Molecular Structures, Vibrational Spectroscopy, and Normal-Mode Analysis of M₂(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 M2(C⋮CR)4(PMe3)4 (M = Mo, W; R = H, Me, But, SiMe3) has been deduced. Metrical data from X-ray crystallographic studies of Mo2(C⋮CR)4(PMe3)4 (R = Me, But, SiMe3) and W2(C⋮CMe)4(PMe3)4 reveal that the core bond distances and angles are within normal ranges and do not differ in a statistically significant way as a function of the alkynyl substituent, indicating that their associated force constants should be similarly invariant among these compounds. The crystal structures of Mo2(C⋮CSiMe3)4(PMe3)4 and Mo2(C⋮CBut)4(PMe3)4 are complicated by 3-fold disorder of the Mo2 unit within apparently ordered ligand arrays. Resonance-Raman spectra (1(δ→δ*) excitation, THF solution) of Mo2(C⋮CSiMe3)4(PMe3)4 and its isotopomers (PMe3-d9, C⋮CSiMe3-d9, 13C⋮13CSiMe3) exhibit resonance-enhanced bands due to a1-symmetry fundamentals (νa = 362, νb = 397, νc = 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 spectra (FT-Raman, 1064 nm excitation, crystalline samples) for the Mo2(C⋮CSiMe3)4(PMe3)4 compounds and for Mo2(C⋮CR)4(PMe3)4 (R = H, D, Me, But, SiMe3) and W2(C⋮CMe)4(PMe3)4 exhibit νa, νb, and νc and numerous bands due to alkynyl- and phosphine-localized modes, the latter of which are assigned by comparisons to FT-Raman spectra of Mo2X4L4 (X = Cl, Br, I; L = PMe3, PMe3-d9)4 and Mo2Cl4(AsMe3)4. Valence force-field normal-coordinate calculations on the model compound Mo2(C⋮CH)4P4, using core force constants transferred from a calculation on Mo2Cl4P4, show that νa, νb, and νc arise from modes of strongly mixed ν(Mo2), ν(MoC), and λ(MoCC) character. The relative intensities of the resonance-Raman bands due to νa, νb, and νc reflect, at least in part, their ν(M2) character. In contrast, the force field shows that mixing of ν(M2) and ν(C⋮C) is negligible. The three-mode mixing is expected to be a general feature for quadruply bonded complexes with unsaturated ligands.