Methylidyne XC≡MX3 (M = Cr, Mo, W; X = H, F, Cl) diagnostic C-H and C-X stretching absorptions and methylidene CH 2=MX2 analogues

Article abstract of DOI:10.1021/om700689e

Laser-ablated Cr, Mo, and W atoms react with di-, tri-, and tetrahalomethanes to form XC≡MX₃ (M = Mo, W; X = H, F, Cl) methylidyne molecules as major products. Dihalomethanes also give a minor yield of CH₂=MX₂ methylidenes. The electronic state and bonding changes in the CH₂=CrCl₂, CH₂= CrFCl, and CH₂-CrF₂methylidene series, but the Mo and W counterparts are calculated to be triplet state CH₂=MX₂ molecules. Identifications of these new carbon-metal multiple bond species are made through isotopic substitution (D, ¹³C) and isotopic frequency calculations using density functional theory. The HC≡MX₃ molecules exhibit C-H stretching frequencies in the 3030-3090 cm^-1 region* and C≡M stretching frequencies in the 1007-980 cm^-1 range, which vary slightly with the carbon hybridization as determined by the substituents employed here. The XC≡MX₃ molecules show very high C-X stretching frequencies in the 1540-1520 cm^-1 region for X = F and 1300-1230 cm^-1 for X = Cl due to strong bonds and the antisymmetric nature of the X-C-M vibrational mode.

Full text of DOI:10.1021/om700689e

Organometallics 2007, 26, 6373-6387
6373
Methylidyne XCtMX3 (M ) Cr, Mo, W; X ) H, F, Cl) Diagnostic
C-H and C-X Stretching Absorptions and
Methylidene CH2dMX2 Analogues
Jonathan T. Lyon,† Han-Gook Cho,‡ and Lester Andrews*
Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904-4319
ReceiVed July 10, 2007
Laser-ablated Cr, Mo, and W atoms react with di-, tri-, and tetrahalomethanes to form XCtMX3 (M
) Mo, W; X ) H, F, Cl) methylidyne molecules as major products. Dihalomethanes also give a minor
yield of CH2dMX2 methylidenes. The electronic state and bonding changes in the CH2dCrCl2, CH2d
CrFCl, and CH2-CrF2 methylidene series, but the Mo and W counterparts are calculated to be triplet
state CH2dMX2 molecules. Identifications of these new carbon-metal multiple bond species are made
through isotopic substitution (D, 13C) and isotopic frequency calculations using density functional theory.
The HCtMX3 molecules exhibit C-H stretching frequencies in the 3030-3090 cm-1 region and CtM
stretching frequencies in the 1007-980 cm-1 range, which vary slightly with the carbon hybridization as
determined by the substituents employed here. The XCtMX3 molecules show very high C-X stretching
frequencies in the 1540-1520 cm-1 region for X ) F and 1300-1230 cm-1 for X ) Cl due to strong
bonds and the antisymmetric nature of the X-C-M vibrational mode.
45.6% s-character in the bond and distortion to Cs symmetry.8
This fundamentally important simple methylidyne complex can
be investigated by high level theoretical calculations to under-
stand the unique bonding and structure around the Re center
including Jahn-Teller distortion and symmetry reduction.
The (HCtW)(PMe3)4Cl complex is a unique host for the
methylidyne ligand, and detailed vibrational spectroscopic
investigations of this species have been performed to explore
the nature of the metal-carbon triple bond.6,7 Both the C-H
and the CtW bonds are deduced to be slightly longer and the
CtW stretching frequency about 100 cm-1 lower than that
found in the laser excitation spectrum of H-CtW formed in
the reaction of laser-ablated W and methane in the gas phase.9
Recently, a new class of electron-deficient triplet state
methyidyne complexes XC÷MX3 has been prepared from group
4 atom reactions with CX4 (X ) F and Cl) molecules.
Chlorofluoromethane reactions were also investigated, and both
possible structural isomers were observed. The thermochemical
driving force for complete R-F transfer dominated these
reactions and provided a very high diagnostic C-F stretching
mode in the 1460-1410 cm-1 region.10-12 Additional work with
methylene halides and haloforms also revealed extensive
R-halogen transfer reactions.13
Introduction
Compounds containing carbon-transition metal triple bonds
have been investigated extensively for their varied chemistry
and catalytic metathesis of alkynes.1-4 These organometallic
complexes necessarily contain the CtM bond, but most also
contain a variety of substituents and ligands that affect the
vibrational frequencies that could characterize the backbone of
these interesting molecules.5-7 A limited number of such
complexes contain the simple MtC-H moiety, and even fewer
incorporate the halogen substituent as MtC-X (M ) transition
metal).1-5 Although the C-H vibrational characteristics are
difficult to investigate due to very low infrared intensity and
interference from other hydrogen stretching absorptions,6,7 the
C-X stretching bands are very intense and appear in a clean
region of the spectrum. Both modes can be used as a probe to
study the hybridization at carbon in the multiple bond. Accord-
ingly, we have prepared the doublet ground-state HCtReH3
complex in solid argon for observation of the C-H stretching
mode at 3101.8 cm-1, and density functional calculations show
* Corresponding author. E-mail: lsa@virginia.edu.
† Present address: Fritz-Haber-Institut der Max-Planck-Gesellschaft,
Faradayweg 4-6, D-14195 Berlin, Germany.
Group 6 metals form the backbone for high-oxidation state
alkylidene (R1R2CdM) and alkylidyne (RCtM) complexes;
however, there are relatively few examples of chromium
alkylidene and alkylidyne complexes.3a Chromium alkylidynes
‡ Visiting scientist: Department of Chemistry, University of Incheon,
177 Dohwa-dong, Nam-ku, Incheon 402-749, South Korea.
(1) Fischer, E. O.; Kreis, G.; Kreiter, C. G.; Mu¨ller, J.; Huttner, G.;
Lorenz, H. Angew. Chem., Int. Ed. Engl. 1973, 12, 564.
(2) McLain, S. J.; Wood, C. D.; Messerle, L. W.; Schrock, R. R.;
Hollander, F. J.; Youngs, W. J.; Churchill, M. R. J. Am. Chem. Soc. 1978,
100, 5962 (carbyne synthesis).
(8) Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 4098 (HCt
ReH3 complex).
(9) Barnes, D.; Gillett, D. A.; Merer, A. J.; Metha, G. F. J. Chem. Phys.
1996, 105, 6168 (WtC-H emission spectrum).
(10) Lyon, J. T.; Andrews, L. Inorg. Chem. 2006, 45, 9858 (Ti + CF4).
(11) Lyon, J. T.; Andrews, L. Organometallics 2007, 26, 2519 (Gr 4 +
CX4, CF2Cl2).
(12) Lyon, J. T.; Andrews, L. Organometallics 2007, 26, 4152 (Gr 4 +
CF3X).
(13) (a) Lyon, J. T.; Andrews, L. Organometallics 2007, 26, 332 (Gr 4
+ CH2Cl2, CHCl3). (b) Lyon, J. T.; Andrews, L. Inorg. Chem. 2007, 46,
4799 (Gr 4 + CH2F2, CHF3).
(3) (a) Schrock, R. R. Chem. ReV. 2002, 102, 145. (b) Herndon, J. W.
Coord. Chem. ReV. 2004, 248, 3. (c) Herndon, J. W. Coord. Chem. ReV.
2005, 24, 999. (d) Herndon, J. W. Coord. Chem. ReV. 2006, 250, 1889.
(4) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John
Wiley and Sons: New York, 1988.
(5) Foulet-Fonseca, G. P.; Jouan, M.; Dao, N. Q.; Fischer, H.; Schmid,
J.; Fischer, E. O. Spectrochim. Acta 1990, 46A, 339.
(6) Manna, J.; Kuk, R. J.; Dallinger, R. F.; Hopkins, M. D. J. Am. Chem.
Soc. 1994, 116, 9793 ((H-CtW)(PMe3)4Cl).
(7) Manna, J.; Dallinger, R. F.; Miskowski, V. M.; Hopkins, M. D. J.
Phys. Chem. B 2000, 104, 10928 ((H-CtW)(PMe3)4Cl).
10.1021/om700689e CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/07/2007

6374 Organometallics, Vol. 26, No. 25, 2007
Lyon et al.
have been prepared with dialkylamino substituents, but these
species are not stable above -50 °C. The isocyanide or trimethyl
phosphate complexes are more stable.3a,14 X-ray analysis has
found CtCr bond lengths in the 1.74 Å range. Chromium is
more thermally accessible than Mo and W, and the CH2dCr
complex has been investigated.15
Methylidyne complexes have appeared as major products in
Mo and W atom reactions with methane and methyl halides,16-21
and similar HCtMX3 methylidyne compounds are expected to
be prominent in group 6 metal reactions with methylene halides,
haloforms, and CX4 precursors in particular because this group
dominates the alkylidyne complex literature.3
Experimental and Computational Methods
Figure 1. Infrared spectra in the 2970-2920 and 590-460 cm-1
regions for the Cr atom and CH2Cl2 reaction products in excess
argon at 8 K. (a) Cr + 0.5% CH2Cl2 in argon co-deposited for 1 h.
(b) After irradiation (λ > 220 nm). (c) Cr + 0.5% 13CH2Cl2 in
argon co-deposited for 1 h. (d) After irradiation (λ > 220 nm). (e)
Cr + 0.5% CD2Cl2 in argon. (f) After irradiation (λ > 220 nm).
The lable m denotes the methylidene complex absorptions, i denotes
the insertion product, and vertical lines indicate chlorine isotopic
splittings.
Laser ablated Cr, Mo, and W atoms (Johnson-Matthey) were
reacted with CH2X2, CHX3, CX4, CF2Cl2, CF3Cl (DuPont), 13CF3-
Cl, 13CF2Cl2 (prepared),22,23 CD2Cl2, CDCl3, 13CH2Cl2, 13CHCl3,
and 13CCl4 (Cambridge Isotope) in excess argon during condensa-
tion at 8 K using a closed-cycle refrigerator (Air Products HC-2).
These methods have been described in detail elsewhere.24,25 Reagent
gas mixtures were typically 0.5-1% in argon. After reaction,
infrared spectra were recorded at a resolution of 0.5 cm-1 using a
Nicolet 550 spectrometer with an Hg-Cd-Te B range detector.
Samples were later irradiated for 10 min periods by a mercury arc
lamp (175 W) with the globe removed using a combination of
optical filters, and then samples were annealed to allow reagent
diffusion and further reaction.
Complementary density functional theory (DFT) calculations
were carried out using the Gaussian 98 package,26 the hybrid
B3LYP density functional,27 the 6-311++G(2d,p) basis sets for
C, H, F, and Cl,28 and the SDD pseudopotential and basis set29 for
the metals to provide a consistent set of vibrational frequencies for
the reaction products. Different spin states were computed to locate
the ground-state product molecule. Additional BPW91 calculations
were performed for selected reaction products to support the B3LYP
results. Geometries were fully relaxed during optimization, and the
optimized geometry was confirmed by vibrational analysis. All of
the vibrational frequencies were calculated analytically with zero-
point energy included in the calculation of reaction energies. Natural
bond order (NBO)26,30analysis was also done to explore the bonding
in new methylidyne molecules.
Results and Discussion
The reaction products of Cr, Mo, and W atoms with
polyhalomethane precursors will be characterized by matrix
infrared spectra and density functional theory calculations. These
experiments also reveal absorptions due to precursor fragment
and parent anion reactive species that have been reported
previously.22,31
(14) (a) Flippou, A. C.; Lungwitz, B.; Wanninger, K. M. A.; Herdtweck,
E. Angew. Chem., Int. Ed. Engl. 1995, 34, 924. (b) Flippou, A. C.; Fischer,
E. O. J. Organomet. Chem. 1990, 382, 143.
(15) Billups, W. E.; Chang, S.-C.; Hauge, R. H.; Margrave, J. L. Inorg.
Chem. 1993, 32, 1529.
(16) Cho, H.-G.; Andrews, L. Chem.-Eur. J. 2005, 11, 5017 (Mo +
CH3F).
(17) Cho, H.-G.; Andrews, L. J. Am. Chem. Soc. 2005, 127, 8226 (Mo
+ CH4).
Cr + CH2X2. Infrared spectra of the reaction product from
laser-ablated Cr and CH2Cl2 are illustrated in Figure 1. Three
sets of product absorptions with different behaviors on ultraviolet
irradiation were observed. Weak new absorptions at 575.7 and
561.0 cm-1 with carbon-12 and -13 samples (labeled i) were
destroyed by >220 nm irradiation. A new band at 2937.2 cm-1
and a strong partially resolved triplet at 513.3, 511.2, 508.0 cm-1
(labeled m) were not affected by >290 nm irradiation, but they
doubled on >220 nm irradiation. A new weaker 487.5 cm-1
band with a 483.3 cm-1 shoulder also appeared on >220 nm
irradiation. The associated m pair was shifted slightly with
carbon-13, as given in Table 1, but the 487.5 cm-1 band was
not shifted. The weak high-frequency band was not observed
with CD2Cl2, but a new band was observed at 545.1 cm-1, and
(18) Cho, H.-G.; Andrews, L. Organometallics 2005, 24, 5678 (Cr and
W + CH3F).
(19) Cho, H.-G.; Andrews, L.; Marsden, C. Inorg. Chem. 2005, 44, 7634
(W + CH4).
(20) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2006, 110, 13151 (Mo,
W + CH3X).
(21) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040 and
references therein (review article).
(22) Prochaska, F. T.; Andrews, L. J. Chem. Phys. 1978, 68, 5577.
(23) (a) Prochaska, F. T.; Andrews, L. J. Am. Chem. Soc. 1978, 100,
2102. (b) Andrews, L.; Willner, H.; Prochaska, F. T. J. Fluorine Chem.
1979, 13, 273.
(24) Andrews, L.; Citra, A. Chem. ReV. 2002, 102, 885 and references
therein.
(25) Andrews, L. Chem. Soc. ReV. 2004, 33, 123 and references therein.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rega,
N.; Salvador, P.; Dannenberg, J. J.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A.
G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.11.4; Gaussian, Inc.:
Pittsburgh, PA, 2002.
the triplet was better resolved at 506.4, 503.7, and 500.8 cm-1
.
(28) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265.
(29) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor.
Chim. Acta 1990, 77, 123.
(30) (a) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys.
1985, 83, 735. (b) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV.
1988, 88, 899.
(31) (a) Milligan, D. E.; Jacox, M. E.; McAuley, J. H.; Smith, C. E. J.
Mol. Spectrosc. 1973, 45, 377. (b) Prochaska, F. T.; Andrews, L. J. Phys.
Chem. 1978, 82, 1731. (c) Andrews, L.; Prochaska, F. T. J. Phys. Chem.
1979, 83, 824. (d) Andrews, L.; Prochaska, F. T. J. Am. Chem. Soc. 1979,
101, 1190. (Cl-)(HCCl2) (e) Maier, G.; Reisenauer, H. P.; Ju, J.; Hess, B.
A.; Schaad, L. J. Tetrahedron Lett. 1989, 30, 4105.
(27) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
Y.; Parr, R. G. Phys. ReV. B 1988, 37, 785.

