Cp*3Co3(mu3-CCH3)(mu3-H) (3) reacted with CO to form the 1:1 carbonyl ethylidyne adduct Cp*3Co3(mu3-CCH3)(mu3-CO)(mu2-H) (5) in 82% yield. Similarly, tert-butyl isocyanide reacted with 3 to form the 1:1 isocyanide ethylidyne adduct Cp*3Co3(mu3-CCH3)(mu3-CNCMe3)(mu2-H) (6) in 87% yield. The reaction of 3 with NO formed the nitrosyl ethylidyne cluster Cp*3Co3(mu3-CCH3)(mu3-NO) (7) in 84% yield. The structures of clusters 5-7 were determined by X-ray crystallography. Each cluster consists of an equilateral triangle of cobalt atoms symmetrically capped on one face by a mu3-CCH3 ligand and capped on the opposite face by a mu3-CO (5), mu3-CNCMe3 (6), or mu3-NO (7) ligand. Line shape analysis of the Cp* peaks in the variable-temperature H-1 NMR spectra of 5 and 6 allowed measurement of the barrier for migration of the hydride ligand between adjacent pairs of cobalt atoms in 5 [DELTAG(double dagger) = 10.3(1) kcal mol-1] and 6 [DELTAG(double dagger) = 12.2(2) kcal mol-1].