A R T I C L E S
Andrews and Wang
Despite the extensive chemistry for group 11 metal dihalide
complexes, the corresponding metal dihydrides are unknown.
The coinage metal diatomic metal hydride molecules have been
characterized in the gas phase by emission spectroscopy.24 An
important theoretical study, however, predicted that AuH will
add H- to form AuH2-.6 This is mechanistically significant, as
the neutral MH2 dihydrides are unstable relative to M + H2, in
contrast to the corresponding MX2 dihalides.2,4 Calculations have
been performed for CuH2, AgH2, and AuH2, and these bent 2B2
ground state molecules range from 6 to 43 to 20 kcal/mol,
respectively, higher in energy than M + H2.14,15 Hence,
preparation of the MH2- anions will be experimentally difficult,
as formation of the MH2 molecules requires excited metal atom
reactions. Although ground state copper (2S) does not react with
H2, excited Cu (2P) reacts spontaneously to form CuH + H in
solid krypton.21 However, the back reaction that might synthe-
size CuH2 on annealing the sample instead restores the lower
energy Cu + H2 reagents. Hence, CuH2 is unstable and
dissociation back to Cu and H2 is favored. Since the AuH bond
is stronger than the CuH bond,24 the AuH2 molecule has a better
chance of survival.
Figure 1. Infrared spectra of copper hydrides in the Cu-H and Cu-D
stretching regions: (a) 5% H2 in argon co-deposited with laser-ablated Cu
at 3.5 K for 60 min, (b) after annealing to 15 K, (c) after λ > 240 nm
photolysis for 15 min, (d) after annealing to 20 K, (e) 5% HD in argon
co-deposited with laser-ablated Cu at 3.5 K for 60 min, (f) after annealing
to 19 K, (g) after λ > 240 nm photolysis for 15 min, (h) after annealing to
25 K, (i) 5% D2 in argon co-deposited with laser-ablated Cu at 3.5 K for
60 min, (j) after annealing to 15 K, (k) after λ > 240 nm photolysis for 15
min, and (l) after annealing to 20 K.
Our investigation of group 11 hydrides involves the reaction
of energetic laser-ablated metal atoms25 and H2 in excess argon,
neon, and hydrogen. All of the group 11 metal hydrides, MH,
and dihydrogen complexes, (H2)MH, have been observed by
matrix infrared spectroscopy and confirmed by comparison to
DFT calculated frequencies.26 We describe here the formation
of the stable linear coinage metal MH2- anions and the related
effective core potential and basis sets for the metals as given in Gaussian
98.34,35 Relativistic effects were included by adjusting the pseudo-
potential parameters to spin-orbit averaged Dirac-Fock energies.35
Vibrational frequencies were computed analytically from the potential
energy surface in the harmonic approximation at the optimized
structures: the calculated frequencies reported here are not scaled. The
BPW91 functional has been recommended for group 11 metal-
containing molecules after comparison of results using several methods,
although the B3LYP functional works almost as well.36 The BPW91
and B3LYP functionals gave frequencies within 1% and bond lengths
within 0.01 Å.
-
square-planar AuH4 anion, and DFT calculations of their
structures and vibrational frequencies.
Experimental and Theoretical Methods
Laser-ablated copper, silver, and gold atoms were reacted with H2,
D2, and HD in excess argon and neon, and with pure H2, HD, and D2
during condensation at 3.5 K using methods described previously for
gold carbonyls27,28 and for vanadium and chromium hydrides using pure
hydrogen as a reactive matrix host.29,30 Infrared (IR) spectra were
recorded at 0.5 or 1.0 cm-1 resolution using an HgCdTeB detector on
a Nicolet 750 FTIR, samples were annealed and irradiated by ultraviolet
and visible light (Philips, 175 W), and more spectra were recorded.
Complementary density functional theory DFT calculations were
performed using the Gaussian 98 program,31 the BPW91 and B3LYP
density functionals,32,33 the 6-311++G(d,p) basis for H, and SDD
Results and Discussion
-
CuH2 and CuD2-. IR spectra in the Cu-H and Cu-D
stretching regions are shown in Figure 1 for laser-ablated Cu
reaction products with H2 and D2 in excess argon, and the
observed frequencies are listed in Table 1. Weak bands are
observed at 1879.8, 1862.5, and 1497.2 cm-1 in the Cu-H
region and at 1354.9, 1343.2, and 1089.4 cm-1 in the Cu-D
region. Annealing to 15 K slightly increases all of these
absorptions, but λ > 240 nm photolysis increases the first,
decreases the second, and destroys the third band in each region.
A subsequent annealing to 20 K sharpens the first, increases
the second, and has no regenerative effect on the third band.
The HD reagent reveals important diagnostic information: the
1879.8 and 1354.9 cm-1 bands are unchanged, the 1862.5 and
1343.2 cm-1 bands are each shifted 0.2 cm-1, and the 1497.2
and 1089.4 cm-1 absorptions are observed along with new,
stronger 1566.9 and 1122.7 cm-1 bands. With H2 + D2 the
1879.8 and 1354.9 cm-1 bands were also unchanged, but the
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