A CASSCF study of the relaxation effects in the lowest 3MLCT excited state of HMn(CO) 3(dab)

The search for minimum energy structures based on the gradient method of optimization and on CASSCF reference wavefunctions is reported for the electronic ground state and the lowest triplet MLCT (metal-to-ligand charge transfer) excited state of HMn(CO)3(dab; dab = 1,4-diaza-1,3-butadiene). A comparison between the structures obtained on the basis of this correlated method with those arising from gradient SCF calculations points to the importance of correlation effects in structure determination. The metal-carbonyl bond lengths are shortened by around 10% when going from the single-determinant approach to the multiconfigurational treatment due to a better description of the π back bonding interaction. A large CASSCF active space including the σ metal-hydrogen bonding orbital and its antibonding counterpart σ∗ gives a good description of the hydrogen trans effect in this molecule. The deformation of the molecule in the lowest triplet MLCT excited state is characterized by an elongation of the order of 18% of the metal-COaxial bond, a small shortening (less than 2%) of the metal-hydrogen bond and a bending of the hydrogen towards the equatorial carbonyls. The metal-COequatorial bonds are only affected slightly when going from the electronic ground state to the lowest triplet MLCT state.