Molecular structures, vibrational spectroscopy, and normal-mode analysis of M-2(C CR)(4)(PMe3)(4) dimetallatetraynes. Observation of strongly mixed metal-metal and metal-ligand vibrational modes

The nature of the skeletal vibrational modes of complexes of the type M-2(C =CR)(4)(PMe3)(4) (M = Mo, W; R = H, Me, Bu-t, SiMe3) has been deduced. Metr ical data from X-ray crystallographic studies of Mo-2(C=CR)(4)(PMe3)(4) (R = Me, Bu-t, SiMe3) and W-2(C=CMe)(4)(PMe3)(4) reveal that the core bond dis tances and angles are within normal ranges and do not differ in a statistic ally significant way as a function of the alkynyl substituent, indicating t hat their associated force constants should be similarly invariant among th ese compounds. The crystal structures of Mo-2(C=CSiMe3)(4)(PMe3)(4) and Mo- 2(C=CBut)(4)(PMe3)(4) are complicated by 3-fold disorder of the Mo-2 unit w ithin apparently ordered ligand arrays. Resonance-Raman spectra ((1)(delta- ->delta*) excitation, THF solution) of Mo-2(C=CSiMe3)(4)(PMe3)(4) and its i sotopomers (PMe3-d(9), C=CSiMe3-d(9), C-13=(CSiMe3)-C-13) exhibit resonance -enhanced bands due to al-symmetry fundamentals (v(a) = 362, v(b) = 397, v( c) = 254 cm(-1) for the natural-abundance complex) and their overtones and combinations. The frequencies and relative intensities of the fundamentals are highly sensitive to isotopic substitution of the C=CSiMe3 ligands, but are insensitive to deuteration of the PMe3 ligands. Nonresonance-Raman spec tra (FT-Raman, 1064 nm excitation, crystalline samples) for the Mo-2(C=CSiM e3)(4)(PMe3)(4) compounds and for Mo-2(C=CR)(4)(PMe3)(4) (R = H, D, Me, Bu- t, SiMe3) and W-2(C=CMe)(4)(PMe3)(4) exhibit v(a), v(b), and v(c) and numer ous bands due to alkynyl- and phosphine-localized modes, the latter of whic h are assigned by comparisons to FT-Raman spectra of Mo2X4L4 (X = Cl, Pr, I ; L = PMe3, PMe3-d(9))(4) and Mo2Cl4(AsMe3)(4). Valence force-field normal- coordinate calculations on the model compound Mo-2(C=CH)(4)P-4, using core force constants transferred from a calculation on Mo2Cl4P4, show that v(a), v(b), and v(c) arise from modes of strongly mixed v(Mo-2), v(MoC), and lam bda(MoCC) character. The relative intensities of the resonance-Raman bands due to v(a), v(b), and v(c) reflect, at least in part, their v(M-2) charact er. In contrast, the force field shows that mixing of v(M-2) and v(C=C) is negligible. The three-mode mixing is expected to be a general feature for q uadruply bonded complexes with unsaturated ligands.