DFT calculations were used to analyze the electronic structures of 15 mono- and dicarbonyl rhodium(I) complexes with bidentate O,O-, N,N-, and N,O-donor ligands. The characteristics describing the metal-CO bond (bond length and spin-spin coupling constant J(CRh)) were found to depend on which of the donor atoms (N or O) is trans to the carbonyl group. Because of this, the trans-effect transmitted along the σ-bonds can be judged from the values of these characteristics. The characteristics whose values are determined by the populations of the π-antibonding orbitals of the CO groups primarily depend on the cis-partners in the coordination sphere.