ELECTROCHEMISTRY AND BONDING IN BISCYCLOPENTADIENYL COMPLEXES OF MOLYBDENUM, TUNGSTEN, AND NIOBIUM WITH O-PHENANTHROLINE AND BIPYRIDINE - CRYSTAL AND MOLECULAR-STRUCTURES OF [NB(ETA-5-C5H5)2(BPY)][PF6]2 AND [M(ETA-5-C5H5)2(O-PHEN)][PF6]2 (M = MO OR W)

The electrochemical behaviour of the new Nb(IV) derivatives [Nb(eta-5-C5H5)2(L-L)][PF6]2 where L-L = 2,2'-bipyridyl or o-phenanthroline and of the molybdenum(IV) and tungsten(IV) analogues was investigated and the reduction of the coordinated ligand was observed along with the redox processes based on the metal. The structure of the new [Nb(eta-5-C5H5)2(bpy)][PF6]2 (1) and of the known related compounds [Mo(eta-5-C5H5)2(o-phen)][PF6]2 (2) and [W(eta-5-C5H5)2(o-phen)][PF6]2 (3) have been characterized by single crystal X-ray diffraction studies. The Mo and the W complexes are isostructural, while the Nb complex crystallizes in a slightly different monoclinic cell. In complex 1 the Nb-N bond distances are 2.226(6) and 2.218(6) angstrom and the bond angle is 72.6(3)-degrees, while in complex 2, Mo-N bond distances are 2.193(7) and 2.186(7) angstrom and the N-Mo-N bond angle is 74.7(3)-degrees and in complex 3, W-N bond distances are 2.181(11) and 2.179(9) angstrom and the N-W-N bond angle is 74.7(4)-degrees. An extended-Huckel molecular-orbital study of the bonding of these complexes showed the absence of back donation to the low-energy, empty orbitals of the nitrogen ligand.