MIXED-DICYCLOPENTADIENYL NIOBIUM AND TANTALUM COMPLEXES - SYNTHESIS AND REACTIVITY X-RAY MOLECULAR-STRUCTURES OF TA(ETA(5)-C(5)ME(5))(ETA(5)-C(5)H(4)SIME(3))CL-2 AND TA(ETA(5)-C(5)ME(5))(ETA(5)-C5H3(SIME(3))(2))H-3

MCpCl-4, (Cp* = eta(5)-C(5)Me(5)) reacts with LiCp (Cp = C5H4(SiMe(3) ) (Cp'); C5H2(SiMe(3))(2) (Cp '')) and sodium amalgam, in 1:1:1 molar ratio to give the paramagnetic dicyclopentadienyl niobium and tantalum (IV) complexes, MCpCpCl(2), (M = Nb, Cp = Cp' 1; Cp '' 2; M = Ta, Cp = Cp' 3; Cp '' 4). Reactions of 3 and 4 with 1/2 equivalent of PCl5 af ford the diamagnetic trichlorocomplexes TaCpCpCl(3), (Cp = Cp' 5; Cp '' 6), while oxidation with dry O-2 gives the diamagnetic dinuclear co mplexes [TaCpCpCl(2)](2)(mu-O), (Cp = Cp' 7; Cp '' 8), and with air a re transformed into the corresponding oxoderivatives TaCpCpCl(O), (Cp = Cp' 9; Cp '' 10). Treatment of compounds 3 and 4 with a slight exce ss of lithium aluminium hydride affords the trihydrido complexes TaCp CpH(3) (Cp = Cp' 11; Cp '' 12). The trihydrido complex 12 reacts with two-electron donor ligands on heating to yield the hydride tantalum(II I) compounds TaCpCp '' H(L) (L = CO 13, C2H4 14, RNC 15). All the new complexes were characterized by usual IR and NMR spectroscopic method s. The crystal structures of 3 and 12 were determined by X-ray diffrac tion studies. Crystals of 3 are orthorhombic, space group P2(1)2(1)2(1 ), with Z = 4 in a unit cell of dimensions a = 11.775(5) Angstrom, b = 12.821(1) Angstrom, c = 13.037(7) Angstrom. Crystals of 12 are tricli nic, space group P (1) over bar with Z = 2 in a unit cell of dimension s a = 7.384(4) Angstrom, b = 10.861(2) Angstrom, c = 16.731(3) Angstro m, alpha = 75.94(2)degrees, beta = 84.75(3) and gamma = 72.57(4)degree s. Both structures were solved from diffractometer data by a combinati on of direct and Fourier methods and refined by full-matrix least squa res fit on the basis of 4088 (3) and 4594 (12) observed reflections to R and R(w), values of 0.040 and 0.064 (3) and 0.022 and 0.033 (12) re spectively.