ABSOLUTE RATE CONSTANTS FOR THE GAS-PHASE SI-H INSERTION REACTIONS OF DIMETHYLSILYLENE WITH SILANE AND THE METHYLSILANES IN THE TEMPERATURE-RANGE 300-600-K

The title reactions have been investigated by means of laser flash photolysis. Dimethylsilylene, SiMe2, was produced by 193-nm photolysis of pentamethyldisilane and detected via its visible absorption at 457.9 nm. Second-order rate constants were obtained for reactions of SiMe2 with SiH4, MeSiH3, Me2SiH2, and Me3SiH. With the exception of SiH4, all the other rate constants decrease with increasing temperature. They represent a classic set of examples of reactions with negative activation energies, increasing to -11.2 ± 1.0 kJ mol-1 for reaction with Me3SiH. The substrate methyl substituent effect suggests the importance of the nucleophilic interaction in the rate-determining step. The measured A factors indicate tight structures for the activated complexes, consistent with kinetic measurements of the reverse reactions. A mechanism proceeding via an intermediate complex is shown to be consistent with the data, with entropy factors dominating the dynamics of the reaction way. Avariational transition-state model (without an intermediate) is not thought to provide such a satisfactory explanation, although it cannot be ruled out. © 1990, American Chemical Society. All rights reserved.