Synthesis and reaction of the novel complex [AsPh4][OsCl5(H2O)]. X-ray structure analysis of [AsPh4][OsCl5(H2O)]·2EtOH and [AsPh4][OsCl5(EtOH)]·EtOH

Article abstract of DOI:10.1021/ic000084c

The synthesis and characterization of the anionic, mononuclear and homobinuclear osmium complexes [AsPh₄]-[OsCl₅L]·xEtOH [L = H₂O, x = 2 (9); L = EtOH, x = 1 (10a); L = py, x = 0 (10b)] and [AsPh₄]₂[Cl₅Os-(pyz)OsCl₅] (12) (pyz = pyrazine) are described. Upon reduction in a chloride-containing medium, OsO₄ (1) affords the osmium(IV) species [OsCl₅(H₂O)]^- (2), which could be isolated by extraction with n-tributyl phosphate (TBP): Complex 9 is the first fully characterized chloroaquo complex of Os(IV). This complex is an effective starting material for the preparation of novel species, such as 10a, 10b, and 12. The X-ray structures of 9 and 10a were determined. Both compounds crystallize in the monoclinic space group P2₁/n. 9: C₂₈H₃₄AsCl₅O₃Os, a = 10.910(4) A, b = 17.127(5) A, c = 17.555(7) A, β = 103.77(2)°, V = 3186(2) A³, and Z = 4. 10a: C₂₈H₃₂-AsCl₅O₂Os, a = 10.7762(2) A, b = 17.3939(1) A, c = 17.1477(3) A, β = 103.645(1)°, V = 3123.45(8) A, and Z = 4. Complexes 9 and 10a crystallize with two and one molecule of EtOH and are bonded via hydrogen bridges to the H₂O and EtOH ligand in 9 and 10a, respectively.

Full text of DOI:10.1021/ic000084c

Inorg. Chem. 2000, 39, 5725-5730
5725
Synthesis and Reaction of the Novel Complex [AsPh4][OsCl5(H2O)]. X-ray Structure
Analysis of [AsPh4][OsCl5(H2O)]‚2EtOH and [AsPh4][OsCl5(EtOH)]‚EtOH
Andrei Maiboroda, Gerd Rheinwald, and Heinrich Lang*
Technische Universita¨t Chemnitz, Institut fu¨r Chemie, Lehrstuhl fu¨r Anorganische Chemie, Strasse der
Nationen 62, D-09111 Chemnitz, Germany
ReceiVed January 26, 2000
The synthesis and characterization of the anionic mononuclear and homobinuclear osmium complexes [AsPh4]-
[OsCl5L]‚xEtOH [L ) H2O, x ) 2 (9); L ) EtOH, x ) 1 (10a); L ) py, x ) 0 (10b)] and [AsPh4]2[Cl5Os-
(pyz)OsCl5] (12) (pyz ) pyrazine) are described. Upon reduction in a chloride-containing medium, OsO4 (1)
affords the osmium(IV) species [OsCl5(H2O)]- (2), which could be isolated by extraction with n-tributyl phosphate
(TBP). Complex 9 is the first fully characterized chloroaquo complex of Os(IV). This complex is an effective
starting material for the preparation of novel species, such as 10a, 10b, and 12. The X-ray structures of 9 and 10a
were determined. Both compounds crystallize in the monoclinic space group P21/n. 9: C28H34AsCl5O3Os, a )
10.910(4) Å, b ) 17.127(5) Å, c ) 17.555(7) Å, â ) 103.77(2)°, V ) 3186(2) Å3, and Z ) 4. 10a: C28H32-
AsCl5O2Os, a ) 10.7762(2) Å, b ) 17.3939(1) Å, c ) 17.1477(3) Å, â ) 103.645(1)°, V ) 3123.45(8) Å, and
Z ) 4. Complexes 9 and 10a crystallize with two and one molecule of EtOH and are bonded via hydrogen
bridges to the H2O and EtOH ligand in 9 and 10a, respectively.
