Cp*3Co3(μ3-CCH 3)(μ3-H) (3) reacted with CO to form the 1:1 carbonyl ethylidyne adduct Cp*3Co3(μ3-CCH3)-(μ 3-CO)(μ2-H) (5) in 82% yield. Similarly, tert-butyl isocyanide reacted with 3 to form the 1:1 isocyanide ethylidyne adduct Cp*3Co3(μ3-CCH3)(μ 3-CNCMe3)(μ2-H) (6) in 87% yield. The reaction of 3 with NO formed the nitrosyl ethylidyne cluster Cp*3Co3(μ3-CCH3)(μ 3-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 μ3-CCH3 ligand and capped on the opposite face by a μ3-CO (5), μ3-CNCMe3 (6), or μ3-NO (7) ligand. Line shape analysis of the Cp* peaks in the variable-temperature 1H 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 [ΔG‡ = 10.3(1) kcal mol-1] and 6 [ΔG‡ = 12.2(2) kcal mol-1]. © 1994 American Chemical Society.