Methylidyne XCtMX3 Diagnostic C-H and C-X Absorptions
Organometallics, Vol. 26, No. 25, 2007 6375
3
Table 1. Observed and Calculated Fundamental Frequencies of CH2dCrCl2 in the Ground A2 Electronic State with C2W
Symmetrya
CH2dCrCl2
13CH2dCrCl2
CD2dCrCl2
approximate
description
obs
calc
int
obs
calc
int
obs
calc
int
C-H str, b2
C-H str, a1
HCH bend, a1
CH2 wag, b1
CdCr str, a1
Cr-Cl str, b2
CH2 twist, a2
Cr-Cl str, a1
3187.1
3074.4
1303.7
700.0
602.2
534.3
438.7
379.4
0
2
1
62
41
81
43
23
3174.3
3069.5
1295.9
693.4
586.4
530.4
436.7
378.0
0
2
1
61
40
81
42
22
2369.1
2221.8
990.2
546.8
562.2
498.5
377.0
357.6
0
2
1
47
32
117
21
7
2937.2
2932.5
not obs
545.1
b
b
-
-
513.3c
512.8c
506.4c
a Frequencies and intensities are in cm-1 and km/mol. Four lower real frequencies are not listed. Observed in an argon matrix. Frequencies and intensities
computed with B3LYP/6-311++G(2d,p) in harmonic approximation using the SDD core potential and basis set for Cr. s2 values are 2.64 before and 2.07
after annihilation. Symmetry notations are based on the C2V structure. b Band covered by precursor. c Frequency for 35Cl isotope. Resolved chlorine isotopic
bands at 511.2 and 508.0 cm-1 for CH2dCrCl2 and at 503.7 and 500.8 cm-1 for CD2dCrCl2.
The profile of the strong new band near 500 cm-1 is nearly
the 9/6/1 relative intensities expected for the vibration of two
equivalent chlorine atoms. A diatomic Cr-Cl oscillator at this
frequency would shift 8.3 cm-1 from Cl-35 to Cl-37, and our
maximum shift is measured as 5.6 cm-1; however, the anti-
symmetric stretching mode of a CrCl2 subunit with a 125°
valence angle would shift 5.9 cm-1, which is very close to what
we observed. Hence, we can assign the triplet with confidence
to the antisymmetric Cr-Cl stretching mode of a molecule
containing a CrCl2 subgroup. This finds support from the
preparation of CrCl2 itself through the reaction of chlorine gas
and hot chromium metal, and its characterization by a chlorine
isotopic triplet absorption at 493.5, 490.4, 487.3 cm-1 in solid
argon.32
The weak 2937.2 cm-1 band shifts just 4.7 cm-1 with carbon-
13, which is about one-half of the shift for one uncoupled C-H
bond vibration. Our calculation for CH2dCrCl2 predicts a 4.9
cm-1 shift for the symmetric C-H and 12.8 cm-1 for the
antisymmetric C-H stretching mode where the average is about
right for a single C-H vibration (10.7 cm-1 for HCtMoF2Cl,
for example). Hence, the small carbon-13 shift characterizes
this motion as the symmetric stretching mode of a CH2
subgroup, and the product is identified as the CH2dCrCl2
methylidene molecule. The CH2 wagging mode is often difficult
to calculate, and this mode predicted at 700 cm-1 is probably
covered by precursor absorption in this region; however, the
CD2 counterpart is observed at 545.1 cm-1. Further support is
found from comparison with the calculated frequencies for triplet
CH2dCrCl2 listed in Table 1. The calculated frequencies are
higher than the observed values by 4.7% and 4.1%, respectively,
which is expected for the B3LYP density functional.33 Finally,
the strong diagnostic Cr-Cl stretching mode predicted for the
4 kcal/mol higher energy quintet single-bonded CH2-CrCl2
species at 468 cm-1 is therefore not compatible with our strong
observed 513.3 cm-1 band.
In addition, the weak 575.7 cm-1 band correlates with the
strongest absorption calculated for the chlorine bridge-bonded
quintet CH2Cl-CrCl insertion product (Table S1) that is within
our instrumental limit. The much stronger Cr-Cl stretching
mode is calculated at 394.9 cm-1 and cannot be observed here.
The 14.7 cm-1 carbon-13 shift observed for the 575.7 cm-1
band is in agreement with the predicted 14.6 cm-1 shift, which
identifies this mode as the mostly C-Cr stretching mode of
the insertion product first formed in the reaction (a pure diatomic
C-Cr oscillator would shift 18.2 cm-1).
The reaction proceeds first through the quintet CH2Cl-CrCl
insertion product, which has a Cl bridged structure and is 66
kcal/mol lower in energy than the Cr and CH2Cl2 reagents. The
triplet methylidene complex CH2dCrCl2 is 8 kcal/mol higher
in energy. This methylidene was computed with different spin
multiplicities, and the quintet ground-state 90° dihedral CH2-
CrCl2 is 12 kcal/mol higher, the singlet coplanar CH2dCrCl2
is 49 kcal/mol higher, and the coplanar quintet structure is 68
kcal/mol higher in energy than the quintet insertion product. A
singlet distorted methylidyne is also computed to be 49 kcal/
mol higher than the insertion product, and such a higher energy
species is unlikely to be formed and trapped here. The reaction
to form the triplet CH2dCrCl2 methylidene is 58 kcal/mol
exothermic.
Cr* + CH2Cl2 f CH2Cl-CrCl* f CH2dCrCl2 (1a)
It is interesting that the reaction product energies are slightly
different for methylene fluoride. The quintet insertion product
CH2F-CrF is 47 kcal/mol lower in energy than the reactants,
and the triplet methylidene is 9 kcal/mol higher, but the quintet
methylidene is now 1.4 kcal/mol lower in energy than the first
quintet insertion product. Hence, the most stable product for
these reagents is the quintet methylidene, reaction 1b. Our
infrared spectra, observed, and calculated frequencies that
validate this conclusion are given in Figure S2 and Table S2.
Because CrF2 itself has been reported to absorb at 749 cm-1 in
solid argon,35 the strong band we observed at 746.4 cm-1 with
matrix site splitting at 754.8 cm-1 is appropriate for the strong
antisymmetric Cr-F stretching mode in the CH2-CrF2 mol-
ecule.
The 487.5 cm-1 band that appears on ultraviolet irradiation
when CH2dCrCl2 increases cannot be identified with certainty,
but it shows no C nor D isotopic shift and a chlorine profile
similar to the nearby methylidene. The CrCl molecule has been
observed in the gas phase, and its fundamental is 397 cm-1 34
.
The 487.5 cm-1 photoproduct is likely to be due to a different
matrix trapping site of isolated chromium dichloride, which is
produced here under different conditions.
Cr* + CH2F2 f CH2F-CrF* f CH2-CrF2 (1b)
These interesting differences between the dichloro- and
difluoromethane products suggested experiments with CH2FCl.
(32) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1969, 51, 4143.
(33) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b)
Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 3937.
(34) Bencheikh, M.; Koivisto, R.; Launila, O.; Flament, J. P. J. Chem.
Phys. 1997, 106, 6231.
(35) Blinova, O. V.; Shklyarik, V. G.; Shcherbe, L. D. Zh. Fz. Khim.
1988, 62, 1640.

