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Hydride derivatives of ((eta)('5)-C(,5)H(,4)CH(,3))(,2)ZrCl(,2) were prepared to examine their sterochemistry and reactivity. {lcub}(eta)('5)-C(,5)H(,4)CH(,3))(,2)ZrH(Cl){rcub}(,x) was prepared, but its characterization was inhibited because it disproportionates in solution to equimolar amounts of ((eta)('5)-C(,5)H(,4)CH(,3))(,2)ZrCl(,2) and {lcub}((eta)('5)-C(,5)H(,4)CH(,3))(,2)ZrH((mu)-H){rcub}(,2), I. I was directly prepared and was completely characterized. The molecular structure of this complex consists of two ((eta)('5)-C(,5)H(,4)CH(,3))(,2)ZrH units linked by two three-center, two-electron Zr-H-Zr bonds. It contains a crystallographic center of inversion and, necessarily, trans terminal hydrides. The important bond angles with the Zr(,2)H(,4) core are H(,t)-Zr-H(,b), 70(1); H(,t)-Zr-H(,b)', 130(1); H(,b)-Zr-H(,b)', 60(1); Zr-H(,b)-Zr', 120(1)(DEGREES). The metal-hydrogen bond distances are Zr-H(,t), 1.78(2); Zr-H(,b), 2.05(3); Zr-H(,b)', 1.94(2) (ANGSTROM). The reactivity of I with diphenylacetylene, phenylacetylene, hexafluoro-2-butyne, and bis(trimethylsilyl)acetylene was examined. Diphenylacetylene and phenylacetylene react with I below 50(DEGREES)C to yield primarily the zirconacyclopentadiene derivatives, ((eta)('5)-C(,5)H(,4)CH(,3))(,2)Zr(C(,4)(C(,6)H(,5))(,4)) and ((eta)('5)-C(,5)H(,4)CH(,3))(,2)Zr(C(,4)(C(,6)H(,5))(,2)H(,2)), respectively and H(,2). If these reactions are repeated at higher temperatures without removing the H(,2), substantial hydrogenation of diphenylacetylene to trans-stilbene and phenylacetylene to ethylbenzene, is observed. The chemistry of I with PhC(TBOND)CH and PhC(TBOND)CPh therefore involves at least two competing reactions - metallacycle formation and acetylene hydrogenation. For F(,3)CC(TBOND)CCF and Me(,3)SiC(TBOND)CSiMe(,3), different results were obtained. For the former the major product is ((eta)('5)-C(,5)H(,4)CH(,3))(,2)ZrF(,2). The latter yields a blue dinuclear zirconocene product which appears to contain two (MeCp)(,2)Zr units bridged by a hydride and the corresponding alkenyl ligand. Both photolysis and thermolysis of I proceed with reductive elimination of H(,2) and concomitant formation of reduced zirconocene species. These reactions were monitored by EPR, which in each case revealed the presence of an intermediate paramagnetic Zr-hydride compound. Its solution EPR spectrum consists of a doublet centered at g = 1.9854 with A(('1)H) = 6.8 G. Although the structure of this paramagnetic species remains to be determined, subsequent EPR experiments have shown that it reacts with disubstituted acetylenes to produce ((eta)('5)-C(,5)H(,4)CH(,3))(,2)ZrH(RC(TBOND)CR), where R = Ph or Me, reacts with phosphines to yield paramagnetic Zr(III)-monophosphine species, and reversibly undergoes H/D exchange. The results of these exploratory studies suggest that paramagnetic zirconocene hydride complexes may participate in the reactions of I.