Introduction
Experimental Section
UV-vis spectra were recorded on a Perkin-Elmer Lambda 40
spectrophotometer. IR spectra were recorded on a Bruker IFS 48
spectrometer as CsI pellets. The Raman spectra were recorded using a
Dilor LabRam spectrometer. Melting (decomposition) points were
determined with a Gallenkamp MFB 595 010 M melting point
apparatus. The UV-vis spectra were recorded on a Perkin-Elmer
Lambda 40 spectrometer. Electrochemical measurements were per-
formed by cyclic voltammetry in solutions of [NnBu4]PF6 (0.1 mol
dm-3) in CH2Cl2 at 290 and 260 K, using a standard three-electrode
Pt-Pt-calomel cell and a Radiometer DEA 101 potentiostat. A scan
rate of 100 mV s-1 was used. All potentials were referenced to the
ferrocene/ferrocenium couple (E1/2 ) 0.000 V). Commercial OsO4,
P(O)(OnBu)3, [PnBu4]Cl, [AsPh4]Cl, pyridine (py), and pyrazine (pyz)
were used without further purification. C2H5OH was distilled from
sodium hydroxide before use. CH2Cl2 was distilled from CaH2.
Microanalyses were performed by the Laboratory of Elemental Analysis
of the Technische Universita¨t Bergakademie Freiberg and the Labora-
tory of Organic Microanalysis, Nesmeyanov Institute of Organometallic
Compounds (Moscow).
Synthesis of [AsPh4][OsCl5(H2O)]‚2EtOH (9). OsO4 (1) (1.0 g,
3.93 mmol) dissolved in 25 mL of a 0.2 M KOH aqueous solution
was added at 25 °C to a solution of 3.0 M H2SO4 (1.0 L) containing
NaCl (58.5 g, 1.0 mol) and Na2SO3 (12.6 g, 0.1 mol). The reaction
mixture was heated to 100 °C for 30 min. After the reaction mixture
was cooled to 25 °C, the obtained product mixture containing
[OsCl5(H2O)]- (2), [OsCl6]2- (3), and [{OsCl3(OH)(H2O)}2(µ-OH)]-
(4) was extracted 10 times with 200 mL of 50 vol % of TBP (5)
in decane (TBP/H2O ratio ) 1:5; TBP ) tributyl phosphate,
P(O)(OnBu)3). The combined extracts were washed 6 times with 0.1
M H2SO4 (TBP/H2O ratio ) 20:1). The aqueous and organic phases
were separated after each washing. Addition of [PnBu4]Cl (6) (0.80 g,
2.70 mmol) to the aqueous phase from the first six washes (Scheme 1)
induces the selective precipitation of [PnBu4]2[OsCl6] (7). The complex
ions 2 and 4 remain in solution. Yield of 7: 0.10 g (0.11 mmol, 3%
based on OsO4). The organic phase, which still contained some
[OsCl5(H2O)]-, was further washed 10 times with 0.1 M H2SO4 (TBP/
H2O ratio ) 10:1). Addition of [AsPh4]Cl (8) (2.40 g, 5.26 mmol) to
the combined aqueous phases from this wash caused the precipitation
[OsCl6]2- is a well-established and characterized complex
anion. The reaction chemistry of this transition metal ion is
dominated by ligand-exchange reactions.1 This process is only
observed when the incoming molecule possesses a more π-acidic
character than Cl- itself. Therefore, ligand exchange is limited
to strong Lewis bases, such as phosphines2 or arsines.3 No
reaction takes place between [OsCl6]2- and weaker donor
molecules such as alcohols. One approach to introduce the latter
type of donors in Os(IV) coordination chemistry is to use
[OsCl5(H2O)]- as the starting material. To date, however, the
latter complex ion has only been generated in situ in chloride-
containing aqueous solutions.4-6 The characterization of this
complex was restricted to UV-vis studies. The same holds true
for H[OsCl5(H2O)]‚TBP, a complex that is formed by TBP
extraction of [OsCl5(H2O)]- from chloride-containing sulfuric
acid solutions.7 However, this complex could not be isolated.
In this paper, the synthesis, isolation, structure, and bonding,
as well as preliminary studies of H2O exchange reactions, of
[OsCl5(H2O)]- are described. The mononuclear and homobi-
metallic species [AsPh4][OsCl5L]‚xEtOH [L ) H2O, x ) 2; L
) EtOH, x ) 1] and [AsPh4]2[Cl5Os(pyz)OsCl5] are the first
examples of osmium(IV) coordination chemistry.
* To whom correspondence should be addressed. E-mail: heinrich.lang@
chemie.tu-chemnitz.de.
(1) Griffith, W. P. Osmium, ComprehensiVe Coordination Chemistry; G.
Wilkinson, Ed.; Pergamon Press: Oxford, 1987; Vol. 4, p 613.
(2) Levinson, J. D.; Robinson, S. D. J. Chem. Soc. A 1970, 2947.
