SYNTHESIS, STRUCTURE, AND NONRIGIDITY OF OS-4(CO)(15), OS-4(CO)(13)(PME(3))[P(OME)(3)], AND OS-4(CO)(14)(CNBU(T))

The binary carbonyl Os-4(CO)(15), 1, has been prepared by the addition of Os(CO)(5) to Os-3(CO)(10)-(cyclooctene)(2) at -15 degrees C. The r elated clusters Os-4(CO)(13)(PMe(3))[P(OMe)(3)], 2, and Os-4(CO)(14)(C NBu(t)), 3, have been synthesized from Os-4(CO)(13)(PMe(3)) and Os-4(C O)(15)(CNBu(t)), respectively. The crystal structures of 1, 2, and 3 r eveal similar planar metal skeletons with short (2.774 (1) -2.793 (2) Angstrom) and long (2.978 (2) -3.019 (2) Angstrom) peripheral Os-Os bo nds; the hinge Os-Os bond in 1-3 ranges in length from 2.936 (2) to 2. 948 (1) Angstrom. The variable temperature C-13 nuclear magnetic reson ance spectra of 1 and 3 show that both are highly nonrigid in solution even at -120 degrees C. The mechanism of nonrigidity is believed to b e an all-equatorial, merry-go-round carbonyl exchange. The variable te mperature C-13 nmr spectra of 2 indicate it is rigid on the nmr time s cale in solution at -45 degrees C. Carbonyl exchange is, however, obse rved in the spectrum at -6 degrees C. From the mode of collapse of the signals it is believed that the lowest energy exchange processes in 3 involve axial-equatorial, merry-go-round CO exchanges in the two plan es that each contain a short Os-Os bond. Crystallographic data for com pound 1: space group C2/c; a = 12.802 (3) Angstrom, b = 10.217 (3) Ang strom, c = 16.380 (5) Angstrom, beta = 91.39 (2)degrees; R = 0.044, 12 04 observed reflections. For compound 2: space group P2(1)/c; a = 11.1 06 (7) Angstrom, b = 16.931 (5) Angstrom, c = 16.481 (5) Angstrom, bet a = 97.71 (5)degrees; R = 0.051, 2117 observed reflections. For compou nd 3: space group P2(1)/n; a = 11.747 (3) Angstrom, b = 18.009 (5) Ang strom, c = 12.448 (2) Angstrom, beta = 92.65 (2)degrees; R = 0.054, 21 31 observed reflections.