VARIATION IN CHARGE-TRANSFER PHOTOCHEMISTRY CLARIFIED BY A CASSCF MR-CCI COMPARATIVE-STUDY OF THE LOW-LYING EXCITED-STATES OF M(R)(CO)(3)(H-DAB) (M = MN, R = H, METHYL, ETHYL, M = RE, R = H, DAB = 1,4-DIAZA-1,3-BUTADIENE)/

The lowest energy electronic transitions of the model complexes M(R)(C O)(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 CASS CF/MR-CCI calculations. On the basis of the excitation energies calcul ated 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 c omplex, the low-lying excited states are nearly pure, corresponding ei ther 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 calc ulated oscillator strengths indicate a large contribution of the secon d MLCT state, corresponding to the 3d(xz) - pi(DAB) excitation, to th e intense visible band observed in this class of complexes. The transi tions to the singlet and tripler MC excited states are calculated at 3 5 900 and 26 380 cm(-1), respectively, and will contribute to the UV a bsorption together with those to the SBLCT states. On going from the h ydride 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-methy l bond. On going from the methyl to the ethyl complex, the SBLCT trans itions are still lowered in energy, due to the weakening of the metal- R bond, but the excitation energies to the MLCT states are not signifi cantly affected. This is a consequence of the more covalent character of the metal-ethyl bond as compared to the metal-methyl bond. The subs titution of hydrogen by an alkyl group is accompanied not only by a re d shift of the low-lying MLCT states from 15 090-26 000 to 13 690-20 4 10 cm(-1) but also by an increase in the density of states in the visi ble energy domain. The second effect that will affect the photophysics and the photochemistry within the molecular series implies an importa nt mixing between the MLCT and SBLCT excited states. A comparison betw een 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 stabil izing interaction between the pi(DAB) and the 6p(z) of the metal cent er. The consequences are a stabilization of the excited states calcula ted between 12 600-27 650 cm(-1) (triplet components) and 15 250-31 34 0 cm(-1) (singlet components) and a significant mixing between the MLC T and SBLCT states.