(3) Harris, A. D.; Robinson, S. D. Inorg. Chim. Acta 1980, 35, 25.
(4) Preetz, W.; Scha¨tzel, G. Z. Anorg. Allg. Chem. 1976, 423, 117.
(5) Miano, R. R.; Gardner, C. S. Inorg. Chem. 1965, 4, 337.
(6) Mu¨ller, H.; Scheible, H.; Martin, S. Z. Anorg. Allg. Chem. 1980, 462,
18.
(7) Maiboroda, A. B.; Troshkina, I. D.; Chekmarev, A. M. Extraction of
Pentachloroaquoosmate(IV) Ion from Sulphuric Acid Solution, Pro-
ceedings of International Conference on SolVent Extraction (ISEC’99),
Barcelona, 1999 (in print).
10.1021/ic000084c CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/17/2000

5726 Inorganic Chemistry, Vol. 39, No. 25, 2000
Maiboroda et al.
Scheme 1. Flow Sheet for the Isolation of 2 and 3 as 7 and 9 from 4
Table 1. Experimental Data for the X-ray Diffraction Studies of
Complexes 9 and 10a
of [AsPh4][OsCl5(H2O)]. The product was recrystallized from CH2Cl2/
EtOH solution (1:1, v/v) at 25 °C to give crystals of [AsPh4][OsCl5-
(H2O)]‚2EtOH (9) suitable for X-ray structure analysis. Yield: 1.05 g
(1.22 mmol, 31% based on OsO4).
9
10a
formula
fw
C28H34AsCl5O3Os
860.92
monoclinic
P21/n
4
10.910(4)
17.127(5)
17.555(7)
103.77(2)
3186(2)
1.795
5.481
Mo KR (0.710 73)
173(2)
0.0704/0.1572
0.1084/0.1331
0.8622/0.6985
53.5
C28H32AsCl5O2Os
842.91
monoclinic
P21/n
4
Mp: 237 °C (dec). IR(Os-Cl): 309 cm-1, (Os-O) 467 cm-1
.
Raman (Os-Cl): 336 cm-1, (Os-O) 444 cm-1. UV-vis (CH2Cl2):
λmax ) 351 nm (ꢀ ) 7.2 × 103 L cm-1 mol-1). Potential of Os(IV)/
Os(III) redox couple: E1/2 ) -0.70 V (∆E ) 140 mV, T ) 260 K).
Anal. Calcd for C28H34AsCl5O3Os: C, 39.06; H, 3.99. Found: C, 39.81;
H, 3.54.
space group
cryst syst
Z
a (Å)
10.7762(2)
17.3939(1)
17.1477(3)
103.645(1)
3123.45(8)
1.792
b (Å)
c (Å)
Synthesis of [AsPh4][OsCl5L]‚xEtOH [L ) EtOH, x ) 1 (10a);
L ) py, x ) 0 (10b)]. [AsPh4][OsCl5(H2O)]‚2EtOH (9) (0.23 g, 0.26
mmol) was dissolved in 5 mL of CH2Cl2. A total of 50 mL of C2H5-
OH (synthesis of 10a) or 0.21 g of py (py ) pyridine) in 50 mL of
CH2Cl2 (synthesis of 10b) was added, and the reaction mixture was
heated to reflux for 30 min. After the mixture was cooled to 25 °C,
volatiles were removed in vacuo to afford [AsPh4][OsCl5L]‚xEtOH
[L ) EtOH, x ) 1 (10a), reaction of 9 with C2H5OH; L ) py, x ) 0
(10b), reaction of 9 with py] as brown solids.
â (deg)
V (Å3)
d
calc (g cm-3
)
µ(Mo KR) (mm-1
radiation (Å)
temp (K)
)
5.586
Mo KR (0.710 73)
173(2)
Ra (I > 2σ(I))/all
0.0417/0.0843
0.0729/0.0856
0.5289/0.3013
87.1
Rw/wR2 (I > σ(I))/all
max/min transm
completeness all data
Yield of 10a: 0.22 g (0.26 mmol, 100% based on 9). Single crystals
of 10a for X-ray structure analysis were obtained by crystallization
from EtOH at 25 °C. Mp: 245 °C (dec). IR(Os-Cl) 316 cm-1, (Os-
a R ) ∑||Fo| - |Fc||/∑|Fo|.
