The paramagnetic tetrahydride cluster Cp*Co-3(3)(mu(2)-H)(3)(mu(3)-H) (1) reacts with acetylene to ultimately form the bis(ethylidyne) cluster Cp*Co-3(3)(mu(3)-CCH3)(2) (2). Two kinetically formed intermediates were isolated from the reaction : the diamagnetic mono(ethylidyne) trihydride cluster Cp*Co-3(3)(mu(2)-H)(3)(mu(3)-CCH3) (3) and the paramagnetic mono(ethylidyne) monohydride cluster Cp*Co-3(3)(mu(3)-H)(mu(3)-CCH3) (4) Cluster 3 loses H-2 at 80 degrees C [Delta G(double dagger) = 29.5(1) kcal mol(-1)] to generate 4, and 4 reacts with H-2 at 80 degrees C [Delta G(double dagger) = 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*Co-3(3)(mu-H)(2)(mu(3)-eta(2)-HCCH) (5), was detected by H-1 NMR spectroscopy at 23 degrees C and rearranges to form exclusively cluster 4. Cluster 1 reacts with DC=CD at 80 degrees C to form Cp*Co-3(3)(mu(2)-H)(2)(mu(2)-D)(mu(3)-CCH2D) (3-CH2D) as the exclusive diamagnetic product. The formation of 3-CH2D is consistent with the presence of the ethylidene dihydride intermediate Cp*Co-3(3)(mu(2)-H)(2)(mu(2)-CDCH2D) (III).