SURFACE-MEDIATED ORGANOMETALLIC SYNTHESIS - HIGH-YIELD PREPARATIONS OF NEUTRAL AND ANIONIC OSMIUM CARBONYL CLUSTERS BY CONTROLLED REDUCTIONOF SILICA-SUPPORTED [OS(CO)(3)CL-2](2) AND OSCL3 IN THE PRESENCE OF NA2CO3 OR K2CO3

[Os-3(CO)(12)], [H4Os4(CO)(12)], [H2Os4(CO)(12)](2-), [Os5C(CO)(14)](2 -), and [Os10C(CO)(24)](2-) have been synthesized selectively and in h igh yields by reductive carbonylation or hydrogenation of OsCl3 or alp ha-[Os(CO)(3)Cl-2](2) supported on silica in the presence of alkali ca rbonates. The selectivity of the reaction is controlled by the choice of (i) the nature and quantity of the alkali carbonate (Na2CO3 or K2CO 3) added to silica, (ii) temperature, (iii) reaction time, and (iv) th e gas-phase composition (CO, CO + H-2, or H-2). These surface-mediated syntheses are often more selective and more efficient and usually req uire less drastic conditions than the best known syntheses in solution , confirming the potential use of the silica surface as a new reaction medium to prepare both neutral and anionic metal carbonyl clusters.