Metal cluster core expansion at tetrahedral group 6-group 9 mixed-metal clusters MIr3(mu-CO)(3)(CO)(8)(eta(5))-L) (M = W, Mo, L = C5H5; M = Mo, L = C5Me5) with the iridium capping reagents Ir(CO)(2)(eta(5))-L') (L' = C5Me5, C5Me4H) in refluxing toluene afforded the trigonal-bipyramidal clusters MIr4(mu-CO)(3)(CO)(7)(eta(5))-C5H5)(eta(5))-L') (M = Mo, L' = C5Me5, 1a; M = W, L' = C5Me5, 1b; M = Mo, L' = C5Me4H, 1c; M = W, L' = C5Me4H, 1d) and MoIr4(mu(3)-H)(mu-CO)(2)(mu-eta(1):eta(5))-CH2C5Me4)(CO)(7)(eta(5))-C5Me5) (2). Related reactions with M2Ir2(mu-CO)(3)(CO)7(eta(5))-L)(2) (M = W, Mo, L = C5H5; M = Mo, L = C5Me5) afforded M2Ir3(mu-CO)(3)(CO)(6)(eta(5))-C5H5)(2)(eta(5))-L') (M = Mo, L' = C5Me5, 3a; M = W, L' = C5Me5, 3b; M = Mo, L' = C5Me4H, 3c; M = W, L' = C5Me4H, 3d), W2Ir3(mu-CO)(4)(CO)(5)(eta(5))-C5H5)(2)(eta(5))-C5Me4H) (4), and Mo2Ir3(mu-CO)(3)(CO)(6)(eta(5))-C5Me5)(3) (5). Single-crystal X-ray diffraction studies of 1a-1d, 2, 3a-3d, and 4 confirmed their molecular structures, including the mu-eta(1)):eta(5)-CH2C5Me4 ligand at hydrido cluster 2, derived from a C-H bond activation of one of the methyl groups. Density functional theory (DFT) studies were employed to suggest the structure of 5. The redox behavior of the new clusters was examined through cyclic voltammetry; all clusters exhibit oxidation and reduction processes (with respect to the resting state), with the oxidation processes being the more reversible, and increasingly so on decreasing Ir content of the clusters, replacing W by Mo, and increasing alkylation of the cyclopentadienyl ligands. In situ IR and UV-vis-near-IR spectroelectrochemical studies of the reversible oxidation processes in 1a and 3a were undertaken, with the spectra of the former suggesting progression to an all-terminal CO geometry concomitant with the first oxidation and a significant structural change upon the second oxidation step. DFT studies of 1a revealed that its crystallographically-confirmed Mo-equatorial core geometry is essentially isoenergetic with a possible Mo-apical isomer, and identified several bridging CO structures for the charged states.