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

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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.
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