Infrared spectra, structure, and bonding of the group 6 and ammonia M:NH3, H2N-MH, and N≡MH3 reaction products in solid argon

Article abstract of DOI:10.1021/om8003459

Laser-ablated chromium, molybdenum, and tungsten atoms undergo oxidative addition reactions with ammonia during condensation in excess argon. The subject molecules were trapped in solid argon and identified by isotopic shifts and DFT frequency calculations. The 1:1 metal-ammonia complexes increased on annealing and photoisomerized to H₂N-MH and then to N≡MoHj and N=WH ₃, but N≡CrH₃ is too high in energy to be formed here. These products also increased slightly on annealing, which indicates a spontaneous reaction between group 6 metal atoms and ammonia. The N≡MoH₃ and N≡WH₃ molecules contain fully developed triple bonds with effective bond orders of 2.91 and 2.92, computed using the B3LYP density functional, and terminal metal nitride bond lengths, in agreement with those measured for larger organometallic complexes.

Full text of DOI:10.1021/om8003459

Organometallics 2008, 27, 4885–4891
4885
Infrared Spectra, Structure, and Bonding of the Group 6 and
Ammonia M:NH3, H2N-MH, and NtMH3 Reaction Products in
Solid Argon
Xuefeng Wang and Lester Andrews*
Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904-4319
ReceiVed April 17, 2008
Laser-ablated chromium, molybdenum, and tungsten atoms undergo oxidative addition reactions with
ammonia during condensation in excess argon. The subject molecules were trapped in solid argon and
identified by isotopic shifts and DFT frequency calculations. The 1:1 metal-ammonia complexes increased
on annealing and photoisomerized to H2N-MH and then to NtMoH3 and NtWH3, but NtCrH3 is too
high in energy to be formed here. These products also increased slightly on annealing, which indicates
a spontaneous reaction between group 6 metal atoms and ammonia. The NtMoH3 and NtWH3 molecules
contain fully developed triple bonds with effective bond orders of 2.91 and 2.92, computed using the
B3LYP density functional, and terminal metal nitride bond lengths, in agreement with those measured
for larger organometallic complexes.
and annealing a high yield of HNdThH2 is formed, suggesting
Introduction
that f orbitals are strongly involved in the bonding.15
Investigations of the oxidative addition of the N-H bond of
ammonia or amines to transition-metal centers are important
for understanding life sciences, catalytic processes, and nitrogen
fixation.1-3 Examples of the resulting transition-metal organo-
metallic complexes have received considerable attention, includ-
ing theoretical computations of the simple nitrido Mo and W
trihydrides.4-10 Previous investigations of the reaction of
thermally produced late-transition-metal atoms (Fe, Ni, Cu) with
NH3 formed the 1:1 complex (M:NH3) adducts, which rearrange
to H2N-MH under ultraviolet irradiation.11-13 Recently laser-
ablated early transition metal atom (Sc, Ti, V, Zr, Hf) reactions
with NH3 gave analogous H2N-MH products, which rearranged
further to HNdMH2.14 The most recent study shows that the
thorium atom reaction with NH3 is very dramatic: on deposition
The chemistry of group 6 metals (Cr, Mo, W) reveals
interesting family trends. Since different d orbitals participate
in bonding, the tungsten and molybdenum compounds favor
high oxidation states, while the chromium counterparts are
stabilized in low oxidation states. For example, group 6 metal
atoms react with H2O2, giving high-oxidation-state H2WO2 and
H2MoO2 and low-oxidation-state Cr(OH)2, respectively.16 Tung-
sten and molybdenum atoms react with H2 in solid neon to give
trigonal-prismatic C3V hexahydrides, WH6 and MoH6, while only
the hydride complex CrH2(H2)2 is obtained for chromium.17
Further explorations of group 6 metal reactions with other
specific molecules will enrich our knowledge of chemical
bonding models.
* To whom correspondence should be addressed. E-mail: lsa@
virginia.edu.
We present here a matrix infrared spectroscopic investigation
of laser-ablated Cr, Mo, and W atom reactions with NH3 in
solid argon. The most interesting new products, the NtMoH3
and NtWH3 complexes, are simple models of larger organo-
metallic nitride complexes18,19 and are isoelectronic with the
methane reaction product methylidyne molecules HCtMoH3,
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D. C.; Adams, W. W. Chem. ReV. 1996, 96, 2817.
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B. O.; Legzdins, P. J. Am. Chem. Soc. 2006, 128, 14762.
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AdVanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999.
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B.; Xiao, Y.; Guo, Y.; Sturhahn, W.; Cramer, S. P.; Wieghardt, K.; Neese,
F. J. Am. Chem. Soc. 2007, 129, 11053, and references therein.
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and references therein.
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1984, 17, 237.
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162, 123.
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106, 11456 (Sc, Ti, V + NH3). (b) Zhou, M.; Chen, M.; Zhang, L.; Lu, H.
J. Phys. Chem. A 2002, 106, 9017 (Zr, Hf + NH3).
(15) Wang, X.; Andrews, L.; Marsden, C. J. Chem. Eur. J. 2007, 13,
5601 (Th + NH3).
