The stoichiometries of high-nuclerity transition metal Carbonyl cluster compounds. 1. Compact close-packed clusters

Crystal of [Ir4Br(CO)11] (PPh4) are orthorhombic, space group P212121, with a 13.276(3), b 18.347(4), c 16.041(4) Å. The structure has been elucidated by the analysis of 2876 observed intensities recorded on an automatic diffractometer, and refined by the least-squares method to R 0.043. The anion contains a tetrahedral cluster of iridium atoms (mean IrIr 2.710 Å). The carbonyl arrangement differs from that of the parent Ir4(CO)12, and is similar to that known for Co4(CO)12 and Rh4(CO)12, with one terminal CO group in the basal M3(CO)9 moiety replaced by the bromide ligand; two of the bridging CO groups become markedly assymetric.