Laboratory studies of the interaction of carbon monoxide with organoactinides result in the formation of isolable complexes such as Cp3UCO derivatives (Cp = cyclopentadienyl) as well as coupling reactions to give derivatives of the oligomeric anions CnOn2- (n = 2, 3, 4). To gain some insight into actinide carbonyl chemistry, binuclear cyclopentadienylthorium carbonyls Cp2Th2(CO)(n) (n = 2 to 5) as model compounds have been investigated using density functional theory. The most favorable such structures in terms of energy and thermochemistry are the tricarbonyl Cp2Th2(eta(2)-mu-CO)(3) having three four-electron donor bridging carbonyl groups and the tetracarbonyl Cp2Th2(eta(4)-mu-C2O2)(eta(2)-mu-CO)(2) having not only two four-electron donor bridging carbonyl groups but also a bridging ethynediolate ligand formed by coupling two CO groups through C-C bond formation. The bridging infrared nu(CO) frequencies ranging from 1140 to 1560 cm(-1) in these Cp2Th2(CO)(n) (n = 3, 4) derivatives indicate extremely strong Th-->CO back bonding in these structures, corresponding to formally dianionic CO2- and C2O22- ligands and the favorable +4 thorium oxidation state. A characteristic of the Cp2Th2(eta(2)-mu-CO)(3) and Cp2Th2(eta(4)-mu-C2O2)(eta(2)-mu-CO)(2) structures is their ability to add terminal CO groups, preferably to the thorium atom bonded to the fewest oxygen atoms. These terminal CO groups exhibit nu(CO) frequencies in a similar range as terminal CO groups in d-block metal carbonyls. However, these terminal CO groups are relatively weakly bonded to the thorium atoms as indicated by predicted CO dissociation energies of 14 kcal/mol for Cp2Th2(CO)(5). Two low energy structures for the dicarbonyl Cp2Th2(CO)(2) are found with two separate four-electron donor bridging CO groups and relatively short Th-Th distances of 3.3 to 3.4 angstrom suggesting formal single bonds and +3 thorium formal oxidation states. However, a QTAIM analysis of this formal Th-Th bond does not reveal a bond critical point thus suggesting a multicenter bonding model involving the bridging CO groups.