(16) Wang, X.; Andrews, L. J. Phys. Chem. A 2006, 110, 10409 (group
6 + H2O2).
(17) (a) Wang, X.; Andrews, L. J. Am. Chem. Soc. 2002, 124, 5636
(WH6). (b) Wang, X.; Andrews, L. J. Phys. Chem. A 2003, 107, 570 (Cr +
H2). (c) Wang, X.; Andrews, L. J. Phys. Chem. A 2005, 9021 (Mo + H2).
(18) Gebeyehu, Z.; Weller, B.; Neumuller, K.; Dehnicke, K. Z. Anorg.
Allg. Chem. 1991, 593, 99 (R3MoN bond lengths)See also: Geyer, A. M.;
Gdula, R. L.; Wiedner, E. S.; Johnson, M. J. A. J. Am. Chem. Soc. 2007,
129, 3900.
(19) Clough, C. R.; Greco, J. B.; Figueroa, J. S.; Diaconescu, P. L.;
Davis, W. M.; Cummins, C. C. J. Am. Chem. Soc. 2004, 126, 7742 (R3WN
bond lengths). See also: Fox, A. R.; Clough, C. R.; Piro, N. A.; Cummins,
C. C. Angew. Chem., Int. Ed. 2007, 46, 973.
10.1021/om8003459 CCC: $40.75
2008 American Chemical Society
Publication on Web 09/10/2008

4886 Organometallics, Vol. 27, No. 19, 2008
Wang and Andrews
Table 1. Infrared Absorptions (cm-1) Observed for Products of the
Reaction of W, Mo, and Cr Atoms and NH3, 15NH3, and ND3
Isotopic Molecules
and HCtWH3 prepared earlier in this laboratory.20 These new
species have been identified from matrix isolation infrared
spectroscopy, comparison to the results of similar investigations
with NF3,21 and theoretical vibrational frequency calculations.
NH3
15NH3
ND3
assignt
W
1924.1
1917.0
1911.1
1772.2
1770.9
1188.0
1092.2
1924.1
1917.0
1911.1
1772.2
1770.9
1181.8
1058.4
1377.0
1377.0
1375.0
1272.3
1271.6
covereda
covered
NWH3 (a1, W-H str)
NWH3 (e, W-H str)
NWH3 (W-H str) (site)
H2NWH (W-H str)
H2NWH (W-H str) (site)
W:NH3 (1:1 complex)
NWH3 (W-N str)
Experimental and Computational Methods
The experiments for laser-ablated Cr, Mo, and W atom reactions
and for ammonia in excess argon at 8 K have been described in
our previous papers.16,17,20-22 Because of wall adsorption, ammonia
concentrations in the matrix are only approximate, but we estimate
the concentrations used here to be near 0.3% ammonia in argon.
The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate
with 10 ns pulse width) was focused onto the rotating metal target
(Johnson Matthey), which gave a bright plume spreading uniformly
to the cold CsI window. The metal targets were polished to remove
oxide coating and immediately placed in the vacuum chamber. The
laser energy was varied about 10-20 mJ/pulse. FTIR spectra were
recorded at 0.5 cm-1 resolution on a Nicolet 750 instrument with
0.1 cm-1 measurement accuracy using an MCTB detector. Isotopic
samples (15NH3, ND3) were used as received, and ND3 was
codeposited directly from the cylinder through a leak valve to avoid
isotopic contamination (we estimate >90% D survival). Matrix
samples were annealed at different temperatures, and selected
samples were subjected to photolysis by a medium-pressure mercury
arc lamp (Philips, 175 W) with the globe removed.
Mo
covered
1815.4
1761
1704.5
1081
1100.8
1097.6
1815.4
NMoH3 (e,Mo-H str)
NMoH
H2NMoH (Mo-H str)
NWH3 (Mo-N str)
Mo:NH3 (1:1 complex)
Mo:NH3 (1:1 complex)
1704.5
covered
1094.9
1091.7
1232.7
covered
covered
covered
Cr
1165.0
covered
covered
1614.3
1100.2
1095.0
1614.3
1095.8
1090.4
H2NCrH (Cr-H str)
Cr:NH3 (1:1 complex)
Cr:NH3 (1:1 complex)
a Unfortunately, owing to the complication of ammonia aggregates
and isotopic H/D exchange, many regions where new bands are
expected are covered. The methane impurity band (from cracking
silicone diffusion pump oil) covers the expected NMoD3 absorption.
species, such as NH2, ArnH+, and metal oxides have been
identified in previous papers.29-31
Following our successful use of density functional theory to
calculate structures and frequencies for analogous group 6 metal
atom reaction products with methane,20 complementary DFT
calculations were performed for the ammonia system using the
Gaussian 03 program,23 the B3LYP and BPW91 functionals, the
6-311+G(3df,3pd) basis set for hydrogen and nitrogen atoms, and
the SDD pseudopotential for metal atoms.24-27 All of the geo-
metrical parameters were fully optimized, and the optimized
geometry was confirmed by vibrational analysis. Harmonic vibra-
tional frequencies were calculated analytically with zero-point
energy included for the determination of reaction energies. Natural
bond orbital (NBO)23,28 analysis was also done to explore the
bonding in new metal nitrido trihydride product molecules.
