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, β-H and β-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 β-Me elimination may be thermodynamically favored over β-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 π donor via its occupied ^πCH₃ orbitals. It is therefore analogous to the well documented hyperconjugation in organic chemistry.