Reactions of transition-metal ions with methylsilanes in the gas phase. The formation and characteristics of strong transition metal-silylene bonds

Abstract

Reactions of transition-metal ions (Ti⁺, V⁺, Cr⁺, Fe⁺, Co⁺, and Ni⁺) with organosilanes are investigated in the gas phase with an ion beam apparatus. Co⁺ and Ni⁺ react with silane to yield metal silylenes as exothermic products. Collision-induced dissociation studies of the product CoSiH₂⁺ and nascent CoSiH₄⁺ adducts provide additional information concerning the product structure and reaction mechanisms. Reactions with methylsilanes lead to formation of metal silylenes as major reaction channels, along with several other processes including hydride abstraction, dehydrogenation, and methane loss. Reactions with hexamethyldisilane are also investigated, with major products indicating Si-Si bond cleavage. An examination of the reaction enthalpies for the observed metal silylene products provides estimates for metal ion-silylene bond energies, which include D⁰(M⁺ - SiH₂) = 67 ± 6 kcal mol⁻¹ (M = Co, Ni). Correlation between the metal ion-silylene bond energies and the electronic structure of the metal ions supports a bonding scheme in which silylene donates its nonbonding lone pair electrons to an empty 4s orbital of the metal center. For Co⁺ and Ni⁺, back-donation of paired 3d electrons from the metal into the empty 3p orbital on silicon is suggested to account for the stronger bond deduced for these metals.