chem.
Soc. 1976, 98 (25), 8037−8041.
(17) Jarchow, O.; Schulz, H.; Nast, R. Structure of the Anion in
Solid K4[Ni2(CN)6]. Angew. Chem., Int. Ed. Engl. 1970, 9 (1), 71−71.
(18) Denninger, U.; Schneider, J. J.; Wilke, G.; Goddard, R.; Kruger,
̈
C. Transition Metal Complexes VII. [(η5 -Cp)Ni(PEt3)]2, a Dinuclear
Organometallic Complex with an Unbridged Ni-Ni Bond; Structure
and Heteronuclear Complexes Thereof. Inorg. Chim. Acta 1993, 213
(1−2), 129−140.
(19) Mokhtarzadeh, C. C.; Carpenter, A. E.; Spence, D. P.; Melaimi,
M.; Agnew, D. W.; Weidemann, N.; Moore, C. E.; Rheingold, A. L.;
Figueroa, J. S. Geometric and Electronic Structure Analysis of the
Three-Membered Electron-Transfer Series [(μ-CNR)2[CpCo]2]n (n
= 0, 1-, 2-) and Its Relevance to the Classical Bridging-Carbonyl
System. Organometallics 2017, 36 (11), 2126−2140.
(20) Stanger, A.; Vollhardt, K. P. C. Synthesis and Fluxional
Behavior of [Bis(Trialkylphosphine)Nickelio]Anthracene (Alkyl = Et,
Bu). Organometallics 1992, 11 (1), 317−320.
(21) Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. Synthesis and
Technique in Inorganic Chemistry: A Laboratory Manual, 3rd ed.;
University Science Books: Sausalito, California, 1999.
(43) Jana, R.; Kumar, M. S.; Singh, N.; Elias, A. J. Synthesis,
Reactivity and Structural Studies of (η5-Methylcyclopentadienyl)(η4-
Tetraphenylcyclobutadiene)Cobalt and Its Derivatives. J. Organomet.
Chem. 2008, 693 (25), 3780−3786.
(44) Ting, C.; Jing-Cherng, T.; Chen, Y.; Hua, S.; Su, T.; Chao, Y.
Metallocene Catalyst for Preparing Olefin Polymer. US 6,207,773 BI,
2001.
(45) Wilcox, C. F.; Craig, R. R. The Preparation and Ground State
Properties of Several Hydro- and Functional Group Derivatives of
Bicyclo[2.2.1]Heptadiene Containing 7-Spiro Substituents 1. J. Am.
Chem. Soc. 1961, 83 (18), 3866−3871.
́
(22) Kelly, R. A.; Scott, N. M.; Díez-Gonzalez, S.; Stevens, E. D.;
Nolan, S. P. Simple Synthesis of CpNi(NHC)Cl Complexes (Cp =
Cyclopentadienyl; NHC = N-Heterocyclic Carbene). Organometallics
2005, 24 (14), 3442−3447.
̈
(23) Hussain, M.; Albert, D.; Wartchow, R.; Butenschon, H. The
First Cyclopentadienylalkylphosphane Nickel Chelates: Synthesis,
Structures, and Reactions. Chem. - Asian J. 2007, 2 (6), 782−793.
(24) Cao, R.; Wang, Q.; Sun, H. Dichloridotris-
(Trimethylphosphine)Nickel(II). Acta Crystallogr., Sect. E: Struct.
Rep. Online 2008, 64, m335.
K
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
́
(46) Bartmann, E.; Bogdanovic, B.; Janke, N.; Schlichte, K.;
Spliethoff, B.; Treber, J.; Westeppe, U.; Wilczok, U.; Liao, S. Active
Magnesium from Catalytically Prepared Magnesium Hydride or from
Magnesium Anthracene and Its Uses in the Synthesis. Chem. Ber.
1990, 123 (7), 1517−1528.
L
https://dx.doi.org/10.1021/acs.organomet.0c00708
Organometallics XXXX, XXX, XXX−XXX