The reactions of the previously reported cluster complexes [Re-6(mu(3)-Se)(8)(PEt3)(5)I] I, trans-[Re-6(mu(3)-Se)(8)(PEt3)(4)I-2], and cis-[Re-6(mu(3)-Se)(8)(PEt3)(4)I-2] with the [Re-6(mu(3)-Se)(8)](2+) core with CO in the presence of AgSbF6 afforded the corresponding cluster carbonyls [Re-6(mu(3)-Se)(8)(PEt3)(5)(CO)][SbF6](2) (1), trans-[Re-6(mu(3)-Se)(8)(PEt3)(4)(CO)(2)][SbF6](2) (2), and cis-[Re-6(mu(3)-Se)(8)(PEt3)(4)(CO)(2)][SbF6](2) (3). Infrared spectroscopy indicated weakening of the bond in CO, suggesting the existence of backbonding between the cluster core and the CO ligand(s). Electrochemical studies focusing on the reversible, one-electron oxidation of the cluster core revealed a large increase in the oxidation potential upon going from the acetonitrile derivatives to their carbonyl analogs, consistent with the depleted electron density of the cluster core upon CO ligation. Disparities between the IR spectra and oxidation potential between 2 and 3 indicate that electronic differences exist between sites trans and cis to the location of a ligand of interest. The active role played by the Se atoms in influencing the cluster-to-CO bonding interactions is suggested through this result and density functional (DF) computational analysis. The computations indicate that molecular orbitals near the HOMO account for backbonding interactions with a high percentage of participation of Se orbitals.