5636/8353) were collected at 170 K on a Bruker SMART CCD
diffractometer for yellow (9) and brown (10a) crystals. Each crystal
was attached to a glass fiber with perfluorinated polyether. The unit
cell parameters were checked for the presence of higher lattice
symmetry.8 Data were corrected for absorption using SADABS.10 The
structures were solved by direct methods (SHELX-97).11 Refinement
was carried out by full matrix least-squares techniques on F2 (SHELXL-
97/2).11 Hydrogen atoms were located from the difference Fourier map.
All hydrogens except H(101), H(201), and H(102) for 9 and H(101)
and H(201) for 10a, which were fully refined, were included on their
ideal positions, riding on their carrier atoms. All non-hydrogen atoms
were refined anisotropically. Crystal data and numerical details of the
structure determination and refinement are listed in Table 1. Selected
geometrical details for 9 and 10a are listed in Table 2.
O) 469 cm-1. Raman (Os-Cl) 349 cm-1. UV-vis (CH2Cl2): λmax
)
353 nm (ꢀ ) 7.0 × 103 L cm-1 mol-1). Potential of Os(IV)/Os(III)
redox couple: E1/2 ) -0.75 V (∆E ) 110 mV). Anal. Calcd for C28H32-
AsCl5O5Os: C, 41.47; H, 3.99. Found: C, 39.78; H, 3.19.
Yield of 10b: 0.21 g (0.26 mmol, 100% based on 8). The
spectroscopic data of 10b are summarized in ref 8.
Synthesis of [AsPh4]2[Cl5Os(pyz)OsCl5] (12). [AsPh4][OsCl5(H2O)]‚
2EtOH (9) (0.33 g, 0.39 mmol) was dissolved in 40 mL of CH2Cl2,
and 15 mg (0.19 mmol) of pyz (pyz ) pyrazine) (11) was added. The
reaction mixture was heated to reflux for 1 h. The solution was
concentrated to ca. 3 mL and passed through a pad of silica gel (CH2-
Cl2 eluent). The first dark fraction was isolated. After removal of solvent
in vacuo a dark-brown solid was obtained. Yield: 0.26 g, 0.17 mmol
(85%).
Mp: 265 °C (dec). IR(Os-Cl): 307 cm-1. UV-vis (CH2Cl2): λmax
) 378 nm (ꢀ ) 1.53 × 104 L cm-1 mol-1). Potentials of the Os(IV)/
Os(III) redox couples: E11/2 ) -0.28 V (∆E ) 100 mV), E21/2 ) -0.59
V (∆E ) 80 mV). Anal. Calcd for C52H44N2As2Cl10O5Os: C, 39.49;
H, 2.80. Found: C, 40.13; H, 3.18.
Results
The pentachloroaquoosmate(IV) [OsCl5(H2O)]- (2) was
obtained in 63% yield when OsO4 (1) in a 0.2 M KOH solution
Structure Determination and Refinement of Complexes 9 and
10a. X-ray structure data (total/unique reflections: 9, 5076/9843; 10a,
(9) Spek, A. L. Acta Crystallogr. 1990, A46, C34.
(10) Area-Detector Absorption Corrections; Siemens Industrial Automation,
Inc.: Madison, WI, 1996.
(8) Skukla, A. K.; Zerbe, H. Z.; Preetz, W. Z. Anorg. Allg. Chem. 1980,
468, 39.
(11) Sheldrick, G. M. SHELX-97, Programs for Crystal Structure Analysis,
release 97-2; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

Mono- and Binuclear Osmium Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5727
Table 2. Selected Geometrical Details of Complexes 9 and 10aa
9
10a
Bond Distances [Å]
2.352(3)
Os1-Cl1
Os1-Cl2
Os1-Cl3
Os1-Cl4
Os1-Cl5
Os1-O1
O1-H101
O1-H201
O1-H102
O1-C1
2.3518(13)
2.2895(13)
2.3282(13)
2.3184(13)
2.3233(13)
2.080(4)
2.280(4)
2.328(3)
2.305(4)
2.332(5)
2.030(17)
0.79(7)
0.83(12)
0.86(5)
1.3(3)
1.500(6)
2.567(5)
O1-O2
2.240(3)
2.530(3)
O1-O3
O3-H101
O2-H101
O1-H102
1.75(6)
1.3(3)
Bond Angles [deg]
179.1(8)
Cl2-Os1-O
178.18(13)
111(4)
Os1-O1-H101
Os1-O1-H201
H101-O1-H201
Os1-O1-C1
105(10)
112(10)
118(10)
127.9
158.5
O1-H101-O3
O1-H101-O2
146.21
a The estimated standard deviations of the last significant digits are
shown in parentheses.