6376 Organometallics, Vol. 26, No. 25, 2007
Lyon et al.
analogous calculation locates a trigonal HCtCrCl3 molecule
14 kcal/mol higher in energy with the frequencies listed in Table
2. The 3038.5 cm-1 band is the lowest C-H stretching mode
we have observed for a methylidyne complex. The calculation
predicts this mode 4.8% too high, which is almost the same as
the 4.7% too high computation for the C-H frequency of CH2d
CrCl2, and both are appropriate for the B3LYP functional.33
The degenerate H-C-Cr deformation mode calculated at 662.8
cm-1 is observed as a sharp band at 637.8 cm-1 with D-C-
Cr counterpart at 533.7 cm-1 (ratio 1.195). The degenerate Cr-
Cl stretching mode computed at 475.2 cm-1 is observed slightly
higher at 488.9 cm-1 with deuterium counterpart at 477.6 cm-1
.
Unfortunately, the CtCr stretching mode is calculated with zero
infrared intensity, and we observed no absorption in the 1000-
1050 cm-1 region for this band. The correlation between
calculated and observed frequencies for the HCtCrCl3 methyl-
idyne molecule (Table 2) substantiates its present formation and
characterization. With highly exothermic reactions like those
under investigation here, it is common to produce and trap
molecules that are slightly higher than the global minimum
energy product.19-21
This reaction also begins with insertion and then is followed
by R-chlorine transfer to chromium to form the methylidene
and the methylidyne complexes. The reaction to give the
methylidene releases 67 kcal/mol, and the overall reaction 2 to
form the methylidyne complex is 53 kcal/mol exothermic. These
reactions are likely initiated by chromium atoms excited in the
laser-ablation process and sustained by the reaction exother-
micities.
Figure 2. Infrared spectra in selected regions for laser-ablated Cr
atom and CHCl3 reaction products with 0.5% reagent in argon co-
deposited for 1 h at 8 K. (a) CHCl3. (b) After irradiation (λ < 220
nm). (c) After annealing to 30 K. (d) CDCl3. (e) After irradiation
(λ > 220 nm). (f) After annealing to 30 K. Arrows denote the
methylidyne absorptions, and P denotes precursor bands.
New absorptions and calculated frequencies using both B3LYP
and BPW91 density functionals are compared in Table S3 for
the triplet ground-state CH2dCrFCl molecule, and the agreement
confirms this identification. In addition, evidence is found for
the quintet CH2Cl-CrF insertion product at 545.8 and 642.8
cm-1, which is calculated to be 6 kcal/mol more stable than
the CH2F-CrCl analogue. These observed frequencies cor-
respond to calculated CH2 rocking (558 cm-1, 16 km/mol
intensity) and Cr-F stretching (648 cm-1, 181 km/mol) modes,
and the latter shifts to 635.9 cm-1 for CD2Cl-CrF (642 cm-1
,
Cr* + CHCl3 f CHCl2-CrCl* f CHCldCrCl2* f
HCtCrCl3 (2)
178 km/mol). Satisfactory matching of the observed and
calculated Cr-F stretching mode position and small deuterium
shift confirms the identification of the CH2Cl-CrF insertion
product and the mechanism for the more energetically favorable
insertion at the C-F rather than the C-Cl bond. Finally, the
CH2Cl-CrF insertion product has a Cl bridged structure similar
to that of CH2Cl-CrCl.
The fluoroform reaction also gave two products, first with
bands at 758.7, 670.3, and 630.5 cm-1 and the second with
stronger absorptions at 753.8, 655.5, and 649.7 cm-1 bands,
which shifted using fluoroform-d to 758.2, 667.8, and 531.8
cm-1 and to 753.2, 652.9, and 541.9 cm-1. Infrared spectra in
Figure S3 show that the first bands increased 60% and the
second set 300% on >290 and >220 nm irradiations. The first
group of absorptions and deuterium counterparts corresponds
with the three strongest bands calculated for the triplet CHFd
CrF2 methylidene product, which is 44 kcal/mol lower in energy
than the reagents. These frequencies computed at 762.6, 668.8,
and 627.5 cm-1 are antisymmetric and symmetric Cr-F
stretching and C-H wagging modes, which shift to 762.3, 667.3,
and 528.0 cm-1 on deuteriation (the strong C-F mode predicted
at 1168 cm-1 is covered by precursor).
The two strongest absorptions in the second group at 753.8
and 655.5 cm-1 are antisymmetric and symmetric Cr-F
stretching modes, which are predicted by our calculation for
trigonal HCtCrF3 at 770.7 and 684.9 cm-1, and for DCtCrF3
at 769.7 and 683.5 cm-1 (Table 2). The degenerate H-C-Cr
bending mode calculated at 622.3 cm-1 is observed at 649.7
cm-1 with D-C-Cr counterpart at 541.9 cm-1 (ratio 1.199).
The B3LYP calculation is 2.2% and 4.5% high for the Cr-F
stretching modes, as is typical for this functional,33 but is 4.2%
low for the more difficult to model bending mode. The
exothermic reaction to form CHFdCrF2 promotes another
R-fluorine transfer to the 13 kcal/mol higher energy HCtCrF3
product in these laser ablation experiments.
Cr* + CH2FCl f CH2Cl-CrF* f CH2dCrFCl (1c)
Cr + CHX3. The chloroform reaction revealed two sets of
product bands. The first bands at 3038.5, 637.8, and 488.9 cm-1
decreased by 50% on >290 nm and another 20% on >220 nm
irradiations and then increased 2-fold on annealing to 30 K,
and decreased in concert by 50% on the final full arc irradiation
as shown in Figure 2. Another band at 463.6 cm-1 increased
on UV irradiation, while the first set decreased. The two lower
bands contain unresolved red shoulders for chlorine isotopic
components of a species with two or more chlorine atoms. The
chloroform-d absorptions for the first photosensitive set were
observed at 533.7 and 477.6 cm-1, and the second band
increased at 469.3 cm-1. The highest frequency component is
lost under the strong precursor C-D stretching band based on
the expected deuterium shift.
Our B3LYP calculations find the triplet methylidene CHCld
CrCl2 to have its strongest infrared absorption at 475.1 cm-1
with a deuterium counterpart blue-shifted to 479.8 cm-1 due to
interaction with a weaker hydrogen deformation mode, which
are in good agreement with our 463.6 and 469.3 cm-1 bands.
A quintet state with a 90° dihedral angle is 1 kcal/mol lower in
energy, but the antisymmetric Cr-Cl mode is too low at 457
cm-1 and red shifts 10 cm-1 with deuterium. Furthermore,
higher spin states are favored by the B3LYP calculation.36 An
Cr + CX4. Infrared spectra following the Cr and CCl4
reaction revealed absorptions at 1230.6 cm-1, with a weaker
1228.2 cm-1 shoulder, and at 495.7 cm-1 that decreased 60%
together on >290 nm irradiation and another 10% on >220
(36) Swart, M. Inorg. Chim. Acta 2007, 360, 179.

Methylidyne XCtMX3 Diagnostic C-H and C-X Absorptions
Organometallics, Vol. 26, No. 25, 2007 6377
1
Table 2. Observed and Calculated Fundamental Frequencies of HCtCrX3 Complexes in the Ground A1 Electronic State with
C3W Structurea
HCtCrCl3
DCtCrCl3
HCtCrF3
DCtCrF3
approximate
description
obs
calc
int
31
obs
calc
int
31
obs
calc
int
obs
calc
int
C-H str, a1
3038.5
3183.9
1054.9
475.2
390.1
662.8
234.4
168.6
118.4
covered
2365.1
1013.5
468.7
389.5
533.7
212.3
143.7
100.4
covered
3190.8
1047.8
770.7
684.9
622.3
294.2
274.0
197.5
39
7
covered
2369.3
1008.5
769.7
683.5
532.1
251.4
273.3
197.5
22
7
HCtCr str, a1
Cr-X str, e
0
0
488.9
637.8
79 × 2
477.6
533.7
62 × 2
753.8
655.5
649.7
200 × 2
753.2
652.9
541.9
187 × 2
Cr-X str, a1
7
7
59
58
H-C-Cr def, e
C-Cr-X def, e
Cr-X3 umb, a1
Cr-X2 bend, e
68 × 2
58 × 2
29 × 2
16 × 2
8
4 × 2
12 × 2
14 × 2
8
4 × 2
15 × 2
7 × 2
0
0
0
0
a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with B3LYP/6-311++G(2d,p)
in the harmonic approximation using the SDD core potential and basis set for Cr. Symmetry notations are based on the C3V structure.
CCl2-CrCl2 complex, which is 6 kcal/mol lower in energy than
the triplet methylidene CCl2dCrCl2 complex and 11 kcal/mol
lower than the ClCtCrCl3 product. These modes are calculated
at 884 and 456 cm-1, which correlate with the observed bands.
The reaction with CF4 produced weak bands at 1475.9, 765.7,
and 759.1 cm-1, which increased slightly on >290 nm irradia-
tion and were affected little by >220 nm irradiation. The bands
near 760 cm-1 are appropriate for Cr-F vibrations, but the
1475.9 cm-1 band is lower than our prediction (Table S4) for
the antisymmetric F-C-Cr stretching mode for the FCtCrF3
complex [this band is probably covered by the intense precursor
absorption at 1550-1530 cm-1].
Investigations were done with Freon compounds because they
are more reactive than CF4, and we have already synthesized
the carbon-13 precursors.31 Figure S4 compares spectra from
the Mo and Cr reactions with CF2Cl2, which give very different
product yields, and spectra from Cr with CF3Cl. Notice first
the strong 1521.6 [1475.7] cm-1 band with Mo that increases
markedly on both UV irradiations, and then find the weak
1528.3 [1483.5] and 1481.8 [1438.5] cm-1 bands with Cr [the
lower band is marked with the lighter arrow], which show
opposite behavior on the two UV irradiations, and the very
strong 1303.1 and 1261.3 cm-1 bands [carbon-13 counterparts
given in square brackets], which exhibit a still different behavior
on irradiation. The lower region reveals weaker 443.4 [441.1]
and 697.8 [693.8] cm-1 bands, which track with the very strong
bands all increasing 35% on >290 nm and none on >220 nm
irradiations. In addition, weaker new bands appear at 830.9
[805.3] and 488.5 [488.5] cm-1 on >220 nm irradiation.
It is clear from the carbon-13 shift percentages, 2.93% and
2.92%, that the weak CF2Cl2 product bands at 1528.3 and 1481.8
cm-1 are due to antisymmetric F-C-Cr vibrations for a linear
linkage. The FCtCrFCl2 complex is predicted to have a very
strong antisymmetric F-C-Cr stretching vibration at 1565.7
cm-1 (Table S4) with a 2.96% carbon-13 shift, and such
agreement is sufficient to identify this methlyidyne complex.
The stronger 1481.8 cm-1 band is best identified as a photor-
eversible structural isomer of this same species. The matrix cage
may assist in the stabilization of these two isomers. Although
the other positional isomer, ClCtCrF2Cl, is 2 kcal/mol more
stable, the strong Cl-C-Cr mode is predicted at 1259 cm-1 in
a region covered with precursor absorptions, which preclude
its detection.
Figure 3. Infrared spectra in selected regions for the group 6 metal
atom and CCl4 reaction products in excess argon at 8 K. 0.5%
13
reagent co-deposited for 1 h with (a) W, (d) Mo, (g) Mo and
-
CCl4 (90% enriched), and (j) Cr spectra (b), (e), (h), and (k) after
irradiation (λ > 290 nm). Spectra (c), (f), (i), and (l) after irradiation
(λ > 220 nm). C denotes bands common to experiments with
different metals. Arrows denote the methylidyne complex absorp-
tions.
nm irradiation. Additional absorptions for different species
appeared at 859.3, 487.7, and 466.6 cm-1 on irradiation. This
spectrum is compared to spectra from Mo and W reactions in
Figure 3. Carbon-13 substitution shifted the former bands to
1189.3 and 495.9 cm-1
.
The strongest absorptions predicted by B3LYP calculations
for the ClCtCrCl3 complex are the antisymmetric Cl-C-Cr
stretching and Cr-Cl stretching modes at 1258.5 and 484.6
cm-1, respectively, which correlate very well with our photo-
sensitive product absorptions at 1230.6 and 496.5 cm-1. The
41.3 cm-1 carbon-13 shift is more than that for a harmonic
diatomic oscillator, but the 2.4 cm-1 shift to the weaker shoulder
appropriate for chlorine-37 in natural abundance is less than
that for the diatomic model. Hence, these complementary shifts
characterize the antisymmetric Cl-C-Cr stretching vibration,
which depends more on the carbon reduced mass. Furthermore,
the relative intensities of the chlorine isotopic peaks and the
residual carbon-12 in the 90% carbon-13-enriched sample
characterize the vibration of single C and Cl atoms. The 496.5
cm-1 band has red shoulders appropriate for the vibration of
two or more chlorine atoms. The above absorptions make a
strong case for identification of the ClCtCrCl3 molecule.
The 859.3 and 466.6 cm-1 absorptions that increase on the
>290 nm irradiation when ClCtCrCl3 absorptions decrease are
appropriate for the strongest C-Cl stretching mode and Cr-
Cl stretching mode of the quintet ground-state methylidene
The very strong 1303.1 and 1261.3 cm-1 bands also show
carbon-13 shifts, 2.56% and 2.57%, that are appropriate for
antisymmetric C-F stretching modes, but with less carbon
participation that comes with a smaller valence angle. The CF2-
CrCl2 methylidene products were computed, and the quintet was
found to be 13 kcal/mol lower in energy than the triplet state
and to have very strong antisymmetric and symmetric C-F

