Direct Observation of β-Methyl Elimination in Cationic Neopentyl Complexes: Ligand Effects on the Reversible Elimination of Isobutene

β-Me elimination from cationic neopentyl complexes, [Cp‘ 2 Zr(CH 2 CMe 3 )] +, has been demonstrated. The more crowded complex (Cp‘ = C 5 Me 5 ) eliminates isobutene instantaneously at −75 °C, whereas the less crowded analogue (Cp‘ = C 5 H 5 ) is stable in solution at 0 °C but undegoes reversible β-Me elimination at 25 °C; strong anion or Lewis base coordination suppresses β-Me elimination, consistent with involvement of a 14-electron species.