Bonding behavior of Co(CO)(3)L (L = CO, PPh(3)) building blocks in platinum-cobalt carbonyl clusters

The reaction of [Co(CO)(4)](-) with [Pt(2)Cl(mu-PPh(2))(PPh(3))(3)] (1:1 ratio), which was prepared in situ by reaction of aqueous HCl with the orthometalated complex [Pt(2)(mu-PPh(2))(mu-o-C(6)H(4)PPh(2))(PPh(3))(2)], afforded the deep green triangular cluster [Pt(2)Co(mu-PPh(2))(CO)(4)(PPh(3))(2)], 7. X-ray crystallographic analysis reveals that this new cluster contains two formally monoanionic fragments, PPh(2-) and Co(CO)(4-), that bridge a d(9)-d(9) 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 PPh(2-). A comparative EHMO bonding analysis of [Co(CO)(3)L](-) (L = CO, PR(3)) fragments is presented which takes into account the possible coordination geometries about cobalt, trigonal bipyramidal (with C(3v) local symmetry) or edge-capped tetrahedral when referring to the ligand polyhedron (with C(2v) or C(s) symmetry). The results support the description of this unusual bridging bonding mode.