6378 Organometallics, Vol. 26, No. 25, 2007
Lyon et al.
Figure 4. Infrared spectra in selected regions for the Mo atom and CH2Cl2 reaction products in excess argon at 8 K. (a) Mo + 0.5%
CH2Cl2 in argon co-deposited for 1 h. (b) After irradiation (λ < 290 nm). (c) After irradiation (λ > 220 nm). (d) After annealing to 30 K.
(e) Mo + 0.5% 13CH2Cl2 in argon co-deposited for 1 h. (f) After irradiation (λ > 220 nm). (g) After annealing to 30 K. (h) Mo + 0.5%
CD2Cl2 in argon. (i) After irradiation (λ > 220 nm). (j) After annealing to 30 K [bands in 900 cm-1 region are common with this precursor
with all metals]. The lable m denotes methylidene, arrows indicate the methylidyne complex absorption, and P denotes precursor bands.
3
Table 3. Observed and Calculated Fundamental Frequencies of CH2dMoCl2 in the Ground A2 Electronic State with C2W
Symmetrya
CH2dMoCl2
13CH2dMoCl2
CD2dMoCl2
approximate
description
obs
calc
int
obs
calc
int
obs
calc
int
C-H str, b2
3170.3
3066.3
1298.0
735.3
729.0
454.1
438.8
376.2
1
17
8
88
35
98
0
3157.8
3061.0
1288.0
729.0
710.3
452.6
438.8
376.0
1
16
7
86
35
99
0
2354.9
2221.0
1027.5
575.5
642.3
444.2
311.4
375.7
1
C-H str, a1
15
18
54
23
103
0
HCH bend, a1
CH2 wag, b1
CdMo str, a1
Mo-Cl str, b2
CH2 twist, a2
Mo-Cl str, a1
688.1
-
446.0
682.4
-
444.9
542.2
628.3
444.6
b
b
25
25
25
a Frequencies and intensities are in cm-1 and km/mol. Four lower real frequencies are not listed. Observed in an argon matrix. Symmetry notations are
based on the C2V structure. Frequencies and intensities computed with B3LYP/6-311++G(2d,p) in harmonic approximation using the SDD core potential
and basis set for Mo. b Band covered by precursor.
stretching modes at 1276.6 and 1238.6 cm-1 with 2.62% carbon-
13 shifts, a weaker 690.6 cm-1 band with 4.1 cm-1 carbon-13
displacement, and a strong Cr-Cl stretching mode at 441.0 cm-1
with 3.5 cm-1 carbon-13 shift (Table S5). Considering the
approximations involved, although the B3LYP functional favors
higher spin states,30 the 1303.1, 1261.3, 697.8, and 443.3 cm-1
bands and carbon-13 shifts correlate very well and identify this
major product as the quintet CF2-CrCl2 methylidene. In
contrast, the higher energy triplet methylidene calculated
frequencies do not fit our observations [the Cr-Cl stretch is
too high at 492 cm-1 and the symmetric C-F stretch is too
low at 1156 cm-1]. The weaker 830.9 and 488.5 cm-1 bands
are most likely due to C-F and Cr-Cl stretching modes in the
isomer quintet CFCl-CrFCl complex, which is calculated to
be 9 kcal/mol higher in energy. Based on our experience with
halomethane reactions, the methylidene reaction product is
expected to be a major product in these systems. What is
important here is that the major lower oxidation state quintet
methylidene product is 24 kcal/mol lower in energy then the
minor singlet FCtCrFCl2 methylidyne product. The absorptions
observed with CF3Cl support these identifications.
CF4 reaction is exothermic by only 31 kcal/mol, and the product
yield is much lower than that for CCl4. The chlorofluorocarbon
reactions appear to go through C-Cl insertion followed by
successive R-halogen transfers.
Mo + CH2X2. The reactions of laser-ablated molybdenum
atoms and methylene chloride are shown in Figure 4 for four
diagnostic regions. The sharp bands marked with arrows at
3054.6, 1864.4, 635.4, and 555.8 cm-1 and a broad band
centered at 983 cm-1 decrease 50% on >290 nm irradiation,
decrease 10% more on >220 nm irradiation, and increase 30%
on annealing to 30 K to allow diffusion. On the other hand,
new bands labeled m at 688.1, 446.0, and 443.1 cm-1 increase
50% on >290 nm irradiation, another 20% on >220 nm
irradiation, and do not change on the final 30 K annealing. These
new absorptions shift with 13CH2Cl2 and CD2Cl2 as given in
Tables 3 and 4. A new m band is observed at 628.3 cm-1 with
CD2Cl2, which suggests that the CH2Cl2 counterpart is covered
by strong precursor absorption near 750 cm-1
.
The analogous reaction with methylene fluoride gave similar
spectra with new absorptions at 665.8 and 639.1 cm-1 that
showed little change on irradiation and new bands that appeared
on >290 nm irradiation at 1883.4, 973.3, 704.7, 658.8, 610.6,
and 561.3 cm-1. With CD2F2, two bands at 664.6 and 643.5
cm-1 were unchanged on irradiation, and the first three bands
produced on photolysis shifted to 1355.1, 932.0, and 692.8 cm-1,
as listed in Tables S6 and S7.
The reaction of Cr with CX4 molecules proceeds as given
for the favorable CCl4 example, where the overall reaction is
exothermic by 73 kcal/mol. The reaction
Cr* + CCl4 f CCl3-CrCl* f CCl2-CrCl2* f
ClCtCrCl3 (3)
passes through the quintet insertion product with R-chlorine
transfer to form the quintet methylidene and a second R-chlorine
transfer to give the final methylidyne product. The corresponding
Extensive experience with transition metal atom and halo-
methane reactions suggests that C-X insertion followed by

Methylidyne XCtMX3 Diagnostic C-H and C-X Absorptions
Organometallics, Vol. 26, No. 25, 2007 6379
1
Table 4. Observed and Calculated Fundamental Frequencies of the HCtMoHCl2 Complex in the Ground A′ Electronic State
with Cs Structurea
HCtMoHCl2
H13CtMoHCl2
DCtMoHCl2
approximate
description
obs
calc
int
obs
calc
int
obs
calc
int
C-H str, a′
3054.6
1864.4
983
3205.8
1980.4
1051.7
811.6
629.3
581.0
578.8
409.8
384.1
24
96
11
5
90
55
26
69
24
3043.4
1864.2
952
3194.0
1980.4
1019.0
804.4
623.0
580.5
578.8
409.7
384.0
24
96
75
65
69
16
47
70
24
2293.2
1340.7
938
2383.2
1409.0
1005.2
640.9
507.5
465.8
419.5
379.5
360.3
19
51
9
Mo-Η str, a′
CtMo str, a′
Mo-Η def, a′
H-C-Mo def, a′
H-C-Mo def, a′′
Mo-H def, a′′
Mo-Cl, str, a′′
Mo-Cl str, a′
5
635.4
555.8
629.4
555.6
514.6
77
50
18
32
19
a Frequencies and intensities are in cm-1 and km/mol. Three lower real frequencies are not listed. Observed in an argon matrix. Frequencies and intensities
computed with B3LYP/6-311++G(2d,p) in the harmonic approximation using the SDD core potential and basis set are for Mo. The symmetry notations are
based on the Cs structure.
R-halogen transfer reactions occur to saturate the metal center
This five-band set also includes diagnostic modes that identify
the product. The sharp band at 3054.6 cm-1 shifts to 3043.3
cm-1 with carbon-13 and to 2293.2 cm-1 with deuterium, which
describes a C-H vibration (H/D frequency ratio 1.3320). The
sharp 1864.4 cm-1 absorption shifts negligibly with carbon-13
and significantly to 1340.7 cm-1 with deuterium (H/D ratio
1.3906), which characterizes a Mo-H vibration. Similar modes
have been observed at 1840.7 and 1835.8 cm-1 for the
HCtMoH2Cl complex.20 The 983 cm-1 band has the profile
for natural molybdenum isotopes, which will be discussed in
detail for the resolved HCtMoCl3 case below. This band shifts
31 cm-1 with carbon-13 and 45 cm-1 with deuterium, which
are in good agreement with our calculated frequencies for a
HCtMo stretching mode (Table 4). The sharp 635.4 cm-1 band
shifts 6.0 cm-1 with carbon-13 and to 514.6 cm-1 (H/D ratio
1.2348) with deuterium, which is appropriate for a H-C-Mo
deformation mode. Finally, the sharp 555.8 cm-1 band shows
a very small carbon-13 shift, and the deuterium counterpart is
below our instrumental cutoff, as it falls just below 572.8, 568.2
cm-1 bands for Mo-H deformation modes in the HCtMoH2-
Cl complex. In summary, the excellent correlation between
observed absorptions and calculated isotopic frequencies and
intensities (Table 4) confirms the present identification of the
methylidyne molecule HCtMoHCl2.
Methylene fluoride produced weaker bands with photochem-
istry different from that of methylene chloride. The bands at
665.8 and 639.1 cm-1 are the strongest and third strongest of
the bands calculated for CH2dMoF2, and the strongest for CD2d
MoF2 shifts only slightly to 664.6 cm-1 as these are antisym-
metric MoF stretching modes. The CD2 wagging mode is
observed at 643.5 cm-1, but the CH2 counterpart is covered by
strong precursor absorptions. Following the above identification
of CH2dMoCl2, these bands are sufficient to show that CH2d
MoF2 is formed in the Mo reaction with CH2F2.
as summarized in reaction 1.10-13,16-21
Mo + CH2X2 f CH2X-MoX* f CH2dMoX2 f
HCtMoHX2 (4)
For the first example of Mo and CH2Cl2, the quintet state
insertion product is 73 kcal/mol lower in energy than the
reagents, but the triplet methylidene is 29 kcal/mol lower in
energy than the insertion product, and the singlet methylidyne
is an additional 5 kcal/mol higher in energy so all of these are
accessible based on the exothermicity of the insertion step (73
kcal/mol) as represented by the asterisk. The quintet CH2-
MoCl2 is 21 kcal/mol higher than the triplet methylidene, so
this is an unlikely contributor to the product spectrum. In
addition, the strongest calculated mode, the antisymmetric Mo-
Cl stretch, is 31 cm-1 below the observed band, and such is
not appropriate. Finally, the reaction to form the triplet
methylidene is 102 kcal/mol exothermic, and the overall reaction
1 to form the singlet methylidyne is exothermic by 98 kcal/
mol.
The spectra in Figure 4 show two products, which exhibit
opposite behavior on UV irradiation. The m bands that increase
include a 446.0 cm-1 band with a 443.1 cm-1 component of
one-half the intensity, which is appropriate for natural chlorine
isotopic splitting. The calculated complete chlorine 35-37 shift
for the antisymmetric vibration of a MoCl2 subunit with the
125.8° angle calculated here for triplet CH2dMoCl2 is 6.4 cm-1
,
and the isotopic splitting for the Mo35Cl2 and Mo35Cl37Cl species
in a 9/6 population ratio is expected near one-half of the
complete shift or 3.2 cm-1, which is in very good agreement
with the 2.9 cm-1 observed splitting. Hence, the 446.0 cm-1
band is due to the antisymmetric vibration of an MoCl2 subunit.
The 688.1 cm-1 band shifts 5.7 cm-1 with carbon-13 and to
542.2 cm-1 on deuteriation, which are appropriate for the CH2
wagging mode that appears at 659.4 cm-1 in the CH2dMoHCl
complex.20 Furthermore, a new band appears at 628.3 cm-1 for
the deuteriated species, which must arise from a product with
a hydrogen counterpart masked by the strong precursor absorp-
tion in the low 700 cm-1 region. Our B3LYP calculation (Table
3) for the CH2dMoCl2 complex places this CdMo stretching
mode appropriately. Hence, the three vibrations characterized
here by isotopic shifts identify MoCl2, CH2, and CdMo subunits
that define the CH2dMoCl2 complex, and the excellent cor-
relation between the observed and strongest calculated infrared
frequencies confirms these assignments.
The second set of absorptions produced on near-UV irradia-
tion includes bands that identify this new product as HCt
MoHF2, which is calculated to be 4 kcal/mol higher in energy
than the triplet methylidene. Unfortunately, the C-H stretching
mode falls under precursor in this case, but the Mo-H stretching
mode is observed at 1883.4 cm-1 with D counterpart at 1355.1
cm-1 (H/D ratio 1.3899). The strongest band, the antisymmetric
Mo-F stretching mode at 704.7 cm-1, shifts to 692.8 cm-1 with
deuterium. The weaker symmetric Mo-F stretching mode and
antisymmetric H-C-Mo and symmetric Mo-H deformation
modes are also observed (Table S7) for the hydrogen compound.
Again, the correlation between observed and calculated spectra
confirms the assignments.
The bands marked by arrows decrease on irradiation, but
increase on subsequent annealing of the argon matrix sample,
which suggests a spontaneous reaction to form a stable product.
Mo + CHX3. Infrared spectra from the series of HCFnCl3-n
(n ) 0, 1, 2, 3) haloform reactions with laser-ablated molyb-

