The paramagnetic tetrahydride cluster Cp*3Co3(μ2-H)3(μ3-H) (1) reacts with acetylene to ultimately form the bis(ethylidyne) cluster Cp*3Co3(μ3-CCH3)2 (2). Two kinetically formed intermediates were isolated from the reaction: the diamagnetic mono(ethylidyne) trihydride cluster Cp*3Co3(μ2-H)3(μ3-CCH3) (3) and the paramagnetic mono(ethylidyne) monohydride cluster Cp*3Co3(μ-H)(μ3-CCH3) (4). Cluster 3 loses H2 at 80 °C [ΔG* = 29.5(1) kcal mol-1] to generate 4, and 4 reacts with H2 at 80 °C [ΔG* = 23.8(1) kcal mol-1] to regenerate 3. A third intermediate in the reaction of 1 with acetylene, the bridging acetylene dihydride cluster Cp*3Co3(μ-H)2(μ-η2-HCCH) (5), was detected by 1H NMR spectroscopy at 23 °C and rearranges to form exclusively cluster 4. Cluster 1 reacts with DC≡CD at 80 °C to form Cp*3Co3(μ2-H)2(μ2-D)(μ3-CCH2D) (3-CH2D) as the exclusive diamagnetic product. The formation of 3-CH2D is consistent with the presence of the ethylidene dihydride intermediate Cp*3-Co3(μ2-H)2(μ2-CDCH2D) (III). © 1995, American Chemical Society. All rights reserved.