Figure 1. Selected UV-vis spectra of the organic extracts of the first
(a), fifth (b), sixth (c), and seventh (d) steps.
is treated with a 3.0 M H2SO4 solution containing 1.0 M NaCl
and 0.1 M Na2SO3 and heated to 100 °C for 30 min (eq 1). The
To obtain the highest concentration of osmium in the organic
phase, a 10-step countercurrent process was used (extraction,
Scheme 1). UV-vis spectroscopic studies show that in addition
to 2, another osmium complex (4) with an absorption maximum
at 378 nm was formed in the aqueous phase and was partly
extracted with TBP. Mu¨ller et al. previously reported the
synthesis of [{OsCl3(OH)(H2O)}2(µ-OH)]- (4), which has a
characteristic absorption band at 378 nm.6 This complex shows
an absorption maximum at 381 nm in the organic phase, due to
a decrease of solvent polarity (Figure 1). UV-vis spectra
measured for all of the 10 extracts clearly show that 2 is almost
completely extracted in the first six steps (Figure 1).
course of the reaction was followed by UV-vis spectroscopy,
where formation of [OsCl5(H2O)]- is indicated by an increase
in the intensity of a characteristic absorption band at 345 nm.6
The complex ion [OsCl5(H2O)]- (2), formed by the reduction
of 1 with Na2SO3 in a chloride-containing sulfuric acid solution,
was separated by solvent extraction with 50 vol % of P(O)-
(OnBu)3 (5) in decane [P(O)(OnBu4)3 ) TBP], as outlined in
Scheme 1.
In the last four steps (out of 10), the extraction of 4 prevails.
Nevertheless, the majority of the homodinuclear complex ion
4 remains in the aqueous phase because of the relatively small
distribution coefficient of 4 (D4 ) 0.4).
Though the extraction of [OsCl6]2- (3) with organophospho-
rus compounds is a well-established method,12 this procedure
has not been used to recover [OsCl5(H2O)]- (2) from aqueous
solutions so far. For our study we choose TBP, which is an
efficient extractant with good Lewis basicity.13 It was found
that 2 could be extracted from chloride-containing sulfuric acid
solutions as H[OsCl5(H2O)]‚TBP solvate.7 The distribution
coefficient of osmium
After the extraction with TBP, the organic phase (extract A,
Scheme 1) contains the complex ions 2, 3, and 4. Therefore, it
was necessary to strip the last two osmium complexes with a
0.1 M solution of H2SO4 to further isolate [OsCl5(H2O)]- (2).
The use of dilute H2SO4 prevents hydrolysis of the correspond-
ing chloroosmium complexes (vide supra), which takes place
when water is used as a stripping agent. The first six steps of
the stripping process remove the complex ion 3 and the
remainder of 4 from the organic extract A (Scheme 1). Complex
ion 2 also goes partially into aqueous phase during the first
stripping. Addition of [PnBu4]Cl (6) as a 0.1 M solution to the
corresponding stripping solution precipitates only [PnBu4]2-
[OsCl6] (7) in an overall yield of 3%. After the precipitation of
7, the complex ions 2 and 4 remain in the aqueous solution,
which can be attributed to their single negative charges, resulting
in increased solubility of the corresponding phosphonium salts.
Further stripping of ion 2 from extract B (Scheme 1) was
achieved with a 0.1 M solution of H2SO4. [OsCl5(H2O)]- (2)
was converted to [AsPh4][OsCl5(H2O)] by addition of [AsPh4]-
Cl (8) to the aqueous solution of the second 10-step stripping
[Os]org
D )
[Os]aq
in this extraction is 14.8, and the equilibrium concentration of
2 in the aqueous phase is [Os]aq ) 5.6 × 10-4 mol/L. The
distribution coefficients of complexes 2 and 3 at various stages
of extraction process are as follows: extraction D2 ) 14.8, D3
) 22.2; first stripping, D2 ) 0.20, D3 ) 0.11; second stripping,
D2 ) 0.46.
(12) Meier, H.; Zimmerhackl, E.; Albrecht, W.; Bo¨sche, D.; Hecker, W.;
Menge, P.; Ruckdeschel, A.; Unger, E. Microchim. Acta 1969, 3, 573.
