Preparation of Tungsten Alkyl Alkylidene Alkylidyne Complexes and Kinetic Studies of Their Formation

An equilibrium mixture of alkyl alkylidyne W(CH2SiMe3)3(⋮CSiMe3)(PMe3) (1a) and its bis(alkylidene) tautomer W(CH2SiMe3)2(CHSiMe3)2(PMe3) (1b) has been found to undergo an α-hydrogen abstraction reaction in the presence of PMe3 to form alkyl alkylidene alkylidyne W(CH2SiMe3)(CHSiMe3)(⋮CSiMe3)(PMe3)2 (2). In the presence of PMe3, the formation of 2 follows first-order kinetics, and the observed rate constant was found to be independent of the concentration of PMe3. The activation parameters for the formation of 2 are ΔH⧧ = 28.3(1.7) kcal/mol and ΔS⧧ = 3(5) eu. In the presence of PMe2Ph, an equilibrium mixture of W(CH2SiMe3)3(⋮CSiMe3)(PMe2Ph) (3a) and its bis(alkylidene) tautomer W(CH2SiMe3)2(CHSiMe3)2(PMe2Ph) (3b) was similarly converted to W(CH2SiMe3)(CHSiMe3)(⋮CSiMe3)(PMe2Ph)2 (4). The observed rate of this reaction was also independent of the concentration of PMe2Ph. These observations suggest a pathway in which the tautomeric mixtures 1a,b and 3a,b undergo rate-determining, α-hydrogen abstraction, foll...