The title compounds can be conveniently prepared by this reaction, for which a plausible mechanism can be suggested. [GRAPHICS] INF>Cl-2 at room temperature results in reduction of the unique carbonyl to a methyl group, giving the ethylidyne cluster (mu-H)(3)Os-3(CO)(9)(mu(3)-CCH3). A single-crystal X-ray structure determination verified the formation of (mu-H)(3)Os-3(CO)(9)(mu(3)-CCH3). No evidence was observed for the formation of intermediate species, such as the vinylidene cluster (mu-H)(2)Os-3-(CO)(9)(mu(3)-eta(2)-C=CH2). Treatment of (mu-H)(2)Os-3-(CO)(9)(mu(3)-eta(2)-C=CH2) with pure, freshly prepared BH3 . THF for 5 days in CH2Cl2 yielded no (mu-H)(3)Os-3-(CO)(9)(mu(3)-CCH3). ?69