Tungsten. Laser-ablated W atoms were codeposited with NH3
in argon; the product absorptions are given in Table 1, and
representative spectra are shown in Figure 1. Many experiments
were performed using different substrate temperatures, laser
energies, and ammonia concentrations in order to optimize
product yields and minimize complications from ammonia
clusters. In the W-H stretching region bands at 1924.1 and
1917.0 cm-1 were observed after deposition, and they increased
slightly on 20 K annealing, increased on >470 nm irradiation,
but decreased on 240-380 nm irradiation. A new weak band
at 1092.2 cm-1 tracks with the upper bands on annealing and
irradiation. A site-split doublet at 1772.2, 1770.9 cm-1 decreases
on annealing, disappears on >470 nm irradiation, but is restored
with 240-380 nm light. A very weak band at 1188.0 cm-1
increases markedly on first annealing, disappears on >470 nm
irradiation, but returns on further annealing. It appears that these
product species are converted into each other on photochemical
rearrangement.
Results
Infrared spectra of products formed in the reactions of laser-
ablated chromium, molybdenum, and tungsten atoms with NH3
in excess argon during condensation at 8 K will be presented
in turn. Density functional calculations were performed to
support the identifications of new reaction products. Common
Isotopic labeling experiments were done using 15NH3 and
ND3 samples (Figures 1 and 2). The W-H stretching frequen-
cies with 15NH3 were essentially the same as those with NH3,
but the weak band at 1092.2 cm-1 shifted to 1058.4 cm-1 in
the ammonia cluster region: these bands increased together on
>470 nm irradiation. With ND3 the upper bands shifted into
(20) (a) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040
(review article). (b) Cho, H.-G.; Andrews, L. J. Am. Chem. Soc. 2005, 127,
8226 (Mo + CH4). (c) Cho, H.-G.; Andrews, L.; Marsden, C. Inorg. Chem.
2005, 44, 7634 (W + CH4).
(21) Wang, X.; Andrews, L.; Lindh, R.; Veryazov, V.; Roos, B. O. J.
Phys. Chem. A 2008, 112, 8030 (group 6 metals + NF3, PF3).
(22) (a) Andrews, L. Chem. Soc. ReV. 2004, 33, 123. (b) Andrews, L.;
Citra, A. Chem. ReV. 2002, 102, 885.
(23) Kudin, K. N., et al. Gaussian 03, Revision D.01, Gaussian, Inc.,
Pittsburgh, PA, 2004.
(24) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
Y.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(25) Perdew, J. P.; Burke, K.; Wang, Y. Phys. ReV. B 1996, 54, 16533,
and references therein.
the W-D stretching region at 1377.0 and 1272.3, 1271.6 cm-1
.
Molybdenum. Infrared spectra of laser-ablated molybdenum
atom reaction products with NH3 in argon are shown in Figure
3, and the absorptions are also given in Table 1. In the Mo-H
stretching region a sharp band at 1815.4 cm-1 appeared on
deposition, increased on annealing, increased markedly on >320
nm irradiation, but decreased on 240-380 nm irradiation. Weak
(26) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265.
(27) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123.
(28) (a) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys.
1985, 83, 735. (b) Reed, A. E.; Curtiss, R. B.; Weinhold, F. Chem. ReV.
1988, 88, 899.
(29) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1965, 43, 4487 (NH2).
(30) (a) Wight, C. A.; Ault, B. S.; Andrews, L. J. Chem. Phys. 1976,
65, 1244. (b) Milligan, D. E.; Jacox, M. E. J. Mol. Spectrosc. 1973, 46,
460 (ArnH+).
(31) Bare, W. D.; Souter, P. F.; Andrews, L. J. Phys. Chem. A 1998,
102, 8279 (metal oxides with resolved Mo isotopic splittings).

Group 6 and Ammonia Reaction Products
Organometallics, Vol. 27, No. 19, 2008 4887
Figure 1. Infrared spectra for the tungsten atom and NH3 reaction products in solid argon at 8 K: (a) W + NH3 deposition for 60 min; (b)
after annealing to 20 K; (c) after >470 nm irradiation; (d) after 240-380 nm irradiation; (e) after annealing to 30 K; (f) W + 15NH3 (75%
survival) deposition for 60 min; (g) after annealing to 20 K; (h) after >470 nm irradiation; (i) after annealing to 25 K; (j) after another
>470 nm irradiation.
Figure 2. Infrared spectra for the tungsten atom reaction products
with ND3 in solid argon at 8 K: (a) deposition for 60 min; (b) after
>530 nm irradiation; (c) after >470 nm irradiation; (d) after >320
nm irradiation; (e) after 240-380 nm irradiation; (f) after >470
nm irradiation; (g) after annealing to 25 K.
