MIXED RHODIUM-OSMIUM CARBONYL CLUSTERS - SYNTHESIS OF THE ANIONS [OSRH5(CO)16]- AND [OSRH3(CO)12]- AND CRYSTAL-STRUCTURES OF THEIR (MU-NITRIDO)BIS(TRIPHENYLPHOSPHORUS)(1+) SALTS

The two new anions [OsRh5(CO)16]- (1) and [OsRh3(CO)12]- (2) have been obtained, in a 1:1 mixture, by acidification of [OsRh4(CO)15]2-. Alternative synthetic routes have been devised: 1 may be also produced by condensation of Rh(I) derivatives on [OsRh4(CO)15]2- or, more conveniently, with a one-pot synthesis, starting from Os3(CO)12 and Rh4(CO)12; 2 may be obtained by oxidative demolition of [OsRh4(CO)15]2- with iodine. Both anions were characterized by single-crystal X-ray diffraction of their [(Ph3P)2N]+ (PPN) salts. (PPN)[OsRh5(CO)16]: space group C2/c, Z = 8, a = 33.665 (4) angstrom, b = 9.752 (1) angstrom, c = 33.785 (5) angstrom, beta = 94.45 (1)degrees, R = 0.030, and R(w) = 0.031 for 5166 independent reflections with I > 3-sigma(I). (PPN)[OsRh3(CO)12]: Space group P2(1)/c, Z = 4, a = 14.707 (3) angstrom, b = 21.371 (6) angstrom, c = 16.095 (3) angstrom, beta = 102.83 (2)degrees, R = 0.037, and R(w) = 0.037 for 3007 independent reflections with I > 3-sigma(I). Anion 1 shows a Rh6(CO)16-like structure with 12 terminal carbonyls (2 on each metal) and 4 triply bridging on the alternate faces of the octahedral skeleton. Anion 2 has a tetrahedral metal frame with an unprecedented ligand disposition, 7 terminal and 5 bridging carbonyls with overall C(s) ideal symmetry. C-13 NMR spectra in the temperature range 323-183 K evidenced a fluxional behavior in 2, from complete CO scrambling to a static situation consistent with the solid-state structure; compound 1 was found to be static at room temperature.