SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF THE OSMIUM CLUSTER DIANIONS [OS17(CO)36]2- AND [OS20(CO)40]2-

The cluster dianions [Os17(CO)36]2- and [Os20(CO)40]2-, the largest osmium carbonyl clusters isolated to date, have been obtained by vacuum pyrolysis of [Os3(CO)10(NCMe)2] at temperatures above 260-degrees-C and initial pressures of ca. 10(-3) Torr. A single-crystal X-ray structure analysis of [AsPh4]2[Os17(CO)36] has established a trigonal-bipyramidal core of 14 osmium atoms which is capped by three additional Os atoms. Carbon-13 NMR studies indicate a high degree of fluxionality in the cluster. The crystal structure analysis of [PBu(n)4]2[Os20(CO)40] has revealed a tetrahedral cubic close-packed metal core of twenty osmium atoms which represents the first example of a totally symmetrical array of metal atoms of this nuclearity, a geometrical arrangement previously proposed for both bare and ligated eicosanuclear metal clusters. The structure of the latter bears close resemblance to the chemisorption of CO on metal (111) surfaces at high density. An interpretation of some aspects of its redox chemical behaviour on the basis of the electronic structure of the compound is given.