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(μ-CO)5]2- and [PtIr4(CO)9(μ-CO)3]2

The 76 CVE (cluster valence electron) species [PtIr4(CO)14]2- (1), obtained by the reductive carbonylation of a mixture of Na2PtCl6 and IrCl3·xH2O 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 of their respective [PPh4]+ and [NEt4]+ salts. [PPh4]2[PtIr4(CO)14] crystallizes in the space group P1 with Z = 2, a = 13.868 (3) Å, b = 21.997 (5) Å, c = 10.804 (2) Å, α = 102.51 (2)°, β = 106.29 (2)°, γ = 98.02 (2)°. Some [Ir6(CO)15]2- (ca. 10%) was found cocrystallized in the cell. The structure was solved and refined to R = 0.033 and Rw = 0.039 for 6545 independent reflections with I > 3σ(I). [NEt4]2[PtIr4(CO)12] crystallizes in the orthorhombic space group Pnma (nonstandard setting of Pnam, n.62) with Z = 12, a = 15.453 (5) Å, b = 22.340 (5) Å, c = 33.422 (8) Å. The structure was solved and refined to R = 0.056 and Rw = 0.057 for 1579 independent reflections with I > 3σ(I). Both anions have a trigonal-bipyramidal frame, the Pt atom in 1 being equatorial and in 2 apical. In 1, where the ideal symmetry is C2, nine carbonyls are terminally bonded (three on each apical Ir atom and one on each of the other metals), and five span the equatorial metal triangle and two opposite Ireq - Irap edges. In 2, with ideal Cs symmetry, nine carbonyls are terminally bonded (two on each of the Ir atoms and one on Pt), while the remaining three span one Ireq-Irap and two Ireq-Pt edges. 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 COs in both anions. © 1989 American Chemical Society.