Figure 3. Infrared spectra for the molybdenum atom and NH3
reaction products in solid argon at 8 K: (a) deposition for 60 min;
(b) after annealing to 25 K; (c) after >530 nm irradiation; (d) after
>320 nm irradiation; (e) after 240-380 nm irradiation; (f) after
annealing to 30 K.
absorption at 1081 cm-1 on top of a broad feature exhibited
the same photolysis behavior. Another band at 1704.1 cm-1
decreased on >320 nm irradiation and annealing to 25 K but
increased on 240-380 nm irradiation, which seems to be at
the expense of the 1815.4 cm-1 band. One band at 1761 cm-1
on deposition increased on near-UV irradiation. These bands
did not yield isotopic shifts with the 15NH3 sample, but with
the ND3 sample all Mo-H bands shifted into the Mo-D
stretching region, as shown in Table 2.
decreases 20% on >420 nm, 80% on >380 nm, and completely
on >320 nm irradiation while the former band increases. Figure
4 illustrates an informative series of infrared spectra. Subsequent
annealing replaces part of the 1100 cm-1 absorption but
decreases the 1614.3 cm-1 band. An investigation with 15NH3
gave the same 1614.3 cm-1 band and a shift of the lower band
to 1095.0, 1090.4 cm-1. With the deuteriated precursor, the
product absorption at 1165.0 cm-1 reveals the same photo-
chemistry as the 1614.3 cm-1 band, but it clears the ND3
precursor absorption more than the hydrogen counterpart (see
Figure 5).
Chromium. Several experiments were performed with chro-
mium and ammonia. Unfortunately, the Cr-H stretching region
is partially covered by the ammonia precursor absorption, but
a new band at 1614.3 cm-1 increases on stepwise irradiation in
concert with the decrease in new absorptions at 1100.2, 1095.9
cm-1. The latter absorption increases on annealing then
Calculations. DFT calculations were done for the W-NH3,
Mo-NH3, and Cr-NH3 systems and the energy profiles and
optimized geometries are illustrated in Figures 6 and 7. The

4888 Organometallics, Vol. 27, No. 19, 2008
Wang and Andrews
Table 2. Observed and Calculated Harmonic Frequencies of the NtWH3 and NtMoH3 Molecules in the Singlet Ground Electronic States with
C3W Structuresa
NtWH3
NtMoH3
approx descripn
obsd
calcd (L)
intens
calcd (W)
intens
obsd
calcd (L)
intens
calcd (W)
intens
M-H str, a1
M-H str, e
MtN str, a1
MH2 bend, e
MH3 def, a1
NMH def, e
1924
1917
1092
n.o.b
1992
1984
1142
857
691
685
48
392
56
60
39
1962
1957
1110
875
666
680
34
304
41
48
32
1926
1924
1154
839
686
662
44
442
68
126
42
1890
1893
1119
858
640
677
31
336
52
118
48
1815
1081c
16
20
55
46
a Frequencies and intensities are in cm-1 and km/mol, respectively. All observed signals were in an argon matrix. Frequencies and intensities were
computed with B3LYP (L) and BPW91 (W) methods in the harmonic approximation. Symmetry notations are for C3V. b Not observed due to masking
by broad absorption. c Weak band on side of broad absorption.
Figure 4. Infrared spectra for chromium atom and NH3 reaction
products in solid argon at 8 K: (a) deposition for 60 min; (b) after
Figure 6. Energy profiles for isomers of M-NH3 computed at the
annealing to 20 K; (c) after >470 nm irradiation; (d) after >380
B3LYP level of theory.
nm irradiation; (e) after 320 nm irradiation; (f) after annealing to
25 K; (g) after annealing to 30 K.
Figure 7. Optimized structures for products of the group 6 metal
Figure 5. Infrared spectra for chromium atom reaction products
with ND3 in solid argon at 8 K: (a) deposition for 60 min; (b) after
>470 nm irradiation; (c) after >380 nm irradiation; (d) after >290
nm irradiation; (e) after 240-380 nm irradiation; (f) after annealing
to 20 K; (g) after annealing to 25 K.
and ammonia reactions. Parameters are given for B3LYP and
BPW91 (in italics). Bond lengths are in angstroms and angles in
degrees.
theory), and H2NMoH is 3 kcal/mol higher. As expected, the
5
molybdenum ammonia complex with a A1 ground state lies
highest in energy (20 kcal/mol). Similarly, the geometry
parameters were calculated for the analogous chromium species,
but the energy profile shows that H2NCrH is the global
minimum species (Figure 6).
NWH3 molecule with C3V symmetry was found to have the
lowest energy, while the HNWH2 and H2NWH molecules are
respectively 9 and 32 kcal/mol higher in energy. The tungsten
5
ammonia complex W:NH3, is calculated to have a A1 ground
state, which is located highest (52 kcal/mol above NWH3) on
the potential energy surface. The analogous NMoH3 molecule
with C3V symmetry is the global minimum, HNMoH2 is almost
identical in energy (only 1 kcal/mol higher at this level of
Discussion
The new absorptions from products of group 6 metal atom
oxidative addition reactions with NH3 will be identified through

Group 6 and Ammonia Reaction Products
Organometallics, Vol. 27, No. 19, 2008 4889
thermochemical and photochemical properties, isotopic substitu-
tion, and comparison with theoretical frequency calculations.
that the NtWF3 molecule exhibits almost the same WtN
stretching fundamental: namely, 1091.1 cm-1 21
.
