The results of Fenske-Hall molecular orbital calculations for cubane-type clusters having Mo3M'S-4 cores show that the electronic structures of the clusters depend on the nature of M'. When M' is a main group metal, as in [Mo-3(SnCl3)S-4(NCS)(9)](6-), Mo3SnS4(S2PEt2)(6), or [Sn(Mo3S4(H2O)(9))(2)](8+) the heterometal is oxidized upon incorporation into the cluster; no M'-Mo bonds are formed, and electrons are transferred from M' to an orbital. localized on the Mo3S4 incomplete cubane core. When M' is a transition metal, as in [Mo3NiS4(CO) (H2O)(9)](4+), [Mo3PdS4(CO) (tacn)(3)](4+) (tacn = 1,4,7-triazacyclononane), or Mo3CoS4(CO)(Cp')(3) (Cp' = methylcyclopentadiene), M' is not oxidized but instead shares electron density with the Mo3S4 core through the formation of metal-metal bonds with the Mo-3 triangle. The relatively high stretching frequencies observed for CO Ligands bound to the Ni and Pd centers in the Mo3NiS4 and Mo3PdS4 clusters arise from the nature of the bonding in the clusters, not from the oxidation of the Ni and Pd atoms. Since the same heterometal orbitals are used both to form the M'-Mo bonds and to back-donate to the CO ligand, the Mo-3 orbitals and the CO pi* orbitals compte for M' electron density. The CO orbitals do not compete effectively for metal electron density in the Ni and Pd clusters, and this results in weak back-donation to the CO pi* orbitals and relatively high CO stretching frequencies. Although it has been proposed that the Mo3NiS4 cluster may serve as a model for NiMoS hydrodesulfurization (HDS) catalysts, the fact that the Ni center in this cluster is not electron rich suggests that it may not provide a suitable model. The electron density at the heterometal can be increased slightly by increasing the donor ability of the ligands attached to the Mo atoms.