SYNTHESIS AND EXPERIMENTAL THEORETICAL INVESTIGATION OF THE HIGH-NUCLEARITY CUBIC T-D[AU6NI12(CO)(24)](2-) CLUSTER, AN INITIAL EXAMPLE OF ADISCRETE GOLD NICKEL BIMETALLIC-BONDED SPECIES - COMPARATIVE-ANALYSISOF THE RESULTS OF ELECTRON-COUNTING METHODS AND THE FENSKE-HALL MO MODEL IN RATIONALIZING THE BONDING INTERACTIONS OF ITS AU6NI12 CORE CONSISTING OF 5 FACE-FUSED METAL OCTAHEDRA

Reactions of the [Ni-6(CO)(12)](2-) dianion with Ph(3)PAuCl have led t o the isolation and crystallographic/IR/electrochemical characterizati on of the [Au6Ni12(CO)(24)](2-) dianion (as the [PPh(3)Me](+) salt). I ts idealized T-d configuration consists of a central Au-6 octahedron w hich is tetrahedrally linked to four triangular Ni-3(CO)(3)(mu(2)-CO)( 3) ligands by the trigonal-antiprismatic (pseudooctahedral) capping of the four Ni-3 triangles on four alternate Au-6 triangular faces. The resulting structurally unprecedented Au6Ni12 core may be envisioned ei ther as five face-fused octahedra or as the composite of four Au3Ni3 o ctahedra joined by the vertex-sharing of each of the six gold atoms be tween two adjacent Au3Ni3 octahedra. This 18-vertex metal cluster, the first known discrete Au-Ni bimetallic-bonded species, was obtained in low yields (<5%) by the remarkable, unexpected cleavage of the Ph(3)P ligand from each gold atom along with concomitant metal-metal condens ation to give the Au6Ni12 framework; each Ni-3(CO)(3)(mu(2)-CO)(3) fra gment retains the pseudo-C-3v architecture found in the two Ni-3(CO)(3 )(mu(2)-CO)(3) moieties comprising the trigonal-antiprismatic [Ni-6(CO )(12)](2-) precursor. A theoretical bonding analysis of the [Au6Ni12(C O)(24)](2-) dianion was performed via the parameter-free Fenske-Hall M O method in order to elucidate the nature of the Au-Ni orbital bonding interactions. It was found that the pi-accepting pi CO ligands withi n the four Ni-3(CO)(3)(mu(2)-CO)(3) fragments play a crucial role in t he formation of this cluster by greatly lowering the energies of the a tomic 4p Ni AOs and thereby providing a favorable energy-matching in t he orbital overlap of the out-of-plane 4p(z) Ni AOs with primarily the 6s Au AOs In the central Au-6 octahedron. Four filled, multicenter, t wo-electron frontier MOs, the a(1) SHOMO (second highest occupied MO) and three t(2) HOMOs under T-d symmetry, provide the essential 'glue' (in addition to presumed relativistic bonding effects due to strongly enhanced 6s-5d gold hybridization) for holding this Au-Ni carbonyl clu ster together; each triply degenerate t(2) HOMO (47% Au, 25% Ni, 28% p i CO) has large 6s Au-4p(z), Ni AO bonding interactions, while the to tally symmetric a(1) SHOMO (67% Au, 25% Ni, 8% pi CO) has both 6s Au- 6s Au and 6s Au-4p(z), Ni AO bonding interactions. In order to reconci le this bonding description obtained from the Fenske-Hall MO method wi th that obtained from several electron-counting schemes, a qualitative delocalized interoctahedral bonding model is proposed involving four S-sigma core-bonding electron pairs for the five face-fused octahedra of this cluster. It is also concluded that this Au6Ni12 cluster furnis hes a prime illustration that electron-counting procedures (although g enerally invaluable in successfully correlating the geometries of smal l-to-moderately large transition metal dusters to their observed numbe rs of cluster valence electrons) do not necessarily provide physically meaningful electronic models for the actual bonding in transition met al clusters.