The strongest absorption of NtMoH3 was observed in laser-
ablated Mo atom reactions with NH3 in solid argon. A weak
band at 1815.4 cm-1 in the Mo-H stretching region appeared
on deposition, increased twofold on annealing to 20 K and
sixfold on >320 nm irradiation, but decreased on 240-380 nm
irradiation. With 15NH3 this band shows no shift. With ND3
the counterpart Mo-D stretching frequency should be around
1305 cm-1, where it would be covered by a CH4 impurity (from
cracking of silicone diffusion pump oil) band at 1305 cm-1 in
solid argon. A weak 1081 cm-1 peak is associated on photolysis,
but the 15N and D isotopic counterparts were not found with
15NH3 and ND3 because of overlap with precursor bands. This
band is broadened by molybdenum isotopic dilution, but recall
that molybdenum isotopic splitting was resolved for the
counterpart absorption at 1075 cm-1 for the analogous NtMoF3
molecule.21 Following this reasoning, the 1081 cm-1 peak
probably corresponds to the most abundant 98Mo counterpart.
The assignments to NtWH3 and NtMoH3 are confirmed
by B3LYP harmonic frequency calculations. For the NtWH3
molecule the calculation predicted a strong degenerate W-H
antisymmetric mode at 1983.9 cm-1, a weak W-H symmetric
mode at 1992.5 cm-1, and a weak W-N stretching mode at
1142.3 cm-1, which are in very good agreement with experi-
mental values (overestimated by 3.5%, 3.6%, and 6.1%). The
observed 7.1 cm-1 symmetric-antisymmetric W-H stretching
mode separation is matched by the calculated 7.6 cm-1
separation. The WtN stretching mode is predicted to shift
3.11% on 15N substitution, which is in excellent agreement with
the observed 3.09% shift. In the NtWD3 case, this mode
separation is predicted to be 0.2 cm-1, and we observed only
one band, at 1377.0 cm-1. Calculation for NtMoH3 gave a
strong degenerate Mo-H antisymmetric mode at 1921.8 cm-1
and a weak Mo-H symmetric mode at 1923.9 cm-1, and we
observe only a single band at 1815.4 cm-1 (overestimated by
5.4%).
Frequencies calculated with the BPW91 functional are slightly
lower (Table 2), and they correlate well with the B3LYP values,
which substantiates the application of density functional theory
to support vibrational assignments for these molecules.33,34
H2N-MH. The sharp band observed to split at 1772.2,
1770.9 cm-1 on deposition of W + NH3 in solid argon
decreased slightly on annealing to 20 K and depleted totally on
>470 nm irradiation. However, this band was recovered on
240-380 nm irradiation. With ND3 this band shifted to 1272.3,
1271.6 cm-1, defining a 1.393 H/D ratio, which is appropriate
for the W-H stretching vibration of the first insertion product
H2NWH. This is by far the strongest absorption calculated for
this molecule. Unfortunately the W-N and NH2 stretching
modes due to this molecule are not observed because of band
weakness.
M:NH3 Complex (M ) W, Mo, Cr). A very weak band at
1188.0 cm-1 appeared in the laser-ablated W atom reaction with
NH3, increased markedly on annealing to 20 K, but totally
disappeared on >470 nm irradiation, which increased the final
product. With 15NH3 this band shifted to 1181.8 cm-1, but with
ND3 this band fell underneath the precursor, which is charac-
teristic of the symmetric bending mode of NH3 perturbed by a
metal atom. The mixed 14NH3 + 15NH3 experiment gave two
bands at 1188.0 and 1181.8 cm-1. This shift, 5.2%, is
characteristic of the ammonia symmetric deformation mode.
Therefore, the 1188.0 cm-1 band is assigned to the W:NH3
complex (1:1), which is the first step in the activation of
ammonia by metal atoms. Note that similar bands in solid argon
were observed for Th:NH3 at 1149.8 cm-1, Zr:NH3 at 1156.0
cm-1, and Hf:NH3 at 1158.8 cm-1 7,8
.
A similar photosensitive band at 1100.8 cm-1 (site 1097.6
cm-1) was observed in laser-ablated Mo atom reactions with
NH3, which appeared weakly on deposition, increased markedly
on annealing to 20 and 25 K, but disappeared on >530 nm
irradiation. The 15N-substituted band shifts to 1094.9 cm-1 (site
1091.7 cm-1), a 5.3% shift. The analogous Mo:NH3 complex
was identified. A similar pair of matrix-split bands at 1100.2,
1095.9 cm-1 is assigned as Cr:NH3.
Our B3LYP functional calculations support these assignments.
The W:NH3 complex was calculated to have a 5A1 ground state
with a strong symmetric N-H deformation mode at 1220 cm-1
,
which is 193 cm-1 above that calculated for ammonia itself.
The complex is observed 213 cm-1 blue-shifted from ammonia
in solid argon, which is in very good agreement with experiment.
Likewise, the Mo:NH3 complex was predicted at 1180 cm-1
,
which is 153 cm-1 above the ammonia calculation, and the
complex at 1101 cm-1 is 126 cm-1 above the argon matrix
ammonia band. In like fashion, the Cr:NH3 complex was
calculated with a 5A1 ground state and a strong symmetric N-H
deformation mode at 1174 cm-1, which is 149 cm-1 above that
calculated for ammonia itself, whereas the observed 1096 cm-1
band is 121 cm-1 above the precursor band in solid argon.