(13) Cox, M.; Douglas, S. F. Metal Extraction Chemistry. In Handbook of
SolVent Extraction; Lo, T. C., Baird, M. H. I., Hanson, C., Eds.;
Krieger: Malabar, 1991; p 68.

5728 Inorganic Chemistry, Vol. 39, No. 25, 2000
Maiboroda et al.
process (Scheme 1). After crystallization from a dichlo-
romethane/ethanol solution, [AsPh4][OsCl5(H2O)]‚2EtOH (9)
was obtained as brownish crystalline material in an overall yield
of 31%. This complex is stable in air for extended periods of
time and is readily soluble in polar organic solvents such as
dichloromethane, chloroform, and acetone.
A weakly bonded H2O ligand is present in 9. Therefore, this
complex is suitable for ligand exchange reactions. One example
is given in eq 2, where EtOH was added to a dichloromethane
solution of 9. This mixture was refluxed for 30 min, and after
Figure 2. ORTEP drawing (drawn at 50% probability level) of
complex 9 (cation is omitted for clarity).
appropriate workup, complex [AsPh4][OsCl5(EtOH)]‚EtOH
(10a) was isolated as a deep-brown solid in quantitative yield
(eq 2). Also, pyridine (py) can be applied as a Lewis base in
the reaction with 9 to produce the complex [AsPh4][OsCl5(py)]
(10b) in quantitative yield.
Treatment of 9 with pyrazine (11) in a 2:1 molar ratio (eq 3)
in dichlormethane as solvent affords the homobinuclear osmium
(IV) complex [AsPh4]2[Cl5Os(pyz)OsCl5] (12), after appropriate
workup, in 85% yield as a dark-brown solid. In complex 12,
Figure 3. ORTEP drawing (drawn at 50% probability level) of
complex 10a (cation is omitted for clarity).
2.240(3) and 2.530(3) Å, which clearly meets the criterion for
the existence of hydrogen bridges (expected: <2.7 Å).15
As with the aquo ligand in 9, the coordinated EtOH group in
10a gives rise to the association of a second EtOH molecule
via a hydrogen bridge, which is formed between O(1) and O(2)
(Figure 3). The distance O(1)-O(2) is 2.567(5) Å, which again
is in accordance with the above-mentioned criterion for hydro-
gen bridges between oxygen atoms.15
-
two identical OsCl5 units are bridged by the π-conjugated
organic group pyz through Os-N dative bonds.
Structures of 9 and 10a in the Solid State. The molecular
structures of [AsPh4][OsCl5(H2O)]‚2EtOH (9) and [AsPh4]-
[OsCl5(EtOH)]‚EtOH (10a) were determined by X-ray diffrac-
tion. Experimental crystal data are listed in Table 1, and
geometric details are listed in Table 2.
In the structures of molecules containing either [PPh4]+ or
[AsPh4]+ as counterion, two packing modes are observed. For
the cation/anion ratio of 1:1, columnlike structures are usually
observed,16 while for the ratio of 2:1 pairs of ([EPh4]+)2 cations
(E ) P, As) are favored.16 However, in the case of 9 and 10a
in which the cation-to-anion ratio is 1:1, neither of these
structures could be observed. The [AsPh4]+ ions in these species
are independent and noncoordinating counterions.
Spectroscopic Studies. Complexes 9, 10, and 12 were
characterized by UV-vis, IR, and Raman spectroscopy.
In the UV-vis spectra of aqueous solutions containing
[OsCl5(H2O)]- (2), one distinct absorption band is observed at
λmax ) 345 nm (ꢀ ) 7.1 × 103 L cm-1 mol-1), while
dichloromethane solutions of 9 absorbed at λmax ) 351 nm with
ꢀ ) 7.2 × 103 L cm-1 mol-1. The UV-vis spectrum of 10a is
very similar to that of 9, with one band at λmax ) 353 nm (ꢀ )
7.0 × 103 L cm-1 mol-1). The UV-vis spectra of both
complexes can be used for a rapid characterization of these
species, since they are quite distinct from the spectra of
[OsCl6]2- (2) and [{OsCl3(OH)(H2O)}2(µ-OH)]- (4).6 The UV-
vis spectrum of 12 shows one absorption band at 378 nm (ꢀ )
1.53 × 104 L cm-1 mol-1).
