Bonding Behavior of Co(CO)3L (L = CO, PPh3) Building Blocks in Platinum−Cobalt Carbonyl Clusters

The reaction of [Co(CO)4]- with [Pt2Cl(μ-PPh2)(PPh3)3] (1:1 ratio), which was prepared in situ by reaction of aqueous HCl with the orthometalated complex [Pt2(μ-PPh2)(μ-o-C6H4PPh2)(PPh3)2], afforded the deep green triangular cluster [Pt2Co(μ-PPh2)(CO)4(PPh3)2], 7. X-ray crystallographic analysis reveals that this new cluster contains two formally monoanionic fragments, PPh2 - and Co(CO)4 -, that bridge a d9−d9 Pt(I)−Pt(I) metal−metal bond. Whereas tetracarbonylcobaltate is generally bonded to only one metal center as a 2e donor ligand, it is best viewed here as a formally 4e donor anionic metalloligand. This model leads then to the usual 16e count for each Pt center and relates this metalloligand to other anionic bridging ligands, such as PPh2 -. A comparative EHMO bonding analysis of [Co(CO)3L]- (L = CO, PR3) fragments is presented which takes into account the possible coordination geometries about cobalt, trigonal bipyramidal (with C 3 v local symmetry) or edge-capped tetrahedral when referring to the ligand polyhedron (with C 2 v or C s symmetry). The results support the description of this unusual bridging bonding mode.