Vibrational Spectroscopy and Normal-Mode Analysis of Tungsten−Methylidyne Complexes. Insight into the Nature of the M⋮CH Bonds

Raman and infrared spectra are reported for the tungsten−methylidyne complex trans -W(⋮CH)(PMe 3 ) 4 Cl ( 1 ) and its deuterated isotopomers W(⋮CD)(PMe 3 ) 4 Cl ( 1 - d 1 ) and W(⋮CH)(PMe 3 - d 9 ) 4 Cl ( 1 - d 36 ). The bands attributable to the vibrational modes of the W⋮C H fragment are clearly identified on the basis of their frequency shifts upon isotopic substitution: ν 1 [ν(C H)] ≅ 2980 cm -1 ( 1, 1 - d 36 ), 2240 cm -1 ( 1 - d 1 ); ν 2 [ν(W⋮C)] ≅ 911 cm -1 ( 1, 1 - d 36 ), 871 cm -1 ( 1 - d 1 ); ν 3 [λ(W⋮C H)] = 788/755 cm -1 ( 1 ), 642/611 cm -1 ( 1 - d 1 ). These data and normal-coordinate calculations on both the W⋮C H and W(⋮CH)P 4 Cl fragments of these compounds reveal that the W⋮C stretch is negligibly coupled either to the C H stretch or to motions involving the ancillary ligands; thus, for 1, the W⋮C force constant given by the pseudodiatomic-oscillator approximation (5.945 mdyne Å -1 ) is within 2% of the best value from the normal-coordinate calculations (6.021 mdyne Å -1 ). The W⋮C stretching frequency is much smaller than those previously assigned for the M⋮CR stretches of metal−alkylidyne complexes with larger R groups (R = Me, Ph; ν ≅ 1270−1600 cm -1 ); normal-coordinate calculations on the general W⋮C B oscillator, where B is a dummy atom of variable mass, reveal that this disparity is the result of large mixing between ν(W⋮C) and ν(C B) for cases where B is heavier than H and, in some cases, of mixing among ν(W⋮C) and internal vibrational modes of B. The W⋮C force constant is consistent with the W⋮C bond length, based on an empirical bond-distance/force-constant correlation for selected third-transition-series metal complexes. The relatively low C H stretching frequency and force constant (5.207 mdyne Å -1 ) are consistent with a C H bond length that is longer than typical for an sp-hybridized carbon atom, suggesting that this bond is correspondingly rich in carbon p-orbital character.