Mechanism of carbene insertion into Group 4 element hydrides via chromium carbene complexes. Part II. Organogermanium and organotin hydrides

Abstract

The kinetics of the reaction between various chromium carbene complexes [Cr(CO)₅{C(X)(C₆H₄Y)}](X = OMe or NC₄H₈; Y =p-OMe, p-Cl, or H) and ER₃H (E = Ge, R = Et or Ph; E = Sn, R = Buⁿ or Ph) in the presence of pyridine in hexane solution have been measured by i.r. spectrophotometry under pseudo-first-order conditions in the range 260–330 K. The results are compared with those for analogous silicon hydrides (E = Si, R = Et or Ph). The variation in the rate of the reaction (E = Si < Ge Sn) is explained in terms of an increasingly favourable concerted process in the second-order contribution to the rate equation, which results from the increasing ability of E to achieve five-co-ordination. This is supported by the observed variation in the activation parameters ΔH₂^‡ and ΔS₂^‡, the influence of substituents at both the carbene carbon and the element E has been examined; it is shown that the reaction is assisted by electron withdrawal from the former and electron donation to the latter site. There is no evidence to support the intervention of free-radical intermediates in these reactions.