SYNTHESIS AND MOLECULAR-STRUCTURE OF NIOBOCENE-(TRICARBONYL)(PI-CYCLOPENTADIENYL)MOLYBDENUM WITH METAL-METAL BOND AND UNUSUAL COORDINATION OF CARBONYL GROUPS

(C5H5)2NbBH4 reacts with C5H5M(CO)3Me in toluene solution in the presence of Et3N to give binuclear complexes (C5H5)2NbM(CO)3C5H5 where M is Mo or W (IV and V, respectively). The structure of IV has been studied by X-ray diffraction (the crystals are orthorhombic, a 12.748(5), b 16.745(6), c 14.314 A/ac> ̊;; Z = 8, space group of Pbca, automatic difractometer Syntex P2I, λ(Mo-Kα, 1382 reflections, R = 0.056, Rw = 0.058). Molecule IV contains a wedge-like sandwich (π-C5H5)2Nb (NbC 2.37-2.48, CC (av) 1.42 A/ac> ̊;, angle between ring planes 49°) linked with the (π-C5H5)Mo(CO) fragment by a direct NbMo bond (3.073 A/ac> ̊;) and two bridging CO groups, one nonsymmetrically bonded through the carbon atom only (CO 1.17, NbC 2.53, MoC 2.02 A/ac> ̊;) and the other σ-bonded to Mo (MoC 1.944 A/ac> ̊;) and π-bonded to Nb (CO 1.22, NbC 2.22, NbO 2.26 A/ac> ̊;). Three types of carbonyl groups present in IV give rise to strong IR bands at 1870, 1700 and 1560 cm-1 assigned to the terminal, μ-bridging and σ, π-bridging CO groups respectively. Complex IV has a similar structure. The electronic structure of IV and its dissociation across the NbMo bond are discussed. © 1979.