Both complexes crystallize in the monoclinic space group
P21/n with two molecules of EtOH (9) or one molecule of EtOH
(10a), which form stable solvates with the corresponding
transition metal complex anions via hydrogen bridges. The metal
center Os1 possesses, as expected, a pseudo-octahedral environ-
ment consisting of the five chloro ligands Cl(1)-Cl(5) and the
H2O molecule (9) or the EtOH (10a) group. The osmium-
chlorine bond lengths for both compounds are in the range
2.305-2.353 Å for the Os(1)-Cl(1,3-5) interatomic bond
distances (Table 2). These values are typical for Os-Cl bonds,
such as those experimentally observed for [OsCl6]2- (2.332-
2.335 Å).14 In contrast to this finding the Os(1)-Cl(2) distances
of 2.280(4) Å for 9 and 2.2895(13) Å for 10a are shorter than
the other ones, which can be explained by the trans influence
of the datively bound H2O (9) or EtOH (10a) groups.
The aquo ligand present in [OsCl5(H2O)]- gives rise to the
formation of the solvate adduct [AsPh4][OsCl5(H2O)]‚2EtOH
(9). Hydrogen bridges between the oxygen atoms of H2O and
EtOH are found [H(101)-O(3), 1.8(2) Å; H(102)-O(1), 1.3-
(3) Å] (Table 2). The distances O(1)-O(2) and O(1)-O(3) are
(15) Jeffrey, G. A.; Maluszynska, H.; Mitra, J. Int. J. Biol. Macromol. 1985,
7, 336.
(14) Kim, E.; Eriks, K.; Magnuson, R. Inorg. Chem. 1984, 23, 393.
(16) Mu¨ller, U. Acta Crystallogr. 1980, B36, 1075.

Mono- and Binuclear Osmium Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5729
Figure 4. Cyclic voltammograms of [AsPh4][OsCl5(H2O)]‚2EtOH (9) at 290 K (left) and 260 K (right).
IR and Raman spectroscopy was used to characterize 9 and
10a. As expected for chloro complexes of platinum metals,18
a
broad band for the Os-Cl stretching frequencies is found at
316 cm-1 for 10a, 309 cm-1 for 9, and 307 cm-1 for 12 in the
IR spectrum. The difference in the frequency for νOs-Cl is the
result of the decrease of π-donating properties of ligands in the
order EtOH > H2O > pyz. Additionally, Os-Cl vibrations are
observed in the Raman spectra of 9 and 10a at 336 cm-1 (9)
and 349 cm-1 (10a). The absorption bands at 467 cm-1 (9) and
469 cm-1 (10a) in the IR spectra can be attributed to the νOs-O
stretching vibrations.19 The νOs-O stretching vibration is also
observable in the Raman spectrum of 9 at 444 cm-1; however,
this band cannot be detected for complex 10a.
Electrochemical Studies. Complex [AsPh4][OsCl5L]‚xEtOH
[L ) H2O, x ) 2 (9); L ) EtOH, x ) 1 (10a)], [PnBu4]2[OsCl6]
(7), and [AsPh4]2[Cl5Os(pyz)OsCl5] (12) were studied by cyclic
voltammetry. It was found that the redox behavior of 9 is
temperature-dependent (Figure 4). At 290 K the one-electron
reduction is irreversible, while at 260 K the Os(IV)/Os(III)
reduction exhibits a reversible behavior at E1/2 ) -0.70 V with
∆E ) 140 mV. Similar observations were made for [NnBu4]2-
[OsCl6].20 In contrast, complex 10a showed a reversible one-
Figure 5. Cyclic voltammogram of [AsPh4][Cl5Os(pyz)OsCl5] (12)
at 290 K.
electron reduction for the Os(IV)/Os(III) redox couple (E1/2
-0.75 V, ∆E ) 110 mV) at 290 K.
)
the reduction process. This anion is formally a Os(IV)-Os(III)
complex with the Os centers bridged through the π-conjugated
organic pyrazine molecule, which has good properties for
intramolecular electron-transfer studies. As expected, further
reduction of this mixed-valence compound takes place at more
negative potential and results in the formation of the Os(III)-
The potentials for the Os(IV)/Os(III) couple in 7, 10a, and 9
shift to more positive values as the π-donating Cl- ligand is
replaced by an EtOH or H2O group [7, E1/2 ) -1.17 V, ∆E )
130 mV (T ) 290 K); 10a, E1/2 ) -0.75 V, ∆E ) 110 mV (T
) 290 K); 9, E1/2 ) -0.70 V, ∆E ) 140 mV (T ) 260 K)].
