Insights into the metathesis reaction involving M-M, C-C, and M-C triple bonds from computations employing density functional theory on model compounds M2(OH)6 and M2(SH)6, where M = Mo and W

Calculations employing density functional theory (Gaussian 98, B3LYP, LANL2DZ, 6-31 G*) have been undertaken to interrogate the factors influencing the metathesis reaction involving M-M, C-C, and M-C triple bonds for the model compounds M-2(EH)(6), M-2(EH)(6)(mu-C2H2), and [(HE)(3)M( dropCH)](2), where M = Mo, W and E = O, S. Whereas in all cases the ethyne adducts are predicted to be enthalpically favored in the reactions between M-2(EH)(6) compounds and ethyne, only when M = W and E = 0 is the alkylidyne product [(HO)(3)W(dropCH)](2) predicted to be more stable than the alkyne adduct. For the reaction M-2(EH)(6)(mu-C2H2) --> [(HE)(3)M(dropCH)](2), the DeltaGdegrees values (kcal mol(-1)) are -6 (M = W, E = O), +5 (M = Mo, E = O), +18 (M = W, E = S), and +21 (M = Mo, E = S) and the free energies of activation are calculated to be AG(double dagger) = +19 kcal mol(-1) (M = W, E = O) and +34 kcal mol(-1) (M = Mo, E = O), where the transition state involves an asymmetric bridged structure M-2(OH)(4)(mu-OH)(2)(CH)(mu-CH) in which the C-C bond has broken; (CC)-C-. . . = 1.89 and 1.98 Angstrom for W and Mo, respectively. These results are discussed in terms of the experimental observations of the reactions involving ethyne and the symmetrically substituted alkynes (RCCR, where R = Me, Et) with M-2((OB)-B-t)(6) and M-2((OBu)-Bu-t)(2)((SBu)-Bu-t)(4) compounds, where M = Mo, W.