SYNTHESIS AND CHEMICAL BEHAVIOR OF THE [CO3NI7(CO)16C2]2- AND [CO3NI7(CO)15C2]3- DICARBIDE CLUSTERS - X-RAY CRYSTAL-STRUCTURE OF [PPH4]2[CO3NI7(CO)16C2]

The synthesis and properties of the new dicarbidocarbonyl bimetallic clusters [Co3Ni7(CO)16C2]2- (I), [Co3Ni7(CO)16C2]3- (II), and [Co3Ni7(CO)15C2]3- (III), are described. Compound I, which is paramagnetic, has been synthesized in high yields by redox-condensation of Co3(CO)9CCl with [Ni9(CO)17C]2-, whereas the diamagnetic trianions II and III have been respectively obtained by reduction of I, with sodium metal in THF and sodium hydroxide in methanol. The anion I has been isolated in a crystalline state in association with several tetrasubstituted ammonium or phosphonium cations, which are characterized by elemental analysis and by a single-crystal X-ray diffraction study, of the tetraphenylphosphonium salt. The anion I has a metal frame based on a 3,4,3-C2h stack of metal atoms, which may be regarded as derived by condensation of two either octahedral or trigonal-prismatic moieties. The resulting deca-vertices metal polyhedron encapsulates a C2 fragment showing a short CC interatomic separation of 1.48 Å. The three cobalt atoms cannot be distinguished from the remaining seven nickel atoms and are probably randomly distributed over the ten vertices. The M-M distances are scattered over the range 2.34-2.80 Å, and each carbide carbon is encapsulated in a seven-vertices cage which may be described as a distorted capped trigonal prism. The carbonyl stereochemistry comprises six carbonyl groups terminally bonded to the atoms of the top and bottom triangular layers, and ten edge-bridging CO which span the ten inter-layer edges. The structure of this decanuclear dicarbide cluster is compared with those reported for related species and a rationalization is offered for the variations of the metal geometry and the CC interatomic separation.