MO calculations have been performed on the complexes fac-XMn(CO)(3)(bpy) (X = Cl, I) and mer-ClMn(CO)(3)-(bpy) in order to establish the influence of X on the electronic transitions of the fac and mer isomers and to interpret the differences in photochemical behavior of the fac and mer complexes. SCF calculations were performed with the Amsterdam density-functional program package ADF. The structural data used for the calculations were partly derived from the X-ray structure of fac-IMn(CO)(3)(bpy) presented in the article. The theoretical data for the fac isomers show that the two highest occupied orbitals have metal-halide antibonding character and that the LUMO is a pi* (bpy) orbital. The first absorption band is therefore assigned to electronic transitions having mixed metal-halide to bpy CT character. The second absorption band is assigned to CT transitions from the corresponding metal-halide bonding orbitals to bpy. From a comparison of the theoretical and spectroscopic data, the intensities of these two absorption bands appear to depend on the metal character of the orbitals from which the electronic transitions originate. The main difference between the theoretical data of the fac- and mer-ClMn(CO)(3)(bpy) complexes is the higher energy of the highest filled orbitals in the case of the mer isomer. This difference is in accordance with the spectroscopic data and may also explain the differences in photochemistry between the two isomers.