6380 Organometallics, Vol. 26, No. 25, 2007
Lyon et al.
98, and 100, respectively, and for the CHF2Cl product at 987.8,
983.8, and 982.6 cm-1. [The Mo 92 to 100 separations are 5.1,
5.2, and 5.2 cm-1 in these three sets of data.] A broad band
centered at 986 cm-1 is observed for the CHF3 product. Notice
the trend of increased CtMo stretching frequency with
increased fluorine substitution, which is predicted by our
calculations (Tables 5 and S8). Molybdenum isotopic resolution
was equally as good with 13CHCl3 where Mo isotopic peaks
were observed at 952.1, 950.7, 950.0, 949.3, 948.0, and 946.8
cm-1, respectively. Notice that the latter Mo 92 to 100 isotopic
separation of 5.3 cm-1 is larger than the value for the carbon-
12 product as Mo vibrates more against the heavier carbon
isotope.
The resolved molybdenum isotopic splittings convincingly
demonstrate that a single Mo atom participates in this vibrational
mode. The Mo 92 to 100 separation for the chloroform product,
5.1 cm-1, is less than the 6.3 cm-1 adjusted value found for
MoN and the 6.0 cm-1 average value for the two stretching
modes of (O2)MoO2 as these mass partners are heavier, and
the latter gives strong, completely resolved Mo isotopic
spectra.38,39 For a harmonic diatomic C-Mo oscillator, the Mo
92 to 100 isotopic shift at 978.1 cm-1 would be 4.6 cm-1, which
is slightly less than our 5.1 cm-1 observed value, and this shift
for C 13 against Mo 98 would be 34.2 cm-1, which is slightly
more that our 30.1 cm-1 observed value. This means that the
Mo atom is also coupled slightly to the three Cl atoms and that
Mo moves in an antisymmetric fashion between HC and three
Cl atoms in this mode. For the major Mo-98 isotope, a simple
C-Mo diatomic oscillator would shift from 978.1 to 943.9 cm-1
on carbon-13 substitution, a shift some 1.4 cm-1 more than that
calculated for the anticipated HCtMoCl3 product (Table 5).
This difference arises because the C-H mode interacts slightly
and DC produces an even larger 45.7 cm-1 observed and 46.1
cm-1 calculated shift for the DCtMoCl3 product absorption.
So our B3LYP calculations predict the Mo isotopic shift within
0.1 cm-1 and demonstrate that we are dealing with the HCt
Mo stretching mode of a carbyne species. The triple bond
stretching mode of acetylene itself exhibits an even larger
deuterium isotopic shift.40
The stronger band at 3058.2 cm-1 shifts to 3048.0 cm-1 on
carbon-13 and to 2296.2 cm-1 on deuterium substitution (H/D
ratio 1.3319), which are appropriate for a C-H stretching mode.
The very strong, very sharp 658.9 cm-1 band shifts to 652.4
cm-1 with carbon-13 and to 533.4 cm-1 with D (H/D ratio
1.2353), which characterizes the degenerate H-C-Mo defor-
mation mode, and the absence of splitting on this band verifies
that the C3V symmetry of the molecule is maintained in the
matrix. The strong 438.7 cm-1 band is appropriate for the
degenerate Mo-Cl stretching mode. Density functional calcula-
tions using B3LYP typically overshoot vibrational frequencies,
and the C-H mode is calculated 5.0% too high by the B3LYP
functional, as is expected,33 but for more difficult to model
modes, the HCtMo mode is calculated 7.6% too high, the
deformation mode 0.12% high, and the Mo-Cl stretching mode
3.0% too low. The calculation of vibrational frequencies,
particularly for a Mo carbyne, is not an exact science, and we
expect density functional theory to provide a very good
Figure 5. Infrared spectra in selected regions for laser-ablated Mo
atom and CHX3 reaction products with 0.5% reagent in argon co-
deposited for 1 h at 8 K and after full arc irradiation. (a) CHF3. (b)
After irradiation (λ < 220 nm). (c) CHF2Cl. (d) After irradiation.
(e) 13CHF2Cl. (f) After irradiation. (g) CHFCl2. (h) After irradiation.
(i) CHCl3. (j) After irradiation. (k) 13CHCl3. (l) After irradiation.
(m) CDCl3. (n) After irradiation. Arrows denote the methylidyne
absorptions, and P denotes precursor bands.
Figure 6. Infrared spectra in the 995-970 cm-1 region for the
laser-ablated Mo atom and CHX3 reaction products with 0.5%
reagent in argon co-deposited for 1 h at 8 K and subjected to
ultraviolet irradiation. (a) CHF3, (b) CHF2Cl, (c) CHFCl2, and (d)
CHCl3. Arrows indicate resolved Mo 92, 98, and 100 isotopic bands.
denum atoms are illustrated in Figure 5 where the first set of
spectra shows new C-H stretching modes marked by arrows.
13
Also included are spectra from the reactions of 13CHF2Cl,
-
CHCl3, and CDCl3 where the isotopic shifts characterize C-H
vibrations. The lower region displays the common H-C-Mo
deformation mode and Mo-F and Mo-Cl stretching vibrations,
and the observed frequencies are listed in Tables 5 and S8. The
weak fluoroform reaction product doubled on full arc irradiation,
did not change on annealing to 30 K, and doubled again on a
second full arc irradiation. The strong chloroform product
increased 10% on the first full arc irradiation, 20% on annealing,
and another 10% on the second irradiation.
Figure 6 shows spectra in the 980 cm-1 region where natural
molybdenum37 isotopic splittings are resolved in the high yield
chloroform product (982.0, 980.7, 980.0, 979.4, 978.1, and 976.9
cm-1 for Mo 92, 94, 95, 96, 98, and 100, respectively) and
partially resolved for the CHFCl2 product at 984.6, 980.7, and
979.4 cm-1 marked by arrows for the major isotopes Mo 92,
(38) Andrews, L.; Souter, P. F.; Bare, W. D.; Liang, B. J. Phys. Chem.
A 1999, 103, 4649 (MoN).
(39) Bare, W. D.; Souter, P. F.; Andrews, L. J. Phys. Chem. A 1998,
102, 8279 (MoO2).
(37) CRC Handbook; Chemical Rubber Publishing Co.: Boca Raton,
FL, 1985.
(40) Herzberg, G. Infrared and Raman Spectra; D. Van Nostrand:
Princeton, NJ, 1945.

Methylidyne XCtMX3 Diagnostic C-H and C-X Absorptions
Organometallics, Vol. 26, No. 25, 2007 6381
1
Table 5. Observed and Calculated Fundamental Frequencies of HCtMoX3 Complexes in the Ground A1 Electronic State with
C3W Structurea
HCtMoCl3
H13CtMoCl3
DCtMoCl3
HCtMoF3
approximate
description
obs
calc
int
35
obs
calc
obs
calc
int
24
obs
calc
int
C-H str, a1
3058.2
978.1b
438.7
3212.2
1051.9
425.0
380.8
660.2
237.7
144.0
100.4
3048.0
948.0
438.6
3200.4
1019.1
425.4
380.7
653.4
232.5
143.7
100.4
2296.2
932.4b
436.4
2387.9
1005.8
423.0
380.7
533.6
212.3
143.7
100.4
3073.1
986.0c
689.2
664.6
645.4
3221.2
1065.0
683.4
651.7
642.7
288.7
228.0
163.6
39
20
HCtMo str, a1
Mo-X str, e
8
6
81 × 2
74 × 2
210 × 2
Mo-X str, a1
9
9
60
H-C-Mo def, e
C-Mo-X def, e
Mo-X3 umb, a1
Mo-X2 bend, e
658.9
76 × 2
652.4
533.4
52 × 2
30 × 2
5 × 2
9
8 × 2
7 × 2
7 × 2
0
0
0
0
a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with B3LYP/6-311++G(2d,p)
in the harmonic approximation using the SDD core potential and basis set for Mo. The symmetry notations are based on the C3V structure. b Band position
for the major 98Mo isotope: see Figure 3. c Band center: see Figure 3.
1
Table 6. Observed and Calculated Fundamental Frequencies of XCtMoX3 Complexes in the Ground A1 Electronic State with
C3W Structurea
ClCtMoCl3
Cl13CtMoCl3
FCtMoF3
approximate
description
obs
calc
int
165
calc
int
154
obs
calc
int
353
anti X-CtMo str, a1
sym X-CtMo str, a1
anti Mo-X str, e
X-C-Mo def, e
sym Mo-X str, a1
Mo-X3 umb, a1
1259.6
1278.6
453.7
432.0
382.5
371.8
130.7
99.1
1215.6
430.2
1233.1
453.6
653.5.0
370.5
372.0
130.5
99.0
1524.2
1566.3
619.0
677.8
464.8
658.8
218.3
161.9
142.9
25
25
1
443.6
96 × 2
98 × 2
682.3
664.3
169 × 2
2 × 2
103
3 × 2
1 × 2
8
0
0
0
7
0
0
0
8
C-Mo-X def, e
Mo-X2 bend, e
8 × 2
78.5
78.5
0
a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with B3LYP/6-311++G(2d)
in the harmonic approximation using the SDD core potential and basis set for Mo. The symmetry notations are based on the C3V structure.
approximation for frequencies and a good approximation for
infrared intensities, and this is in fact achieved here for HCt
MoCl3.
reaction is spontaneous with ground-state metal atoms, which
was the case for Li in the simple abstraction reaction.41
Mo + CHX3 f CHX2-MoX* f CHXdMoX2 f
HCtMoX3 (5)
Five absorptions in the CHF3 reaction correlate well with the
above CHCl3 product bands, and the calculated frequencies
(Table 5) support the assignment to HCtMoF3. The Mo-F
stretching modes are, of course, higher than the Mo-Cl
counterparts, and the C-H stretching and H-C-Mo deforma-
tion modes show slight changes. The trend in C-H stretching
mode will be later related to hybridization of carbon in these
molecules.
The ability to tune the C-H stretching frequency with
halogen substituents at the metal center prompted inclusion of
the fluorochloroform precursors with this investigation. The
Mo-F and Mo-Cl stretching modes of the latter reaction
products are near the values discussed above (Tables 5 and S8).
The H-C-Mo deformation modes do not form a smooth trend
because the now nondegenerate stretching and deformation
modes mix more in the molecules with lower symmetry although
the average of the two deformation modes for the F, Cl species
falls between the values for the trigonal species. However, the
C-H stretching modes are isolated and virtually uncoupled, and
the 3058.2, 3063.5, 3068.7, and 3073.1 cm-1 trend is almost
uniform with the number of F’s replacing Cl on the Mo center.
The C-H stretching frequency is expected to increase with
s-character in the carbon hybrid orbital, and the s,p-character
will depend on polarization of the CtMo triple bond by the
halogen substituents.
Mo + CX4. The reactions of laser-ablated molybdenum atoms
and tetrahalomethanes gave a product with very high C-F or
C-Cl stretching frequencies. Infrared spectra for the reaction
product of Mo with CCl4 revealed new absorptions at 1259.6,
1253.3, and 443.6 cm-1, which increased slightly on ultraviolet
irradiation but not on annealing. With 13CCl4, these bands shifted
to 1215.6, 1209.4, and 442.5 cm-1 (Figure 3). The reaction with
CF4 gave much weaker product absorptions at 1524.2, 682.3,
and 664.3 cm-1. The spectra in Figure S4 show that the product
yield increases with the CF3Cl and CF2Cl2 precursors, which
gave strong new bands at 1523.8 and 1521.6 cm-1 that shifted
to 1476.6 and 1475.7 cm-1 with the carbon-13-enriched
precursors. These Freon reactions also provided new bands at
685.0 and 667.0, and at 670.6 and 473.9 cm-1, respectively.
Ultraviolet irradiation slightly increased the CF4 and substan-
tially increased the CF3Cl and CF2Cl2 product absorptions.
The reaction with CCl4 formed a single new metal-containing
product with two strong absorptions at 1259.6 and 443.6 cm-1
,
which correlate well with the two strongest absorptions calcu-
lated for the trigonal ClCtMoCl3 molecule. Table 6 compares
the observed and calculated frequencies. First, the lower 1253.3
cm-1 satellite is due to a matrix site splitting similar to that
observed in the titanium system.10 The major band at 1259.6
cm-1 has a 1257.7 cm-1 shoulder, which are appropriate for
the vibration of 35Cl and 37Cl in natural abundance and verifies
the participation of a single chlorine atom in this mode. Second,
the weak carbon-12 band at 1259.6 cm-1 in the 90% carbon-
13 experiment with the strong 1215.6 cm-1 carbon-13 coun-
The reaction proceeds following the mechanism suggested
above with C-X insertion and two successive R-halogen
transfer reactions to completely halogenate the metal center as
shown in reaction 5. For chloroform, the methylidyne is 40 kcal/
mol lower in energy than the methylidene complex, and this
overall reaction is 147 kcal/mol exothermic; hence, the ther-
mochemical driving force to form the carbyne is compelling.
The increase on annealing suggests that the Mo and CHCl3
(41) Carver, T. G.; Andrews, L. J. Chem. Phys. 1969, 50, 4235.

