SYNTHESIS AND STRUCTURES OF THE MU-VINYLIDENE BINUCLEAR NICKEL-COMPLEXES [NI-2(MU-CCH2)(DMPM)(2)CL-2] AND [NI-2(MU-CCH2)(DPPM)(2)BR-2] - COMPARISON OF THE ELECTRONIC-STRUCTURES OF NICKEL A-FRAMES

The reaction of bis(cycloactadiene)nickel(0) with bis(dimethylphosphin o)methane (dmpm) and 1,1-dichlorovinylidene in THF yields the vinylide ne-bridged binuclear nickel A-frame complex [Ni-2(mu-C=CH2)(dmpm)(2)Cl -2] (1a). The structure of 1a was determined by X-ray crystallography. The Ni-Ni separation is 2.898(2) Angstrom, and the vinylidene carbon- carbon bond distance is 1.35(2) Angstrom. The structure of the bis(dip henylphosphino)methane (dppm) bridged mu-vinylidene complex [Ni-2(mu-C =CH2)(dppm)(2)Br-2] (2b) was also determined by X-ray crystallography. The Ni-Ni separation of 2.874(2) Angstrom for 2b is comparable to tha t of 1a. The series of Ic-vinylidene nickel A-frames with substituted terminal ligands [Ni-2(mu-C=CH2)(PR(2)CH(2)PR(2))(2)X(2)] (X = Cl, Pr, I, R = Me; X = Cl, Pr, I, NCS, OCN; R = Ph) were prepared and charact erized. The nickel A-frames with different bridging ligands [Ni-2(mu C =E)(dppm)(2)Cl-2] (E = O, NPh) were also examined. Extended Huckel mol ecular orbital (EHMO) calculations indicate that the HOMO in these nic kel A-frame complexes is primarily metal based and that the LUMO is de rived primarily from the pi system of the mu-C=CH2 ligand. The sigma- donating ability of the diphosphine ligands affects the electronic str ucture of these complexes primarily by stabilizing interactions betwee n the filled d-orbital manifold and the b(2) orbital of the vinylidene fragment.