NtMH3 (M ) W, Mo). The very high frequency absorption
observed at 1917.0 cm-1 on deposition of laser-ablated W atoms
with NH3 tracks with a weak band at 1924.1 cm-1 in the W-H
stretching region and a weak band at 1092.2 cm-1 in the W-N
stretching region.17,32 These bands increased twofold on >470
nm irradiation and decreased on 240-380 nm irradiation, which
caused an increase of another product absorption at 1771.5 cm-1
(Figure 1). In another experiment this group of absorptions
doubled in intensity on >530 and >380 nm irradiation,
decreased on 240-380 nm irradiation, and increased again with
>530 nm irradiation. The two W-H stretching absorptions are
slightly lower than this mode for WH6 in argon but higher than
for the remaining tungsten hydrides.17 With ND3 the two upper
bands shift into one band, 1377.0 cm-1, which defines the 1.392
H/D ratio that is expected for tungsten hydrides. The W-N
stretching frequency is 33 cm-1 higher than the diatomic WtN
molecule fundamental in solid argon,32 suggesting triple-bond
character. With 15NH3 the two upper W-H stretching modes
show no shift, but the W-N stretching mode shifts to 1058.4
cm-1 (3.09%), which is similar to that for the WtN diatomic
molecule. This group of bands can be assigned to W-H and
W-N stretching modes of the NtWH3 molecule. It interesting
The analogous Mo-H stretching mode of H2NMoH was
observed at 1704.5 cm-1 and decreased on >320 nm and
240-380 nm irradiation. With ND3 the counterpart band was
found at 1232.7 cm-1, giving a 1.383 H/D ratio, which is
essentially the same as the ratio found in molybdenum hydrides.
Again, the strongest band calculated for this species is the
Mo-H stretching mode at 1776 cm-1
.
The chromium photochemical product absorption at 1614.3
cm-1 shifts to 1165.0 cm-1 on deuterium substitution (H/D ratio
1.386), which is reasonable for a Cr-H stretching mode.17 The
(32) Andrews, L.; Souter, P. F.; Bare, W. D.; Liang, B. J. Phys. Chem.
A 1999, 103, 4649 (W + N2).
(33) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
(34) Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 3937.

4890 Organometallics, Vol. 27, No. 19, 2008
Wang and Andrews
Table 3. Observed and Calculated Vibrational Frequencies of the Group 6 H2N-MH Insertion Product Molecules in the 5A′ Ground Electronic
States with Cs Structuresa
H2N-CrH
H2N-MoH
H2N-WH
approx descripn obsdb calcd (L) intens calcd (W) intens obsdb calcd (L) intens calcd (W) intens obsdb calcd (L) intens calcd (W) intens
N-H str
N-H str
M-H str
NH2 bend
M-N str
NH2 rock
NH2 def
M-H def
torsion
3577
3496
1710
1542
650
551
551
382
362
13
15
267
17
122
26
110
382
97
3485
3400
1702
1493
645
557
547
382
356
12
11
3568
3483
1776
1536
637
611
509
437
332
21
21
283
10
106
17
105
88
3481
3400
1769
1486
636
606
490
432
323
19
15
3567
3484
1847
1548
655
632
494
447
373
23
19
232
17
87
16
140
23
22
3487
3403
1837
1498
652
626
479
440
360
19
11
205
13
67
13
141
12
19
1614
225 1704
12
91
18
91
232 1772
7
80
14
108
63
144
77
51
41
a Frequencies and intensities are in cm-1 and km/mol, respectively. Frequencies and intensities were computed with B3LYP (L) and BPW91 (W)
methods in the harmonic approximation. b Observed in an argon matrix.
Table 4. Natural Orbital Occupation Numbers for EtMH3
complex rearranges exothermically to the insertion product H2N-
MH for all three metals (see Figure 6), and further rearrangement
through HN)MH2 to NtMH3 is exothermic for Mo and W
only (reaction 2).
Molecules and the Resulting Effective Bond Order (EBO)a
molecule
σ
π
σ*
π*
EBO
NtCrH3 (L)
NtMoH3 (C)
NtMoH3 (L)
NtMoH3 (W)
NtWH3 (C)
NtWH3 (L)
NtWH3 (W)
1.94
1.96
1.96
1.96
1.98
1.97
1.98
3.89
3.93
3.96
3.97
3.93
3.96
3.97
0.09
0.08
0.08
0.08
0.06
0.06
0.06
0.10
0.06
0.04
0.03
0.06
0.04
0.03
2.82
2.88
2.90
2.91
2.89
2.92
2.93
M:NH3 f H2N-MH
(2)
Upon >530 nm irradiation the H2N-MH intermediate
photochemically isomerizes to NtMH3, but with 380-240 nm
irradiation, the rearrangement reverses (reaction 3).
