BRIDGED BIS(PENTADIENYL) LIGANDS - CONFORMATIONALLY ALTERED OPEN METALLOCENES

The reactions of the bridged bis(pentadienyl) dianion, K2[4,4'-(CH2)2(2-C6H8)2] (2-C6H8 = 2-methylpentadienyl) with chlorides of divalent vanadium, chromium, or iron lead to the expected bridged open metallocene complexes, M [4,4'-(CH2)2(2-C6H8)2], whose stabilities and reactivities differ significantly from those of the M(2,4-C7H11)2 Counterparts. Thus, the chromium complex readily forms a variety of ligand adducts, whereas Cr(2,4-C7H11)2 forms none. Structural data have been obtained for the vanadium and chromium compounds. For V[4,4'-(CH2)2(2-C6H8)2], the space group is orthorhombic, Pbcn, with a = 10.941(3) angstrom, b = 13.579(2) angstrom, c = 8.424(2) angstrom, and V = 1251.53 angstrom3 for z = 4. Discrepancy indices of R = 0.027 and R(w) = 0.028 were obtained for 804 observed reflections. For Cr[4,4'-(CH2)2(2-C6H8)2], the space group at -130-degrees is orthorhombic, Pbca, with a = 8.499(2) angstrom, b = 17.091(2) angstrom, c = 33.569(6) angstrom, and V = 4875.92 angstrom3 for Z = 16 (two independent molecules). Discrepancy indices of R = 0.054 and R(w) = 0.058 were obtained for 2790 observed reflections. The structural results reveal that a great deal of strain in the organic fragment is required to bring about effective bonding with the metal centers.