Quantum chemical calculations at DFT (BP86) and ab initio levels (CCSD(T)) have been carried out for transition metal carbon complexes [MX2(PR3)2(C)] with various combinations of M = Fe, Ru, Os, X = F, Cl, Br, I, and R = H, Me, Ph, Cyc. Calculations have also been performed for [RuCl2(PMe3)(NHC)(C)] and [RuCl2(NHC)2(C)] where NHC = N-heterocyclic carbene and for [M(Por)(C)] (M = Fe, Ru, Os; Por = porphyrin). The properties of the carbon complexes as donor ligands were studied by calculating the geometries and bond dissociation energies of the Lewis acid-base adducts with the Lewis acids M(CO)5 (M = Cr, Mo, W), PdCl2SMe2, BH3, BCl3, and Fe2(CO)8. The latter species are compared to the analogous CO complexes. The nature of the donor-acceptor interactions between the Lewis acids LA and carbon complexes [TM]C-LA is compared to the bonding in OC-LA. The bonding analysis was carried out with charge- and energy-partitioning methods. The bond strength and the donor-acceptor properties of metal carbon complexes closely resemble those of CO, and thus carbon complexes may be considered as electronically tuneable analogues of carbon monoxide. Similar properties are also calculated for the porphyrin carbon complexes 10MC, which bind more strongly and are slightly stronger pi acceptors than the [(X2(R)2M(C)] species. The carbon complexes [(X2(R)2M(C)] are slightly weaker pi acceptors than CO, and thus they tend to have slightly weaker bonds than CO in group-6 donor-acceptor complexes. The calculations suggest that bond energies of carbon complexes as ligands with d10 transition metals are larger than those of CO. The theoretical results let it seem possible that adducts with more than one carbon complex as ligands may be synthesized and that even homoleptic complexes may be prepared.