The homoleptic mono- and multinuclear carbonyls for Mo, Tc, Ru, and Rh, namely, Mo(CO)6, Ru(CO)5, Tc2(CO)10, Ru3(CO)12, Rh4(CO)12, and Rh6(CO)16, are investigated theoretically by the Hartree-Fock method and three density functional theory (DFT) methods, i.e., BP86, B3LYP, and MPW1PW91, along with the SDD ECP basis sets. The results predicted by all the methods are basically in agreement with each other. The MPW1PW91 and BP86 methods predict geometric parameters and vibrational spectra, respectively, closest to the experimental values. For Ru3(CO)12 the relative energies of the D3h isomer with only terminal CO groups and the C2v isomer with two bridging CO groups are within 3 kcal/mol of each other with the lower energy isomer depending upon the computational method used. For Rh4(CO)12 the global minimum is predicted to have C3v symmetry, with three bridging and nine terminal carbonyls, in accord with experiment. The Rh6(CO)16 structure has Td symmetry and satisfies the Wade-Mingos rules for an octahedral cluster. Using the MPW1PW91 method the Rh-Rh distances in Rh4(CO)12 are found to be 2.692 Å and 2.750 Å and those in Rh6(CO)16 to be 2.785 Å.