Why is β-me elimination only observed in d⁰ early-transition-metal complexes? An organometallic hyperconjugation effect with consequences for the termination step in Ziegler-Natta catalysis

Two processes, beta-Hand beta- Me migration, have been observed to compete in the termination of alkene oligomerization (Ziegler-Natta catalysts) in certain d0 early transition metal systems. ECP ab initio calculations have been performed to study these processes. Models of intermediates (X2MR(q): X = Cl, Cp; M = Zr, Hf, Ti, q = +1; Zr, Nb, q = 0) have been optimized at the HF level with additional single-point energy calculations at the MP2 level. It is shown that the beta-Me elimination may be thermodynamically favored over beta-H elimination for strongly electron-deficient metal centers. This preference is attributed to the presence of multiple bonding between a d0 transition metal and the methyl group, which behaves like a weak pi donor via its occupied pi(CH3) orbitals. It is therefore analogous to the well documented hyperconjugation in organic chemistry.