HCtMoH3 (L)
HCtMoH3 (W)
HCtWH3 (L)
HCtWH3 (W)
1.96
1.96
1.98
1.98
3.94
3.97
3.97
3.95
0.07
0.07
0.05
0.05
0.04
0.04
0.04
0.04
2.89
2.90
2.91
2.92
H2N-MH T N≡NH3
(3)
Remember that annealing increases the NtWH3, H2N-MoH,
NtMoH3, and H2N-CrH products: this means that the reaction
of naked group 6 metal atoms with ammonia is spontaneous. A
similar conclusion was reached for thorium, as HNdThH2 was
formed on annealing.15 Here, however, we do not trap the
HNdMoH2 and HNdWH2 intermediates as the reaction goes
on to the more stable final products.
a Values for π orbitals summed over both components; (C) denotes
the CCSD method, (L) the B3LYP functional, and (W) the BPW91
functional.
first insertion product, H2NMoH, is the most stable species in
this system, and the strong Cr-H stretching mode is calculated
as 1710 cm-1 (Table 3), which is 5.9% higher than the observed
value and is in accord with the other first insertion products.
Our B3LYP calculations predicted the W-H stretching mode
of H2NWH molecule at 1847 cm-1, an overestimate of 4.2%,
which is the expected agreement with the experimental value.20
Recall the calculated W-H stretching modes of tungsten
hydrides are 3-4% higher than observed values.17 The calcu-
lated Mo-H stretching frequency of H2NMoH at 1776 cm-1
is 4.1% high and correlates with experiment very well. The trend
in calculated and observed M-H stretching frequencies for these
three insertion products (Table 3) shows that density functional
theory is a very good approximation for these molecules.
NMoH. A weak band at 1761 cm-1 doubled in intensity on
>530 nm irradiation and remained constant throughout the rest
of the experiment. This band is 54 cm-1 below the strong
Mo-H stretching mode of the NtMoH3 molecule, and our
calculation predicts the strong Mo-H stretching modes of
NMoH to fall 104 cm-1 lower. This band shows no 15N shift,
but it was too weak to observe on deuterium substitution. We
have insufficient information to identify NMoH conclusively,
but this possibility should be considered.
M + NH3 f M:NH3 f H2N-MH f N≡MH3
(4)
In the laser-ablation process the NtMH3 molecule can be
photodecomposed to give NtMH. Reaction 5 is endothermic
by 115 and 20 kcal/mol for W and Mo, respectively, which
may account for the possible observation of the Mo and not
the W species.
N≡MH3 + photons f N≡MH + H2
(5)
Bonding. Since the d electrons of tungsten and molybdenum
are strongly involved in σ bonding, reactions of Mo and W with
NH3 gave major stable high-oxidation-state products, NtMoH3
and NtWH3, which are in agreement with the DFT calculated
energy profile (Figure 6). By the same rationale tungsten
(molybdenum) hexahydride, WH6, has been identified in the
W atom reaction with H2 in a low-temperature matrix.17
Tungsten atom reactions with H2O2, giving tungsten hydride
oxide, H2WO2, is another example of d orbital participation in
σ bonding.16
Natural orbital occupations have been calculated for the
NtMoH3 and NtWH3 molecules using the CCSD wave
function based method23 and two density functionals, and the
results are given in Table 4. These occupations are approximate,
as 6.03 electrons occupy this six orbital set instead of the
expected 6.00. The effective bond order is total bonding minus
total antibonding divided by 2. We see that the EBO for the
WtN triple bond is 2.89, 2.92, or 2.93 and for the MotN triple
bond is 2.88, 2.91, or 2.90, depending on the theoretical method
employed. These are fully developed triple bonds, and the
EBO’s are essentially the same for HCtMoH3 and HCtWH3,
computed in the same manner. A slight increase in EBO for
Reaction Mechanisms. Combination of chromium, molyb-
denum, or tungsten atoms with NH3 gives the straightforward
intermediate M:NH3 complex trapped in the cold argon matrix.
M + NH3 f M:NH3
(1)
Reaction 1 is exothermic by 7, 7, and 20 kcal/mol for Cr, Mo
and W, respectively. The more exothermic value for W can be
reasoned by the notion that the lower W atom ground state holds
the electron pair from NH3 with less repulsion. This intermediate

Group 6 and Ammonia Reaction Products
Organometallics, Vol. 27, No. 19, 2008 4891
characterized by a C3V skeleton.38 However, tungsten and
molybdenum hexahalides favor Oh structures because of strong
ligand-ligand repulsion and available π bonding, as shown in
Figure S2 (Supporting Information).
The terminal MotN and WtN triple bonds investigated here
have computed bond lengths (Figure 7) which are in agreement
within the error of calculation with measurements made on
larger organometallic L3MotN and L3WtN complexes, 1.634
and 1.669 Å, respectively.18,19 In addition, the computed triple-
bond lengths for our H3MotN and H3WtN complexes are
essentially the same as calculated by the B3LYP density
functional and at the higher CASSCF/CASPT2 level of theory
for the analogous F3MotN and F3WtN complexes21 and are
about 2% shorter than bond lengths taken from tabulated triple-
bond radii.39 Hence, the B3LYP density functional does a very
good job of describing the terminal MotN and WtN triple
bonds investigated here.
