SURFACE ORGANOMETALLIC CHEMISTRY - UNDERSTANDING THE MULTISTEP PROCESS OF SILICA-MEDIATED SYNTHESIS OF VARIOUS OSMIUM CARBONYL CLUSTERS FROM SUPPORTED ALPHA-[OS(CO)(3)CL-2](2)

Reaction of silica-supported alpha-[Os(CO)(3)Cl-2](2) in the presence of alkali-metal carbonates affords reactive surface osmium(II) species . The nature of the latter depends on the basicity given to the silica surface, with K2CO3 behaving as a stronger base than Na2CO3 when supp orted on silica. Infrared evidence suggests that with a low basicity ( for instance, molar ratio Na2CO3:Os = 2:1), surface species such as [O s(CO)(3)(OR)(2)](n) (R = H, Si<right attachments>) are initially forme d; an increase of the surface basicity (molar ratio (Na2CO3 or K2CO3): Os = (10-20):1) leads to the formation of probably anionic ([Os(CO)(3) (OR)(2)](m)(OR))(-) (R = H, Si drop; m > 1) entities up to the less re active species [Os(CO)(3)(OH)(3)](-). The high reactivity of these sur face species is confirmed by the controlled reduction by CO or Hp Of s ilica-supported [Os(CO)(3)(OH2)2](n) in the presence of alkali-metal c arbonates, which leads selectively to either neutral ([OS3(CO)(12)], [ H4Os4(CO12]) or anionic ([H3Os4(CO)(12)](-), [Os10C(CO)(24)](2-)) clus ters, in accord with results obtained with supported alpha-[Os(CO)(3)C l-2](2). There is direct and indirect evidence that the aggregation pr ocess occurs via silica-anchored [HOs3(CO)(10)(OSi drop)] or silica-su pported [HOs3(CO)(10)(OH)] species, followed by further condensation t o [H4Os4(CO)(12)] or [H3Os4(CO)(12)]- according to the basicity of the surface. The nature and the quantity of added alkali carbonate (Na2CO 3 or K2CO(3)), together with the temperature, influence the formation of either [H3Os4(CO)(12)](-) or [H2Os4(CO)(12)](2-), which can act as intermediates for further condensation to cluster anions of higher nuc learity. In addition to these reaction parameters, the amount of H-2 i n the gas phase is also crucial in defining the relative stability and the reactivity of the surface species [H3Os4(CO)(12)](-) and [H2Os4(C O)(12)](2-) and their further condensation to specific carbonyl cluste r anions.