72-76 Valence electron clusters: an extension of the 16-18 electron rule: synthesis, crystal structure, and solution dynamics of the trigonal-bipyramidal pentanuclear clusters [PtIr4(CO)9(.mu.-CO)5]2- and [PtIr4(CO)9(.mu.-CO)3]2-

The 76 CVE (cluster valence electron) species [PtIr4(CO)14]2- (l), obtained by the reductive carbonylation of a mixture of Na2PtC16 and IrC13.xH20 or Ir4(CO)12, is in equilibrium with the 72 CVE species [PtIr4(CO)12]2-(2) through the dissociation-association of two carbonyl ligands. Both anions were characterized by the single-crystal X-ray diffraction. Both anions have a trigonal-bipyramidal frame, the Pt atom in 1 being equatorial and in 2 apical. The ligand disposition on the apical metals, considering an additional ideal direction as that pointing toward the central triangle, appears trigonal bipyramidal in 1 and square planar in 2, representative of saturated 18-electron and unsaturated 16-electron configurations, respectively. 13C and 195Pt NMR showed temperature dependent fluxional behavior of the CO's in both anions.