ELECTRONIC-SPECTRA OF RMN(CO)3(ALPHA-DIIMINE) (R=H, CH3) - A CASSCF CCI COMPARATIVE-STUDY OF THE LOWEST SINGLET EXCITED-STATES

A theoretical study, based on CASSCF/CCI calculations of the lowest part of the electronic spectra of RMn(CO)3(alpha-diimine) (R = H, CH3), is reported. These systems are a model for a rather general class of transition metal complexes with low-lying Metal-to-Ligand Charge Transfer (MLCT) states. The theoretical electronic spectrum of HMn(CO)3(alpha-diimine) is characterized by a high density of states below 35000 cm-1 (285 nm). The lowest 1A' and 1A'' singlet states, directly accessible through allowed transitions from the 1A' ground state, correspond to d --> pi* excitations, between 171 10 and 22300 cm-1. These values are in excellent agreement with the vis-UV absorption spectra usually obtained for this family of molecules with a MLCT band at around 500 nm (20000 cm-1). The next 1A' and 1A'' states ( above 27800 cm-1) correspond to d --> d excitations and should contribute to a shoulder on the absorption spectra at around 350 nm. The first excited 1A' sigma --> pi* state, susceptible to play a role in the homolysis of the Metal-R bond, is calculated at 32150 cm-1. This state is not directly accessible upon irradiation at 500 nm, but may interact with the lowest 1MLCT states early on the reaction path corresponding to the homolysis of the Metal-R bond. In the light of these results, the photochemical behavior of other transition metal (alpha- diimine) complexes with low-lying MLCT excited states is discussed. The excitation energies for the lowest 1A' states only slightly differ when going from the hydride to the alkyl complex.