The chemistry of recoiling silicon atoms. V. Product-forming reactions in phosphine-silane mixtures

Product studies on reactions of recoiling silicon atoms in binary mixtures of phosphine, the silicon precursor, and silane, disilane, and trisilane, respectively, support a mechanism for the product-forming steps involving silylene ⁸¹SiH₂ as the principal reactive intermediate which gives rise to the observed products. In each case the major product is a next-higher homolog of the starting silane, the expected product of Si-H insertion by ³¹SiH₂. Four other mechanisms are discussed and rejected on the basis of product studies and scavenger experiments. Competition experiments in ternary phosphine-silane-disilane and quaternary phosphine-silane-disilane-nitric oxide mixtures are correlated by a kinetic scheme for the product-determining steps involving a single intermediate. It is suggested that the product-determining steps are the product-forming steps and that the single kinetically important intermediate is ³¹SiH₂. Formation of lower homologs of the major products is explained by the unimolecular dissociation of vibrationally excited silylene insertion products.