NIOBIUM(III) MONOCYCLOPENTADIENYL ISOCYANIDE, ACETYLENE, AND DIENE COMPLEXES - X-RAY CRYSTAL-STRUCTURES OF NB(ETA-5-C5ME5)CL4(CN-2,6-ME2C6H3) AND NB(ETA-5-C5ME5)CL2(CN-2,6-ME2C6H3)3-CENTER-DOT-1/2MEC6H5

NbCp*Cl4 (Cp* = eta5-C5Me5) reacts with isocyanides to give the pseudooctahedral adducts NbCp*Cl4(CNR) (R = (t)Bu, 1; R = 2,6-Me2C6H3,2). Reduction of these adducts or alternatively NbCp*Cl4 with 2 equiv of Na/Hg in the presence of stoichiometric amounts of isocyanides gives the diamagnetic pseudooctahedral niobium(III) complexes NbCP*Cl2(CNR)3 (R = (t)Bu, 3; R = 2,6-Me2C6H3,4). A similar reduction of NbCp*Cl4 in the presence of acetylenes and dienes leads to the isolation of new niobium(III) derivatives NbCP*Cl2(R'C=CR'') (R' = R'' = Me, 5; R' = R'' = SiMe3,6; R' = Me, R'' = Et, 7; R' = Ph, R'' = SiMe3, 8) and NbCp*Cl2(diene) (diene = isoprene, 9; diene = methyl methacrylate, 10. Complexes 5-8 can also be prepared by reactions with acetylenes of the niobium(III) phosphine complexes NbCp*Cl2L2(L = PMe3,PMe2Ph) and by addition of acetylenes to the dichloroniobium(III) dimer [NbCp*Cl2]2. All the complexes were characterized by mass spectrometry and IR and NMR spectroscopy, and the molecular structures of 2 and 4 were confirmed by X-ray diffraction methods. Crystals of 2 are monoclinic, space group P2(1)/c, with Z = 4 in a unit cell of dimensions a = 8.274(4) angstrom, b = 33.313(10) angstrom, c = 8.656(3)angstrom, and beta =116.71(2)degrees. Crystals of the toluene solvate of 4 are monoclinic, space group P2(1)/c, with Z = 4 in a unit cell of dimensions a = 17.130(8) angstrom, b = 15.781(7) A, c = 14.922(5) angstrom, and beta = 108.43(2)degrees. Both structures were solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares on the basis of 1509 (2) and 4164 (4.1/2MeC6H5) observed reflections to R and R(w) values of 0.0365 and 0.0432 (2) and 0.0435 and 0.0602 (4.1/2MeC6H5), respectively.