Neither dimethylhafnocene nor [1, 2-bis (tetrahydroindenyl) ethane] dimethylzirconium, 1, reacts at anappreciable rate with phenylsilane. In the case of the hafnium compound, this results in complete masking of catalytic activity toward dehydrocoupling of phenylsilane, while in the case of 1 activity is greatly suppressed. This behavior is governed by the low reactivity of the M-C bonds toward σ-bond metathesis with silanes. Catalysts generated by reaction of MeLi and PhSiH3 with hafnocene dichloride or [1,2-bis (tetrahydroindeny1)-ethune] dichlorozirconium, 2, show high activities toward PhSiH3coupling. The molecular weights of the resulting polymers are significantly higher than those of polymers derived from CpzTiMez and CpzZrMez catalysts, and the hafnocene catalyst gives unusually low proportions of low molecular weight cyclic products. It is proposed that the catalyst species, probably a hydrido silyl complex, in these new systems are generated by reaction of a reactive lithium hydride, resulting from rapid reaction of MeLi with PhSiH3, with the chloro complexes.