The difference in the redox potentials for 9 and 10a is attributed
to the H2O and EtOH ligands. EtOH is a stronger electron-
donating group than H2O, leading to a shift of the Os(IV)/Os-
(III) redox couple to a more negative value.
Os(III) complex [Cl5Os(pyz)OsCl5]4-
.
Discussion
The osmium(IV) complex [OsCl5(H2O)]- (2) can easily be
prepared from OsO4 (1) by reaction with NaCl in sulfuric acid
using Na2SO3 as the reducing agent. The product is isolated by
solvent extraction with TBP. [AsPh4][OsCl5(H2O)]‚2EtOH (9)
is the first isolated and characterized chloroaquo complex of
osmium(IV). This complex can be used as an effective starting
material for the synthesis of new pentachloroosmate(IV) species
containing only weakly bound π-donating ligands. Thus, the
reaction of 9 with two-electron donors L afforded [AsPh4]-
[OsCl5L]‚xEtOH [L ) EtOH, x ) 1 (10a); L ) py (10b), x )
0] in quantitative yield. Similarly, the reaction of 9 with 11
produces the homobinuclear complex [AsPh4]2[Cl5Os(pyz)-
OsCl5] (12).
The cyclic voltammogram of 12 (Figure 5) shows two
reversible one-electron reduction processes at E11/2 ) -0.28 V
and E2 ) -0.59 V. The formation of the intermediate
1/2
[Cl5Os(pyz)OsCl5]3- ion can be considered as the first step of
(17) Conradi, E.; Bohrer, R.; Weber, R.; Mu¨ller, U. Z. Kristallogr. 1987,
181, 187.
(18) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds; John Wiley and Sons: New York, 1997; p 218.
(19) Petrov, K. I.; Kravchenko, V. V.; Sinitzin, N. M. Russ. J. Inorg. Chem.
1970, 15, 2732-35.
(20) Taylor, K. J.; Yellowlees, L. J. Molecular Electrochemistry of
Inorganic, Bioorganic and Organometalic Compounds. NATO ASI Ser.,
Ser. C 1993, 69.

5730 Inorganic Chemistry, Vol. 39, No. 25, 2000
Maiboroda et al.
While the reactions of [OsCl6]2- with strong π-acid ligands
are usually not selective and afford a mixture of mono-, di-,
tri-, etc. substituted derivatives, complex 9 reacts very smoothly
and selectively with Lewis bases to produce mononuclear
osmium(IV) complexes of the general type [OsCl5L]- (L ) two-
electron donor ligand) or dinuclear species, such as [AsPh4]2[Cl5-
Os(pyz)OsCl5] (12).
osmium(IV) complexes of the general types [AsPh4][OsCl5L]
(L ) two-electron donating group) and [AsPh4]2[Cl5Os(LL)-
OsCl5] (LL ) bidentate ligand). Since 9 contains a weakly
bound H2O group, this complex is suited for the synthesis of
further Os(IV) complexes, which contain weakly bonded L
groups. Therefore, 9 presents a good opportunity to systemati-
cally study [OsCl5L]- complexes.
Complexes 9, 10a, 10b, and 12 are stable toward air. They
are highly soluble in polar organic solvents such as dichlo-
romethane, chloroform, and acetone. The H2O and the EtOH
ligands in 9 and 10a give rise to the formation of solvation
spheres around the octahedrally coordinated osmium(IV) centers,
as could be shown by X-ray structure analysis. In 9, as a result
of hydrogen bonding, two EtOH molecules are bound to the
H2O ligand, while in 10a a second EtOH molecule is bound
via a hydrogen bridge to the datively coordinated EtOH ligand.
The reaction chemistry of 9 is the topic of current studies.
Acknowledgment. This work was supported in part by the
Volkswagenstiftung, Fonds der Chemischen Industrie and the
DAAD (A.M.). We are grateful to Dr. S. Back for many fruitful
discussions and to Degussa-Hu¨ls AG for many chemicals gifts.
Supporting Information Available: Tables of crystal data and
details of the structure determinations, final coordinates and equivalent
isotropic parameters of non-hydrogen atoms, hydrogen atoms positions
and isotropic thermal parameters, anisotropic thermal parameters, and
bond distances and bond angles for 9 and 10a. This material is available
free of charge via the Internet at http://pubs.acs.org.
Conclusions
[AsPh4][OsCl5(H2O)]‚2EtOH (9) can be used as a starting
material for the preparation of a large variety of different
IC000084C