The rate of hydrogenolytic alkane liberation from permethylzirconocene neopentyl halide compounds, (C S (CH 3)shZr(X)CH 2 C(CH 3 h, X = F, CI, Br), is greatly reduced if the ring ligands are interconnected by an ethylene bridge, as in C2H4(CS(CH3)4hZr(X)CH2C(CH3h Hydrogenolysis of the corresponding permethylzirconocene neopentyl hydride compounds (X = H), on the other hand, is too fast for kinetic measurements at room temperature, even with the ethylene-bridged derivative. These observations, and the inverse kinetic isotope effect observed for reaction with D 2 , are interpreted with the assumption that H 2-induced alkane liberation from permethylzirconocene alkyl halides proceeds via an indirect ringmediated hydrogen transfer reaction which is feasible only with freely rotating ring ligands; hydrogenolysis of permethylzirconocene alkyl hydrides, on the other hand, apparently occurs without such limitation, by direct H 2-to-alkyl hydrogen transfet.