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Electronically unsaturated diene complexes Cp*M(diene)Cl (Cp* = q5-CsMes; M = Zr, Hf; diene = 2,3-dimethyl-l,&butadiene, 2-methyl-l,3-butadiene) react with CO through an intermediate adduct to form (di)methylcyclopentadiene and [Cp*M(O)Cl],. Incorporation of the CO carbon atom in the cyclopentadiene was confirmed by 13CO-labeling experiments. The less oxophilic Ti is unable to perform this CO bond cleavage. Cp*Ti(2,3-dimethy1-1,3-butadiene)Cl reacts with CO to form the dimeric titanaoxirane (Cp*Ti[p-OCCH2C(Me)=C(Me)CH2]C1),, which was characterized by X-ray analysis (space group Pi, a = 10.293 (3) A, b = 10.445 (2) A, c = 8.409 ( 3 ) A, a = 103.12 (2)O, p = 101.58 (3)O, y = 68.29 (2)' at 170K, 2 = 1). The molecule contains a highly symmetrical central Tiz02 unit, and the C-0 distance of 1.408 (4) A suggests CO reduction to a single C-0 bond.

Chemical Society, for support of this research. S. D. acknowledges a Rumble fellowship provided by Wayne State University for support of his graduate studies. Registry No. Trimesitylaluminum-tetrahydrofuran, 99654-Supplementary Material Available: Complete listings of bond lengths (Table SI), bond angles (Table S-11), anisotropic thermal parameters (Table S-III), and hydrogen atom positional coordinates and isotropic thermal parameters (Table S-IV) and a packing diagram (Figure S-1) (7 pages); a listing of observed and calculated structure factors (Table S-V) (23 pages). Ordering information is given on any current masthead page.

The polyhydride [Cp*HfHzC1]42 waa prepared by high-pressure hydrogenation of Cp*Hf(C6Hlo)C1 (C6H10 = 2,3-dimethyl-l,3-butadiene) or Cp*Hf(COT)Cl (COT = cyclooctatetraene). The structure of [Cp*HfH2C1], was determined by X-ray diffraction. The compound crystallizes in the tetragonal space group P42'm with a = 21.454 (3) A, c = 10.525 (3) A, and Z = 4. The structure was refined to R = 0.030 and R, = 0.039 for 303 parameters and 2839 reflections with I > 2.5a(T). The molecule has four Hf atoms in a butterfly arrangement with alternating Cp* and C1 ligands (Hf-Ct = 2.15,2.23, and 2.27 A; Hf-Cl = 2.397 (3), 2.438 (3), and 2.432 (3) A). Hydride positions could not be found due to disorder in one of the Cp* ligands probably caused by the presence of a benzene solvate molecule. IR and NMR studies indicate the presence of two sets of four equivalent hydrides bridging the edges of a Hf4 butterfly arrangement. Bridging hydride ligands account for the low reactivity of the complex.

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