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

Abstract

Raman and infrared spectra are reported for the tungsten−methylidyne complex trans-W(⋮CH)(PMe3)4Cl (1) and its deuterated isotopomers W(⋮CD)(PMe3)4Cl (1-d1) and W(⋮CH)(PMe3-d9)4Cl (1-d36). The bands attributable to the vibrational modes of the W⋮CH fragment are clearly identified on the basis of their frequency shifts upon isotopic substitution: ν1[ν(CH)] ≅ 2980 cm-1 (1, 1-d36), 2240 cm-1 (1-d1); ν2[ν(W⋮C)] ≅ 911 cm-1 (1, 1-d36), 871 cm-1 (1-d1); ν3[λ(W⋮CH)] = 788/755 cm-1 (1), 642/611 cm-1 (1-d1). These data and normal-coordinate calculations on both the W⋮CH and W(⋮CH)P4Cl fragments of these compounds reveal that the W⋮C stretch is negligibly coupled either to the CH 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⋮CB 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(CB) 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 CH stretching frequency and force constant (5.207 mdyne Å-1) are consistent with a CH 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.