6382 Organometallics, Vol. 26, No. 25, 2007
Lyon et al.
terpart demonstrates the presence of a single carbon atom in
the product species. The antisymmetric Cl-C-Mo stretching
mode is predicted at 1278.6 cm-1 with a 45.5 cm-1 carbon-13
shift, and our strong product band is observed at 1259.6 cm-1
with a 44.0 cm-1 shift. The high intensity and large carbon-13
shift for this mode are due in large part to the antisymmetric
motion of carbon between the Cl and Mo centers. Correspond-
ingly, the 1.9 cm-1 chlorine isotopic shift is less than the 8.8
cm-1 value expected for a diatomic C-Cl motion, but our
B3LYP calculation predicts this to be 1.7 cm-1, which is
appropriate for the antisymmetric Cl-C-Mo stretching vibra-
tion.
The much weaker symmetric Cl-C-Mo stretching mode
predicted at 453.7 cm-1 is not observed, but the degenerate
antisymmetric Mo-Cl stretching mode calculated at 432.0 cm-1
with a 1.8 cm-1 carbon-13 shift is observed slightly higher at
443.6 cm-1 with a 1.1 cm-1 shift. The breadth of this band is
appropriate for the contribution of natural Mo and Cl isotopes
for the vibration of two or more chlorine atoms. This excellent
correlation between calculated and observed frequencies and
isotopic characteristics for the ClCtMCl3 molecule, which has
a unique very high-frequency C-Cl stretching mode over 450
cm-1 higher than CCl4 and other normal C-Cl single bond
vibrations, confirms our assignment. Finally, this verifies that
complete R-chlorine transfer reaction to saturate the Mo center
has occurred and that the 162 kcal/mol exothermic overall
reaction 6 has gone to completion. Again, energy released in
the initial insertion reaction helps drive the successive R-chlorine
transfers.
Figure 7. Infrared spectra in selected regions for the W atom and
CH2Cl2 reaction products in excess argon at 8 K. (a) W + 0.5%
CH2Cl2 in argon co-deposited for 1 h. (b) After irradiation (λ >
220 nm). (c) After annealing to 30 K. (d) W + 0.5% 13CH2Cl2 in
argon co-deposited for 1 h. (e) After irradiation (λ > 220 nm). (f)
After annealing to 30 K. (g) W + 0.5% CD2Cl2 in argon. (h) After
irradiation (λ > 420 nm). (i) After irradiation (380-240 nm). (j)
After annealing to 30 K. The label m denotes methylidene, arrows
denote the methylidyne absorptions, and P denotes precursor bands.
at 3083.2, 1959.0, 1007.4, 638.9, 597.1, and 588.7 cm-1, which
decreased slightly on >420 nm irradiation, increased 10% on
>290 nm and another 30% on >220 nm irradiation, and
increased 20% on 30 K annealing and another 10% on final
>220 nm irradiation. These new bands and their 13CH2Cl2 and
CD2Cl2 counterparts are shown in Figure 7, and the frequencies
are listed in Table 7. They are comparable to the Mo
counterparts except that the 1007.4 cm-1 band is sharp (full-
width at half-maximum 0.9 cm-1) as natural tungsten isotopes
do not broaden the band like those for molybdenum.37
Mo + CCl4 f CCl3-MoCl* f CCl2dMoCl2 f
ClCtMoCl3 (6)
The deuteriated sample also reveals a new 550.5 cm-1 band
(labeled m) that increases on the initial >420 nm irradiation
when the above band set decreases. Again, the CH2Cl2
counterpart probably falls under the strong precursor absorption.
This band is due to a different product. Following the Mo/
halomethane chemistry described above, and the related work
on methyl halides,16,18,20 the CD2dWCl2 molecule through
which the reaction passes must be considered. Our calculations
find the methylidene product CH2dWCl2 to be 6 kcal/mol
higher in energy than the associated HCtWHCl2 methylidyne
molecule, and the frequencies map those in Table 3 for the
CH2dMoCl2 complex with some frequencies being slightly
lower. The above 550.5 cm-1 band is analogous to the 542.2
cm-1 band assigned above to the CD2 wagging mode of the
CD2dMoCl2 methylidene where the CH2 counterpart is masked
by precursor absorption and the W-Cl stretching modes are
too low to be observed here.
Substantially weaker bands observed with CF4 are assigned
to the analogous vibrational modes, the antisymmetric F-C-
Mo and Mo-F stretching modes, based on the calculated FCt
MoF3 product frequencies as collected in Table 6. The sym-
metric F-C-Mo stretching mode is calculated at 619.0 cm-1
with practically no intensity. Reaction 6 for the fluorine system
is exothermic by 114 kcal/mol, which reflects the stronger C-F
bond energy. Analogous antisymmetric F-C-Mo stretching
modes were observed at 1523.8 and 1521.6 cm-1 for the
FCtMoF2Cl and FCtMoFCl2 products in the Freon-13 and
12 reactions. These latter bands exhibit the large carbon-13
shifts, 47.2 and 45.9 cm-1, which are characteristic of these
antisymmetric vibrational modes. In addition, two Mo-F
stretching modes (685.0, 667.0 cm-1) and one Mo-F and one
Mo-Cl stretching mode (670.6, 473.9 cm-1) are observed for
these products. In the group 4 metal reactions with these
chlorofluoromethanes, two structural isomers were observed
with different numbers of chlorine and fluorine atoms transferred
to the metal center.11,12 In these experiments, the higher energy
FCM form was favored on the deposition reaction with laser-
ablated metal atoms, and subsequent UV photolysis reduced
this in favor of the lower energy ClCM form. In the present
case, it appears that only the 2 kcal/mol higher energy FCt
MoFCl2 isomer is produced, and the same occurs for FCtMoF2-
Cl. Other possible lower oxidation state products in this system,
the triplet ground-state methylidene complexes CF2dMoCl2,
CFCldMoFCl, and CCl2dMoF2, are 37, 42, and 42 kcal/mol
higher in energy, and the quintet state analogues are 6, 9, and
12 kcal/mol higher still in energy, and these higher energy
intermediate species are not observed.
The above six-band set that increases on UV irradiation is
assigned to the associated HCtWHCl2 methylidyne complex
based on its relationship to the Mo counterparts and comparison
with the calculated frequencies in Table 7. The C-H stretching
frequency at 3083.2 cm-1 is just above that for the related HCt
WH2Cl methylidyne complex at 3077.4 cm-1 [found by
reanalysis of the spectrum].20 The W-H stretching frequency
at 1959.0 cm-1 may be compared to the 1926.2 and 1921.4
cm-1 values observed for HCtWH2Cl, and the frequencies for
other functional groups are also comparable.20 The final
methylidyne complex is 7 kcal/mol more stable than the
methylidene intermediate and 172 kcal/mol lower in energy than
the reagents. Reaction 8 is photoreversible, but not nearly as
W + CH2X2. The reaction of laser-ablated tungsten atoms
and methylene chloride gave a set of weak product absorptions

