Abstract Me 3NO activation of the tetrairidium cluster Ir 4(CO) 12 ( 1) in presence of the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) furnishes the bpcd-substituted clusters Ir 4(CO) 10(bpcd) ( 3) and Ir 4(CO) 8(bpcd) 2 ( 4) as the minor and major products, respectively. Cluster 3 has been isolated as the sole observable product from the reaction of [Ir 4(CO) 11Br][Et 4N] ( 2) with bpcd in presence of AgBF 4 at room temperature. Both 3 and 4 have been isolated and fully characterized in solution by spectroscopic methods. The solid-state structure of 3 reveals that the ancillary bpcd ligand is bound to a single iridium center, with chelating and bridging bpcd ligands found in the X-ray structure of cluster 4. Cluster 4 is unstable at room temperature and slowly loses CO to afford the hydride-bridged cluster HIr 4(CO) 4(μ-CO) 3(bpcd)[μ-PhP(C 6H 4)C C(PPh 2)C(O)CH 2C(O)] ( 5). Cluster 5 has been fully characterized in solution by IR and NMR spectroscopies, and the C–H bond activation attendant in the ortho metalation step is shown to occur regioselectively at one of the aryl groups associated with the bridging bpcd ligand. The redox properties of clusters 3– 5 have been explored and the electrochemical behavior discussed with respect to extended Hückel MO calculations and related diphosphine-substituted cluster compounds prepared by our groups.