High-yield syntheses, under mild conditions, of various neutral and anionic ruthenium and osmium carbonyl clusters by controlled reduction in ethylene glycol of [M(CO)(3)Cl-2](2) or MCl3 (M=Ru, Os) in the presence of Na2CO3 or K2CO3

[M3(CO)12], [H4M4(CO)12], [H3M4(CO)12]- (M=Ru, Os) and [Ru6C(CO)16]2- have been synthesized in high yields by one-pot controlled reduction at atmospheric pressure of MCl3 or [M(CO)3Cl2]2, dissolved in t-amyl alcohol or ethylene glycol, working in the presence of alkali carbonates. The selectivity of the reduction is controlled by the: (i) nature and quantity of the alkali carbonate (Na2CO3 or K2CO3); (ii) nature of the solvent; (iii) gas-phase composition (CO or CO+H2); (iv) temperature and (v) reaction time. Yields are so high and reaction conditions so mild that, in most cases, these new syntheses are more convenient than those previously reported either in solution or on the silica surface as reaction medium.