Methylidyne XCtMX3 Diagnostic C-H and C-X Absorptions
Organometallics, Vol. 26, No. 25, 2007 6383
Figure 8. Infrared spectra in selected regions for the W atom and CHCl3 reaction products in excess argon at 8 K. (a) W + 0.5% CHCl3
in argon co-deposited for 1 h. (b) After irradiation (λ > 220 nm). (c) After annealing to 30 K. (d) Mo + 0.5% CDCl3 in argon. (i) After
irradiation (λ > 220 nm). (j) After annealing to 30 K. Arrows denote the methylidyne absorptions, and P denotes precursor bands.
1
Table 7. Observed and Calculated Fundamental Frequencies of the HCtWHCl2 Complex in the Ground A′ Electronic State
with Cs Structurea
HCtWHCl2
H13CtWHCl2
DCtWDCl2
approximate
description
obs
calc
int
obs
calc
int
obs
calc
int
C-H str, a′
3083.2
1959.0
1007.4
3232.6
2042.7
1052.4
794.3
630.8
613.8
599.7
388.0
383.8
23
73
11
9
96
56
19
77
22
3072.1
1959.0
974.4
3220.7
2042.7
1017.5
787.3
625.5
613.5
599.7
387.8
383.8
22
73
11
8
94
56
20
77
22
2317.3
1409.9
-
2404.2
1449.5
1003.5
625.8
498.0
454.3
438.8
382.1
364.7
18
38
9
W-Η str, a′
CtW str, a′
b
W-H def, a′
H-C-W def, a′′
H-C-W def, a′
W-H def, a′′
W-Cl str, a′′
W-Cl str, a′
8
638.9
597.1
588.7
633.5
597.0
588.7
506.1
460.5
56
50
22
26
40
a Frequencies and intensities are in cm-1 and km/mol. Three lower real frequencies are not listed. Observed in an argon matrix. Frequencies and intensities
computed with B3LYP/6-311++G(2d,p) in the harmonic approximation using the SDD core potential and basis set are for W. The symmetry notations are
based on the Cs structure. b Band covered by precursor.
much so as the dihydridomonochloro analogue, which has more
hydrogen atoms available for transfer.20
957.2, 534.2, and 416.3 cm-1, and these bands increased 10%
on >290 and 220 nm irradiations and another 30% on annealing
to 30 K. Representative spectra are shown in Figure 8 except
for the lowest frequency bands (0.02 and 0.01 absorbance units,
respectively). The observed frequencies are listed in Table 8
and compared to frequencies calculated for the HCtWCl3
methylidyne product, which is 52 kcal/mol lower in energy than
the corresponding methylidene complex. The C-H stretching
frequency is predicted 26 cm-1 higher than for the Mo
counterpart, and it is observed 29 cm-1 higher. The B3LYP
calculations do not fit as well for the CtW and CtMo
stretching modes as the CtW mode is predicted 2 cm-1 higher
and observed 28 cm-1 higher (shift between major metal isotopic
positions). In addition, the W-Cl stretching mode is observed
18 cm-1 higher than calculated, and the Mo-Cl stretching mode
is observed 14 cm-1 higher than calculated. However, both
H-C-M deformation modes are observed within 2 cm-1 of
the calculated values. Overall, the correlation between calculated
and observed frequencies for the HCtMCl3 methylidyne
complexes is very good and confirms our assignments.
W + CH2Cl2 f CH2Cl-WCl* f CH2dWCl2 f
HCtWHCl2 (7)
HCtWHCl2 + >420 nm f CH2dWCl2
CH2dWCl2 + >290 nm f HCtWHCl2
(8a)
(8b)
Our reaction of W with methylene fluoride produced weak
new bands at 1978.6, 1015.6, 705.0, 675.3, and 601.3 cm-1 upon
>290 nm irradiation, which increased 3-fold on >220 nm
photolysis. These bands relate to the above sets assigned to the
HCtWHCl2 and HCtMoHF2 methylidyne complexes, and
their assignment follows accordingly. The higher W-H fre-
quency at 1978.6 cm-1 follows the higher values of 1936.4 and
1930.4 cm-1 for the HCtWH2Cl methylidyne.18 In contrast to
the methylene chloride case, the 8 kcal/mol higher energy CH2d
WF2 complex was not observed in these experiments.
The CH2dMX2 complexes show no evidence of agostic
distortion at the B3LYP computational level of theory, in
contrast to the group 6 CH2dMH2 and CH2dMHX methylidene
complexes prepared from methane and methyl halides.16-21 This
observation is probably due in part to the 0.03 Å longer CdM
bond lengths in the dihalide complexes.
The fluoroform reaction gives weaker product bands at
3103.2, 683.2, 677.6, and 651.4 cm-1, which increase 100%
over the course of >290 nm, >220 nm irradiation, 30 K
annealing, and a second >220 nm irradiation. These bands are
1, 2, 4, and 7 × 10-3 absorbance units, respectively, which
matches very well the predicted infrared intensities of the
computed frequencies in Table 8. As mentioned above, the W-F
stretching frequencies are computed slightly low, and in this
case the most intense degenerate H-C-W deformation mode
W + CHX3. Infrared spectra from the tungsten and chloro-
form reaction give bands similar to those found for Mo but
displaced to 3087.7, 1006.4, 669.9, and 416.5 cm-1, and again
the 1006.4 cm-1 band is sharp (0.9 cm-1 bandwidth). The
chloroform-carbon-13 product shifted to 3076.1, 973.5, 664.1,
and 416.2 cm-1. The chloroform-d product shifted to 2319.7,

6384 Organometallics, Vol. 26, No. 25, 2007
Lyon et al.
1
Table 8. Observed and Calculated Fundamental Frequencies of HCtWCl3 and HCtWF3 Complexes in the Ground A1
Electronic State with C3W Structurea
HCtWCl3
H13CtWCl3
DCtWCl3
HCtWF3
approximate
description
obs
calc
int
30
obs
calc
int
29
obs
calc
int
22
obsb
calc
int
C-H str, a1
3087.7
1006.4
669.9
3238.5
1053.5
670.9
398.7
386.5
238.8
133.1
98.5
3076.1
973.5
664.1
416.2
3226.6
1018.5
664.7
398.6
386.5
233.2
133.0
98.5
2319.7
957.2
534.2
416.3
2408.5
1004.7
533.7
398.5
386.5
215.7
3103.2
3250.0
1073.0
646.0
660.7
662.6
289.2
216.4
158.4
35
19
154 × 2
73 × 2
65
HCtW str, a1
H-C-W def, e
W-X str, e
9
9
7
82 × 2
74 × 2
10
81 × 2
75 × 2
10
48 × 2
72 × 2
10
651.4
677.6
683.3
416.4
W-X str, a1
C-W-X def, e
W-X3 umb, a1
W-X2 bend, e
3 × 2
2 × 2
3 × 2
1 × 2
0
0
133.0
98.5
0
7
0
0
0
6 × 2
a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with B3LYP/6-311++G(2d,p)
in the harmonic approximation using the SDD core potential and basis set for W. The symmetry notations are based on the C3V structure. b Relative intensities
1, 2, 4, 7 × 10-3 absorbance units, respectively, in order of decreasing frequency.
Table 9. Comparison of Bond Lengths and Characteristic Frequencies for Related H-CtW Compounds
b
c
d, Å or fr, cm-1
H-CtWa
H-CtWH3
H-CtWCl3
H-CtW(PMe3)4Cld
d(C-H)
1.0765
1.7367
1.081
1.742
3076
1003
727
1.081
1.751
3087
1006
670
2320
957
534
1.092-1.093
1.79-1.80
2994, 2978
909
788, 755
2244, 2234
871
d(CtW)
fr(C-H)
fr(HCtW)
fr(H-C-W)
fr(C-D)
1006
660
fr(DCtW)
fr(D-C-W)
953
501
953
571
642, 611
a Gas-phase measurements, ref 9. b B3LYP calculated bond lengths and argon matrix frequencies, ref 19. The C-H stretching frequency was too weak
to observe, but the 3076 cm-1 value is extrapolated from that for the related HCtWH2Cl methylidyne complex at 3077 cm-1 c B3LYP calculated bond
.
lengths and argon matrix frequencies, this work. d Microcrystalline sample, ref 7.
is computed about 5 cm-1 below the observed value. Overall,
the correlation between calculated and observed frequencies for
this demanding HCtWF3 subject molecule is gratifying.
The tungsten reaction with chloroform is overall even more
exothermic, 220 kcal/mol, than the molybdenum counterpart,
and the methylidene intermediate is 52 kcal/mol higher in energy
than the final methylidyne product, and as before the fluoroform
reaction is less exothermic (211 kcal/mol). These B3LYP energy
calculations underscore the stability of the final very stable
methylidyne product.
precursor. These high-frequency bands are due to C-F stretch-
ing modes in FCtWF2Cl and FCtWFCl2, respectively. These
Freon precursors also provided new bands at 654.2 and 638.9,
and at 711.8, 675.4, and 585.7 cm-1, respectively. Ultraviolet
irradiation substantially increased the product absorptions.
Reaction 10 for tungsten is even more exothermic, 234 kcal/
mol, than the molybdenum counterpart, but the W reaction with
CF4 is less exothermic, 189 kcal/mol, all calculated at the
B3LYP level of theory.
W + CCl4 f CCl3- WCl* f CCl2dWCl2 f
ClCtWCl3 (10)
W + CHCl3 f CHCl2-WCl* f CHCldWCl2 f
HCtWCl3 (9)
C-H, CtM, and X-C-M Stretching Frequencies. After
the observation and characterization of these new trigonal HCt
MX3 and XCtWX3 methylidyne molecules, the most interesting
and informative measurable parameters are the C-H, CtM,
and X-C-M stretching frequencies. The C-H stretching
frequencies in the HCtMoFnCl3-n (n ) 0, 1, 2, 3) series
increase from 3058.2, 3063.5, 3068.7, to 3073.1 cm-1 with the
number of F’s replacing Cl on the Mo center. This is opposite
W + CX4. Spectra from the reaction products of laser-ablated
tungsten atoms and tetrahalomethanes are similar to those for
molybdenum, and the spectra are compared in Figures 3 and
S6. The CCl4 product absorptions are higher at 1292.8, 1287.3
cm-1 and lower at 416.0 cm-1, and the carbon-13 shifts are
46.2, 46.1, and 0.5 cm-1, respectively. The 1287.3 cm-1
component is a matrix site splitting, but a partially resolved
shoulder on the major 1292.8 cm-1 band at 1291.1 cm-1 forms
precisely the 3/1 pattern expected for natural abundance chlorine
isotopes and the vibration of a single chlorine atom. The larger
carbon-13 and smaller chlorine-37 shifts, as compared to the
molybdenum counterparts, are due to the involvement of a
heavier metal and further characterize this antisymmetric Cl-
C-M vibration. Our B3LYP calculation predicts the carbon and
chlorine isotopic shifts as 47.3 and 1.7 cm-1, and we observe
them as 46.2 and 1.7 cm-1. The 416.0 cm-1 band is due to the
antisymmetric W-Cl stretching vibration as shown in Table
S9.
the trend in the CHCl3 (3053.4 cm-1) and CHF3 (3043.5 cm-1
)
precursors. These C-H stretching frequencies are higher than
normal alkane values (3028.5 cm-1 for methane in solid argon),
which suggests an adjacent multiple carbon-metal bond to
provide higher s-character to the C-H bond,4,42 although
acetylene (3288.9 cm-1 in solid argon) and methylacetylene
(3322.8 cm-1 in solid argon)43 are considerably higher. Table
S10 shows that the s-character from NBO analysis26,30 increases
in the above series with the increase in C-H stretching
frequencies and decrease in computed C-H bond length,
although this decrease is only 0.0009 Å. The calculated
frequencies, of course, and the infrared intensities follow the
same trend. The natural charges show that the CtM bond
The CF4 product absorptions are higher such that the upper
band is covered by precursor and the lower bands are at 676.1
and 671.2 cm-1. With the CF3Cl and CF2Cl2 reagents, strong
new bands were observed at 1541.2 and 1538.2 cm-1, and the
former shifted to 1492.9 cm-1 with the carbon-13-enriched
(42) Pavia, D. L.; Lampman, G. M.; George, S. K. Introduction to
Spectroscopy, 3rd ed.; Brooks Cole: New York, 2000.
(43) Andrews, L.; Johnson, G. L. J. Phys. Chem. A 1982, 86, 3380.

