The reaction between the tetrahedral cluster Co2Rh2(CO)12 (1) and a 2-fold excess of dimethyl acetylenedicarboxylate (dmad) proceeds rapidly at room temperature to give alkyne-substituted clusters Co3Rh(CO)10(μ-dmad) (3) and CoRh3(CO)9(μ-dmad)3 (4) as the major products in good yield. Cluster 3 was characterized in solution by IR and 1H NMR spectroscopies and by comparison to an independently prepared sample of 3 from the reaction of dmad with the known tetrahedral cluster Co3Rh(CO)12 (2). Cluster 4 was characterized by IR and NMR (1H and 13C) spectroscopies and FAB mass spectrometry, and the solid-state structure was established by X-ray diffraction analysis. The structure of CoRh3(CO)9(μ-dmad)3 consists of a square planar array of metals where two of the three dmad ligands cap both of the CoRh3 faces and the remaining dmad ligand bridges one of the Rh−Rh edges. The nine ancillary CO groups are all terminally bound, with one Co(CO)3 and three Rh(CO)2 moieties structurally found. The facile cluster fragmentation/metal redistribution observed upon reaction with dmad is briefly discussed relative to other alkyne ligands that react with cluster 1 to furnish the arachno clusters Co2Rh2(CO)10(μ4-alkyne).