Surface-Mediated Organometallic Synthesis:  High-Yield Preparations of Neutral and Anionic Osmium Carbonyl Clusters by Controlled Reduction of Silica-Supported [Os(CO)₃Cl₂]₂and OsCl₃in the Presence of Na₂CO₃or K₂CO₃

[Os(3)(CO)(12)], [H(4)Os(4)(CO)(12)], [H(2)Os(4)(CO)(12)](2)(-), [Os(5)C(CO)(14)](2)(-), and [Os(10)C(CO)(24)](2)(-) have been synthesized selectively and in high yields by reductive carbonylation or hydrogenation of OsCl(3) or alpha-[Os(CO)(3)Cl(2)](2) supported on silica in the presence of alkali carbonates. The selectivity of the reaction is controlled by the choice of (i) the nature and quantity of the alkali carbonate (Na(2)CO(3) or K(2)CO(3)) added to silica, (ii) temperature, (iii) reaction time, and (iv) the gas-phase composition (CO, CO + H(2), or H(2)). These surface-mediated syntheses are often more selective and more efficient and usually require 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.