Reactions of the tetrahedral clusters MoIr3(mu-CO)(3)(CO)(8)(eta-L) (L = C5HMe4, C5Me5) with the carbonylmetalate anions [Mo(CO)3(eta-L)](-) afford the trigonal bipyramidal clusters Mo2Ir3(mu(3)-H)(mu-CO)(2)(CO)(9)(mu-L)(2) (L = C5HMe4 (3c), 74%; L = C5Me5 (3d), 55%) in which the group 6 metal atoms occupy the apexes; reaction of the cyclopentadienylmolybdenum-containing analogues or their cyclopentadienyltungsten-containing homologues failed to afford analogous products. Reactions of MIr3(mu-CO)(3)(CO)(8)(eta-C5H5) (M = Mo, W) with [M(CO)(3)(eta-L)](-) (L = C5HMe4, C5Me5) afford the core-expanded heteroapex clusters M2Ir3(mu(3)-H)(mu-CO)(2)(CO)(9)(eta-C5H5)(eta-L) (M = Mo, L = C5HMe4 (5c), 9%, L = C5Me5 (5d), 4%; M = W, L = C5Me5 (6d), 5%) in low yield, together with the homoapex clusters M2Ir3(mu(3)-H)(mu-CO)(2)(CO)(9)(eta-L)(2) (M = Mo, L = C5HMe4 (3c), 81%, L = C5Me5 (3d), 60%; M = W, L = C5Me5 (4d), 5%) in much higher yield for the Mo-containing examples. The identities of clusters 3c, d, 4d, and 5c, d have been confirmed by single-crystal X-ray diffraction studies, with the same disposition of ligands about the trigonal bipyramidal cluster cores being observed in each case, a ligand arrangement that has been examined by complementary density functional theory studies. While cluster 5d is accessible as above, no reaction is observed from MoIr3(mu-CO)(3)(CO)(8)(eta-C5Me5) and [M(CO)(3)(eta-C5H5)](-). Treating MoIr3(mu-CO)(3)(CO)(8)(eta-C5H5) with 1 equiv of [M(CO)(3)(eta-C5Me5)](-) affords 5d as the major product, a further 1 equiv affording some MoIr3(mu-CO)(3)(CO)(8)(eta-C5Me5) and a third 1 equiv giving a good yield of 3d. This is consistent with reaction proceeding by apex fragment addition, followed by apex fragment elimination, and finally a further apex fragment addition, the homometallic incoming apexes being distinguished from the departing vertices by their highly methylated cyclopentadienyl ligands. Spectroscopic data suggest that the electron density at these disparate-metal-containing cluster cores is tunable by progressive ( conceptual) cyclopentadienyl alkylation.