SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF NEW ISOCYANATE AND IMIDO NIOBOCENE COMPLEXES - CRYSTAL-STRUCTURES OF [(NB(ETA-C(5)H(4)SIME(3))(2)CL)(2)] AND [NB(ETA-C(5)H(4)SIME(3))(2)(=NPH)CL]

The isocyanate complexes [Nb(eta-C(5)H(4)SiMe(3))(2)(RNCO)Cl] (R = Ph 2, Bu(t) 3, Pr-n 4, Pr-i 5) have been prepared from the reaction of [Nb(eta-C(5)H(4)SiMe(3))(2)Cl] 1 with the appropriate ligands. Treatment of 1 with the corresponding amines NH(2)R afforded the imido complexes [Nb(eta-C(5)H(4)SiMe(3))(2)(=N R)Cl] (R = Ph 6. Bu(t) 7, Pr-n 8 or Pr-i 9) which can also be prepared by thermal treatment of the corresponding isocyanate complexes 2-5. All complexes have been characterised in solution by both H-1 and C-13 NMR spectroscopy. Complex 1 crystallises in the monoclinic space group P2(1)/c showing two crystallographic independent molecules with Z = 8, a = 12.968(4), b = 23.988(7), c = 25.546(7) Angstrom A and beta = 103.22(3)degrees. It has a binuclear structure with two almost symmetrical chloride bridges, giving a closed-shell, 18-electron configuration. Complex 6 crystallises in the monoclinic space group P2(1)/ln with Z = 4, a = 7.343(2), b = 16.880(3), c = 20.227(4) Angstrom A and beta = 96.41(2)degrees. The structure shows a value of 165.1(2)degrees for the Nb-N-C angle and an Nb-N bond distance of 1.792(2) Angstrom A. intermediate between that expected for double and triple bonds.