The lowest energy electronic transitions of the model complexes M(R)(CO)(3)(H-DAB) (M = Mn, R = H, CH3,C2H5; M = Re, R = H, alpha-diimine = H-DAB = 1,4-diaza-1,3-butadiene) are investigated with the use of CASSCF/MR-CCI calculations. On the basis of the excitation energies calculated for the low-lying nd --> pi*(DAB) (metalto-ligand-charge-transfer), sigma(M-R) --> pi*(DAB) (sigma-bond-to-ligand-charge-transfer), and nd --> nd (metal-centered) excited states, it is shown how the three-center interaction between the R group, the metal center, and the pi* acceptor DAB ligand controls the nature and the energies of the lowest electronic transitions of these molecules. In the manganese hydride complex, the low-lying excited states are nearly pure, corresponding either to MLCT states in the visible energy domain between 15 090 and 26 000 cm-(1) or to SBLCT states calculated at 34 390 and 37 950 cm(-1) for the triplet and for the singlet components, respectively. The calculated oscillator strengths indicate a large contribution of the second MLCT state, corresponding to the 3d(xz) - pi*(DAB) excitation, to the intense visible band observed in this class of complexes. The transitions to the singlet and tripler MC excited states are calculated at 35 900 and 26 380 cm(-1), respectively, and will contribute to the UV absorption together with those to the SBLCT states. On going from the hydride to the methyl complex, the main change is a drastic lowering of the transition energies, which may exceed 0.5 eV for the SBLCT states. This effect is largely due to the weakening of the metal-R bond, the basicity of CH3-, and the more polarized character of the metal-methyl bond. On going from the methyl to the ethyl complex, the SBLCT transitions are still lowered in energy, due to the weakening of the metal-R bond, but the excitation energies to the MLCT states are not significantly affected. This is a consequence of the more covalent character of the metal-ethyl bond as compared to the metal-methyl bond. The substitution of hydrogen by an alkyl group is accompanied not only by a red shift of the low-lying MLCT states from 15 090-26 000 to 13 690-20 410 cm(-1) but also by an increase in the density of states in the visible energy domain. The second effect that will affect the photophysics and the photochemistry within the molecular series implies an important mixing between the MLCT and SBLCT excited states. A comparison between the lowest part of the spectrum of Mn(PI)(CO)(3)(H-DAB) and Re(H)(CO)(3)(H-DAB) points to a large influence of the metal center, mainly due to the relativistic destabilization of the d shells and the stabilizing interaction between the pi*(DAB) and the 6p(z) of the metal center. The consequences are a stabilization of the excited states calculated between 12 600-27 650 cm(-1) (triplet components) and 15 250-31 340 cm(-1) (singlet components) and a significant mixing between the MLCT and SBLCT states.