It is also interesting to compare the product energy profile
for methane reactions20 with the products from ammonia. With
chromium, the insertion product is the global minimum energy
species for both reactions. With molybdenum, all products are
closer in energy but the insertion product is lowest for methane
and highest for ammonia. With tungsten, the strength of the
W-H bond contributes to promote maximum hydrogen transfer
and make the triple-bonded species the global minimum for both
precursors.
Figure 8. HOMO’s for triple bonds in the NtCrH3, NtMoH3,
and NtWH3 molecules calculated with B3LYP/SDD/6-
311++G(3df,3pd). The isodensity level is 0.03 e/au3.
the heavier metal species was also observed for the analogous
NtMoF3 and NtWF3 molecules.21
The bonding molecular orbitals for NtCrH3, NtMoH3, and
NtWH3 are plotted in Figure 8. We see mixing of the M-H
and M-N bonding orbitals in varying degrees. Finally, the EBO
for NtCrH3 using the B3LYP method is 2.82, but this molecule
is not formed, owing to weakness of the Cr-H bonds relative
to the N-H bonds in the ammonia precursor.
The bonding and structure of the NtMoH3 and NtWH3
molecules have been considered theoretically by Firman and
Landis,10 and our computed DFT structures are in agreement.
As discussed by these authors, the two π bonds in NtWH3
form a cylinder of electron density around the W-N bond, and
maximization of the π bonding locates the W-H bonds in the
plane perpendicular to the W-N bond.
Conclusions
Reactions of laser-ablated chromium, molybdenum, and
tungsten atoms with ammonia during condensation in excess
argon give the H2N-MH insertion product, which rearranges
to NtMoH3 and NtWH3, but NtCrH3 is too high in energy
to be formed here. The subject molecules were trapped in solid
argon and identified by isotopic shifts and DFT frequency
calculations. The 1:1 metal-ammonia complexes were formed
on annealing and photoisomerized to the above products.
Annealing increases the sharp product absorptions, which
indicates a spontaneous reaction between group 6 metal atoms
and ammonia molecules. The NtMoH3 and NtWH3 molecules
contain fully developed triple bonds with effective bond orders
near 2.9, computed using CCSD and density functional methods.
Furthermore, the triple-bond lengths calculated here for the
simple NtMoH3 and NtWH3 molecules are in agreement with
experimental measurements for larger NtMoR3 and NtWR3
organometallic complexes.18,19
It is interesting to compare the structures of NtWH3 and
NtWF3 (Figure S1 in the Supporting Information). First, the
H-W-H angle in NtWH3 is 4.5° larger than the F-W-F
angle in NtWF3. Second, the WtN triple bond is highly
polarized in NtWF3 because of fluorine electronegativity,
although the WtN bond lengths are essentially the same for
both molecules. These molecular structures clearly violate the
traditional VSEPR model, since stronger F-F atom repulsion
in NtWF3 yields a smaller F-W-F angle than weaker H-H
atom repulsion in NtWH3 yields a larger H-W-H angle. The
π bonding between ligand and metal center and ligand-ligand
repulsion must play critical roles in the bonding.35 For NtWF3
the p electron pair from the F atom donates to d orbitals on the
W center to form a π bond, which leads to the regular VSEPR
structure. However, the F-F ligand repulsion makes the
F-W-F angle larger than 90°. For NtWH3 the H atom does
not have p electrons and only H-H repulsion leads to a larger
H-W-H bond angle. Similar F-W-F bond angles in NtWF3
and PtWF3 suggest that polarization of the outmost core shell
of the tungsten center is not very important. Note that for main-
group-metal hydrides the core polarization favors bent structures
of BaH2 and SrH2.36
Acknowledgment. We gratefully acknowledge financial
support from NSF Grant No. CHE 03-52487 and NCSA
computing Grant No. CHE07-0004N to L.A.
Supporting Information Available: Figures S1 and S2, com-
paring structures with H and F substituents. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM8003459
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1993, 98, 508. (b) Kang, S. K.; Tang, H.; Albright, T. A. J. Am. Chem.
Soc. 1993, 115, 1971. (c) Tanpipat, N.; Baker, J. J. Phys. Chem. 1996,
100, 19818. (d) Kaupp, M. J. Am. Chem. Soc. 1996, 118, 3018. Shen, M.;
Schaefer, H. F., III; Partridge, H. J. Chem. Phys. 1993, 98, 508 (WH6
theoretical calculations).
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S.; Pfennig, V.; Seppelt, K. Chem. Eur. J. 1998, 4, 1687. (c) Haaland, A.;
Hammel, A.; Rypdal, K.; Volden, H. V. J. Am. Chem. Soc. 1990, 112,
454.
Many tungsten and molybdenum complexes have been found
to violate the traditional VSEPR bonding model. For example
valence-bonded WH6 and MoH6 have been confirmed both
experimentally and theoretically as trigonal-prismatic C3V
structures.17,37 The analogues W(CH3)6 and Mo(CH3)6 have been
(35) (a) Kaupp, M. Angew. Chem., Int. Ed. 2001, 40, 3534. (b) Straka,
M.; Hrobarik, P.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 2591.
(36) Wang, X.; Andrews, L. J. Phys. Chem. A 2004, 108, 11500.
(39) Pyykko, P.; Riedel, S.; Patzschke, M. Chem. Eur. J. 2005, 12, 3511.