Synthesis and molecular structure of niobocene(tricarbonyl)(π-cyclopentadienyl)molybdenum with metalmetal bond and unusual coordination of carbonyl groups

(C 5 H 5 ) 2 NbBH 4 reacts with C 5 H 5 M(CO) 3 Me in toluene solution in the presence of Et 3 N to give binuclear complexes (C 5 H 5 ) 2 NbM(CO) 3 C 5 H 5 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 P 2 I , λ(Mo- K α , 1382 reflections, R = 0.056, R w = 0.058). Molecule IV contains a wedge-like sandwich (π-C 5 H 5 ) 2 Nb (NbC 2.37–2.48, CC (av) 1.42 A/ac> ̊ ;, angle between ring planes 49°) linked with the (π-C 5 H 5 )Mo(CO) fragment by a direct NbMo bond (3.073 A/ac> ̊ ;) and two bridging CO groups, one nonsymmetrically bonded through the carbon atom only (CO 1.17, NbC 2.53, MoC 2.02 A/ac> ̊ ;) and the other σ-bonded to Mo (MoC 1.944 A/ac> ̊ ;) and π-bonded to Nb (CO 1.22, NbC 2.22, NbO 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 NbMo bond are discussed.