An unusual exchange between alkylidyne alkyl and bis(alkylidene) tungsten complexes promoted by phosphine coordination: Kinetic, thermodynamic, and theoretical studies

d0 Tungsten alkylidyne alkyl complex (Me3SiCH2)3W(≡CSiMe3)(PMe3) (4a) was found to undergo a rare, PMe3-promoted exchange with its bis(alkylidene) tautomer (Me3SiCH2)2W(=CHSiMe3)2(PMe3) (4b). Thermodynamic studies of the exchange showed that 4b is favored and gave Keq and the enthalpy and entropy of the equilibrium: ΔH° = -1.8(0.5) kcal/mol and ΔS° = -1.5(1.7) eu. Kinetic studies of the α-H migration between 4a and 4b by variable-temperature NMR gave rate constants k1 and k-1 for the reversible reactions and activation enthalpies and entropies: ΔH1‡ = 16.2(1.2) kcal/mol and ΔS1‡ = -22.3(4.0) eu for the forward reaction (4a → 4b); ΔH2‡ = 18.0(1.3) kcal/mol and ΔS2‡ = -20.9(4.3) eu for the reverse reaction (4b → 4a). Ab initio calculations at the B3LYP level revealed that PMe3 binds with the bis(alkylidene) tautomer relatively more strongly than with the alkylidyne tautomer and thus stabilizes the bis(alkylidene) tautomer. Copyright © 2004 American Chemical Society.