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stantial 7r,v4-character, more so than is present in the (v5-C5H6),M(diene) (M = Zr, Hf) complexes. This in turn suggests that the M(I1) oxidation state contributes significantly to the bonding scheme of the derivatives M-These new group 4 butadiene complexes display fluxional behavior that is best described as diene rotation rather than the envelope-flipping mechanism normally associated with early metal diene derivatives. That this fluxional process is more akin to that found for an electron-rich, late-transition-metal complex provides further support for the proposal that phosphine donors can stabilize lower oxidation states of the early transition metals.31 (31) For recent examples of this strategy, see: (a) Wielstra, Y.; Gambarotta, s.; Roedelof, J. B.; Chiang, M. Y.

. Can. J. Chem. 69, 264 (1991). A systematic comparison between the dimethylzirconocene (DMZ) and dimethyltitanocene (DMT) catalyzed dehydrocoupling of organosilanes is reported. The polymerization rates of primary silanes are about ten times faster for DMZ than for DMT, but the characteristics of the polymers produced are essentially identical. Variations of molecular weight and molecular weight distribution with varying monomer and reaction time, and the relative amounts of linear and cyclic polymer are also essentially the same for the two catalysts.Oligomerizations of phenylmethylsilane, diphenylsilane, and 1,2-diphenyldisilane are described. The compound Cp2Zr-(SiPhMeH)(p,-H),(SiPhMe2)ZrCp, (16) is isolated from the reaction of phenylmethylsilane with DMZ and its structure confirmed by crystallography and 'H-NMR spectroscopy. The compound Cp2Zr(Me)(p,-H)2(SiPh2H)ZrCp2 ( I d ) is isolated from the reaction of diphenylsilane with DMZ and its structure assigned by NMR spectroscopy. A number of other hydride bridged species are observed by 'H-NMR in the reactions of DMZ with phenylmethyl-, diphenyl-, and n-butylmethylsilanes and their structures are tentatively assigned. The implications of these observations on the mechanism of transformation of DMZ into active catalyst and our current views of the polymerisation mechanism are also discussed.

The structures of some poly(organosilylenes), [Formula: see text] (R = Ph, p-tolyl, n-hexyl, and benzyl), produced by catalytic dehydrogenative coupling of primary silanes have been studied by infrared, nuclear magnetic resonance, and mass spectroscopies. These results, combined with data on molecular weights and molecular weight distributions from vapour pressure osmometry and gel permeation chromatography, lead to the conclusion that the polymers are linear and have SiH2R end groups. The polymers all have degrees of polymerization of ca. 10 and very narrow molecular weight dipersions. Some possible features of the mechanism that gives rise to this behaviour are discussed.

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