A dramatic isotope effect in the reaction of ClSiH with trimethylsilane-1-d: experimental evidence for intermediate complexes in silylene Si-H(D) insertion reactions

A kinetic isotope effect (kD/kH) of 7.4 has been found for the reaction of chlorosilylene with trimethysilane (Me3SiD vs Me3SiH). Such a value can be accounted for by theoretical modelling, but only if an internal rearrangement of the initially form complex is included in the mechanism. This provides the first concrete evidence for such complexes. The insertion reaction of silylenes into Si-H bonds is one of the fundamental and most widely studied reactions of silylenes. The mechanism of this process is thought to involve weakly bound intermediate complexes. The evidence for this comes largely from the interpretation of gas-phase kinetic studies, allied to theoretical (ab initio) calculations. The main arguments in favour of such complexes are the existence of low energy minima on the calculated potential energy surfaces for such reactions and the experimental finding of negative activation energies. Despite these arguments the experimental evidence may still be regarded as inconclusive. The complexes themselves have not been seen directly nor has any process unique to the complex itself been identified. We report here an experiment which provides unambiguous evidence of such a process. The basis of our choice of this experiment was the prediction from ab initio calculations that, in the reaction of ClSiH with Me3SiH, not only does the reaction proceed via a complex, but the complex has a degenerate rearrangement process as shown in scheme 1 below, indicated by the exchange of H and H*, via a low energy transition state in which both Hatoms are coordinated to each Si-atom.