Methylidyne XCtMX3 Diagnostic C-H and C-X Absorptions
Organometallics, Vol. 26, No. 25, 2007 6385
Figure 9. Structures of the CH2dCrX2, CH2-CrX2, and CH2Cl-CrCl complexes optimized at the level of B3LYP/6-311++G(2d,p)/
SDD. The bond lengths and angles are in Å and deg. Spin multiplicities are given underneath.
Figure 12. The structures of CH2dWX2, HCtWX3, and XCt
Figure 10. Structures of the HCtCrX3, XCtCrX3, CHXdCrX2,
WX3 molecules optimized using B3LYP/6-311++G(2d,p)/SDD
and CF2-CCl2 complexes optimized at the level of B3LYP/6-
methods. The bond lengths and angles are in Å and deg.
311++G(2d,p)/SDD. The bond lengths and angles are in Å and
deg. Spin multiplicities are given underneath.
However, re-examination of the HCtWH2F spectrum18
reveals the weak C-H stretching mode at 3081.6 cm-1, and
likewise for the related HCtWH2Cl methylidyne complex20 at
3077.4 cm-1 and the bromine analogue at 3076.0 cm-1. Our
calculation for HCtWHCl2 predicted the C-H stretching mode
5 cm-1 higher than for HCtWH2F, and we observed this mode
2 cm-1 higher, which is in very good agreement. We have now
observed C-H stretching modes for the last three members of
the HCtWClnH3-n (n ) 0, 1, 2, 3) series of methylidynes
(3077.4, 3083.2, and 3087.0 cm-1). Our calculations show that
the all-hydrogen n ) 0 species, which we were unable to observe
due to low reaction yield, extrapolates 1 cm-1 lower than
HCtWH2Cl at 3077.4 cm-1, and they allow us to evaluate this
mode for HCtWH3 as 3076 cm-1 (Table 9).
The computed s-character is highest in the HCtMoF3 and
Figure 11. The structures of CH2dMoX2, HCtMoX3, and XCt
HCtWF3 molecules (48.5%, 48.4%, respectively), and the C-H
MoX3 molecules optimized using B3LYP/6-311++G(2d,p)/SDD
stretching frequencies are the highest we observe (3073, 3103
methods. The bond lengths and angles are in Å and deg.
cm-1). Our calculations predict the C-H stretching mode for
polarization increases with F substitution, and the C-H stretch-
ing infrared intensity follows. The same rationale applies to the
HCtMoFnH3-n (n ) 0, 1, 2, 3) series of methylidynes16,17 where
the calculated C-H frequency and infrared intensity increase
with fluorine substitution, and this mode has been observed only
for the most intense n ) 3 member of the series.
the tungsten methylidyne to be 29 cm-1 higher than that for
the molybdenum analogue, and we observe it to be 30 cm-1
higher (Table S10).
Notice that the CtM stretching frequencies also follow the
same increasing trend with fluorine substitution. The π bonds
show no variation in hybridization in the series, but a small

6386 Organometallics, Vol. 26, No. 25, 2007
Lyon et al.
Table 10. Comparison of Characteristic Frequencies for Related HCtMCl3 Methylidyne Molecules in Solid Argon
mode
HCtCrCl3
DCtCrCl3
HCtMoCl3
DCtMoCl3
HCtWCl3
DCtWCl3
C-H str
3038.5
not obs
637.8
covereda
not obs
533.7
3058.2
978.1
658.9
438.7
2296.2
932.4
533.4
436.4
3087.7
1006.4
670.0
2319.7
967.2
534.2
416.3
CtM str
H-C-M def
C-X3 str
488.9
477.6
416.5
a Covered by precursor absorption.
decrease in carbon 2s character is accompanied by a large
increase in Mo 5s character in the carbon-molybdenum σ bond.
The C-H stretching frequency of the related HCtMoHCl2
methylidyne is 3 cm-1 lower than that for HCtMoCl3, and the
s-character is slightly, 0.7%, lower. The same consistency is
observed within the tungsten pair HCtWCl3 and HCtWHCl2,
where the latter C-H frequency is 4 cm-1 lower and the
s-character 0.5% lower. However, the comparison between Mo
and W is not consistent as the C-H frequency is higher for the
W species but the s-character lower, although the differences
are small. We believe that this is due to an imperfect application
of the SDD pseudopotential and basis for this particular
comparison.
The very high C-X stretching frequencies and large carbon-
13 shifts observed for the XCtMX3 methylidynes indicate that
these are in fact due to antisymmetric stretching of the C atom
between the halogen and metal masses. The carbon-13 shift for
a diatomic C-Cl stretching mode at the 1259.6 cm-1 frequency
observed for ClCtMoCl3 would be 36.9 cm-1, and the observed
carbon-13 shift is 44.0 cm-1. The pure antisymmetric stretch
of a linear Cl-C-Cl linkage would shift 42.3 cm-1, and our
B3LYP calculation for this Cl-C-Mo mode finds 45.5 cm-1
for the antisymmetric motion of C between Cl and Mo. This
point is substantiated by comparing the higher 1292.6 cm-1
frequency for ClCtWCl3 and the larger 46.2 cm-1 carbon-13
shift (3.57% for C moving against W as compared to 3.49%
with Mo and to 3.35% with Cr). Hence, the very high C-X
stretching frequency with such a very large carbon-13 shift is
also diagnostic of the unique bonding in the XCtMX3 methyl-
idyne molecules.
WCl3 complex, the W-Cl stretching modes are too low (416
cm-1 and lower) to perturb significantly the important diagnostic
HCtW modes, and accordingly our HCtW group frequencies
approach the gaseous values. Finally, and perhaps the most
interesting, is the C-H stretching mode, which is also split in
the solid state and about 100 cm-1 lower than in the HCtWCl3
methylidyne molecule in solid argon. We suggest that our simple
methylidyne molecule is more representative of the methylidyne
C-H stretching modes, as it is only encumbered by argon matrix
and not ligand and solid-state effects.
Bonding and Structure Comparisons. Several comparisons
within the important group 6 transition metal family methylidene
and methylidyne molecules are of interest. First, consider the
calculated structures illustrated in Figures 9-12. The CtCr
triple bonds are all computed to be in the 1.592-1.635 Å range,
which is slightly shorter than the 1.735-1.745 Å values
measured by X-ray for heavily ligated chromium carbyne
complexes.3a,14 This is outside of the error range of our B3LYP
calculation, and it shows that the ligated complex triple bonds
are longer than the CtCr triple bonds prepared in these simple
matrix isolated methylidyne molecules. Second, notice that
fluorine substitution for chlorine on the chromium center very
slightly elongates the double and triple carbon-chromium
bonds. This is due to contraction of the metal 3d orbitals by
the inductive effect of fluorine, which makes them less effective
in forming π bonds to carbon. Third, notice that there is no
evidence for agostic distortion in the methylidene complexes,
which have C2V computed structures. This is in contrast to the
monohalo methylidene complexes of Mo and W, which are
agostic.16-21
A final comparison of the new HCtWCl3 molecule is of
particular interest because the gaseous HCtW species, the
simple hydrogen derivative HCtWH3, and the thoroughly
investigated microcrystalline HCtW(PMe3)4Cl complex7,9,19
provide a wide range of tungsten methylidyne compounds for
comparison of methylidyne frequencies and bond lengths. The
photophysics of these latter complexes is tunable by changing
substituents44 like the C-H stretching frequency described
above. Important parameters are collected in Table 9. First, the
bond lengths are measured to four decimal places for the gaseous
HCtW species,9 and these are all slightly shorter than our
B3LYP calculated values for the methylidynes and estimated
values for the solid-state complex.7 The frequencies measured
for HCtW have to be taken as benchmarks, and the HCtW
mode (1006 cm-1) and its DCtW (953 cm-1) counterpart show
that there is coupling with H(D). Our methylidyne complex
values, 1003(953) and 1006(957) cm-1, are extremely close.
However, the microcrystalline complex frequencies are 97 and
82 cm-1 lower, and other methylidyne complex values are in
the 1300-1400 cm-1 range,5 which demonstrates considerable
coupling to other modes in these organometallic complexes. The
H-C-W and D-C-W deformation modes for the gaseous
species are 660 and 501 cm-1. As the data in Table 9 show,
these modes increase with molecular complexity including
ligands and even split in the solid state. In the case of our HCt
Table 10 compares the frequencies for the series of the simple
methylidyne molecules HCtMCl3 for M ) Cr, Mo, and W.
The C-H stretching frequency increases with metal size as the
calculated C-H length decreases from 1.0874 to 1.0841 to
1.0814 Å. The s-character in the carbon hybrid orbital increases
from 44.3% for the Cr methylidyne to 46.3% for the Mo and
46.0% for the W species. (We are unsure why the W is not
higher than the Mo value, but we are using SDD pseudopotential
and basis sets developed from other atomic and molecular data.29
We do note that the values for Mo and W are similar, and Cr
is lower than both.) We also point out that the natural charges
on C and Cr (0.18, 0.28) indicate very little bond polarity, in
marked contrast to the Mo and the even more polar W examples.
This is also accompanied by an increase in the H-C-M
deformation frequency. The H-C-M/D-C-M frequency ratio
also increases slightly, 1.195, 1.235, 1.254, which is a measure
of harmonic character in the vibrational potential function. The
H/D ratio for the analogous mode in HCtCrF3 is nearly the
same, 1.199, so this quantity is metal dependent. Unfortunately,
the CtCr stretching mode is calculated to be too weak to
observe, but in the same region as the CtMo and CtW
stretching modes, which were observed with natural metal
isotopic profiles.
The increasing stability of the heavier metal methylidynes is
underscored by the reaction exothermicities collected in Table
S11. Notice that the chromium product formation reaction
(44) Da Re, R. E.; Hopkins, M. D. Coord. Chem. ReV. 2005, 249, 1396.

Methylidyne XCtMX3 Diagnostic C-H and C-X Absorptions
Organometallics, Vol. 26, No. 25, 2007 6387
exothermicities are about one-half or less than those for
molybdenum, which are substantially less than the values for
tungsten. The fluoride reactions are less exothermic due to the
stronger C-F bond energy as compared to C-Cl. Furthermore,
the Cr and Freon reactions give predominantly methylidene with
little methylidyne product. This trend is an extension of the
energy profile presented for the group 6 metal and CH3F reaction
products, which showed that chromium methylidenes and
methylidynes are higher energy species.18
It is also of interest to compare the carbon-13 and chlorine-
37 isotopic shifts in the antisymmetric Cl-C-M stretching
mode for the ClCtMCl3 molecules observed at 1230.6, 1259.6,
and 1292.8 cm-1, respectively, for Cr, Mo, and W. The isotopic
shifts are expressed as % of the frequency for C and Cl,
respectively, as follows: Cr (3.35, 0.20), Mo (3.49, 0.15), and
W (3.57, 0.13). In this antisymmetric stretching mode, carbon
is moving most, and C will move more against the heavier W
resulting in less motion for the Cl atom.
and show the effect of substituents and the solid state on the
important diagnostic methylidyne vibrational frequencies.
In contrast to the group 6 CH2dMH2 and CH2dMHX
methylidene complexes prepared from methane and methyl
halides,16-21 the CH2dMX2 complexes show no evidence of
agostic distortion at the B3LYP computational level of theory.
This may be in part due to the 0.03 Å longer CdM bond lengths
in the dihalide complexes.
The Cr reactions with methylene halides gave triplet CH2d
CrCl2 and CH2dCrFCl or quintet CH2-CrF2 methylidene
complexes as major products, in contrast to previous investiga-
tions with methane and methyl halides, which formed only
quintet CH3-CrX insertion products.18-21 The observation of
methylidynes in Cr reactions with haloforms and carbon
tetrahalides highlights the importance of halogen atoms as
R-transfer reagents to increase the reaction exothermicity and
thus to favor the formation of the higher oxidation state
methylidyne molecules. The C-H stretching mode at 3038.5
cm-1 and the 44.3% carbon 2s character for HCtCrCl3 are the
lowest we have observed for a group 6 metal. The present simple
methylidyne complex CtCr bond lengths calculated in the
1.59-1.63 Å range demonstrate the effect of bulky substituents
and the solid state on these fundamental molecular parameters,3a,14
and our simple complexes should be used as the benchmark
for comparison of chromium methylidyne physical constants.
Conclusions
Reactions of laser-ablated Cr, Mo, and W atoms with di-,
tri-, and tetrahalomethanes have produced a series of new
methylidene CH2dMX2 and methylidyne molecules HCt
MHX2, HCtMX3, and XCtMX3 (X ) F, Cl). Matrix-isolation
infrared spectra and quasi-relativistic density functional methods
have been used to identify the new products formed by highly
exothermic metal atom reactions in these experiments. Sym-
metrical trigonal structures were computed for the latter methyl-
idyne molecules. Bonding analyses were performed using natural
bond orbitals, and the C-H stretching frequencies were shown
to increase with increasing s-character in the carbon hybrid
orbitals in the HCtMoFnCl3-n (n ) 0, 1, 2, 3) series.
Comparisons with the gaseous HCtW species, the simple
hydrogen derivative HCtWH3, the new HCtWCl3 molecule,
and the microcrystalline HCtW(PMe3)4Cl complex7,9,19 un-
derscore the close chemical relationship among these species
Acknowledgment. We gratefully acknowledge financial
support from NSF Grant CHE 03-52487 to L.A.
Note Added after ASAP Publication. Table 2 contained
errors in the version published ASAP November 7, 2007; the
corrected version published ASAP November 12, 2007.
Supporting Information Available: Tables of calculated
frequencies and figures of matrix infrared spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM700689E