While other third-row transition metals react more readily with CO, rhenium carbon monoxide is found to be relatively less stable at both the complete active space multiconfiguration self-consistent field (CASMCSCF) and the multireference singles + doubles configuration interaction (MRSDCI) computation levels. The (4)Sigma(-) state was found to be the ground state for both TcCO and ReCO complexes. Although at the CASSCF level this state has negative D-e relative to the M(a(6)S) + CO((1)Sigma(+)) dissociation limit (M = Tc or Re), at a more accurate MRSDCI level, the D-e＇ s of the (4)Sigma(-) state were computed as 0.002 eV for TcCO, and 0.012 eV for ReCO, with respect to the same dissociation limit. Spin-orbit effects for ReCO and TaCO split the (4)Sigma(-) nonrelativistic ground state into 1/2 and 3/2 Omega states. The energy difference for the two states is computed as 981 cm(-1). For the TaCO complex, the spin-orbit effects enlarge the energy difference between (4)Delta(1/2) and (6)Sigma(1/2)(+) to 1742 cm(-1), compared with 691 cm(-1) in the absence of spin-orbit effects. The computed properties of all M-CO species (M = second- and third-row transition metals) and their nature of bonding for TcCO and ReCO are discussed. It is shown that the curve crossing of the ground and excited states is an important factor in the nature of charge transfer in these species. This combined with the extent of charge transfer from CO to M, and the back transfer from M to CO through pi-bonding, is found to be important to result in stable complexes.