Linkage Photoisomerization Mechanism in a Photochromic Ruthenium Nitrosyl Complex: New Insights from an MS-CASPT2 Study

The N -> O linkage photoisomerization mechanism in a ruthenium nitrosyl complex, [RuCl(NO)(py)(4)](2+), for which a quasicomplete photoconversion between the stable nitrosyl (N-bonded) and metastable isonitrosyl (O-bonded) isomers has been observed under continuous irradiation of the crystal at 473 nm (Cormary et al. Acta Cryst. B 2009 , 65 , 612-623 ), is investigated using multiconfigurational second-order perturbation theory (CASPT2). The results support efficient intersystem crossing pathways from the initially excited singlet states to the lowest triplet excited state of metal-to-ligand charge transfer character ((MLCT)-M-3). The topology of the involved potential energy surfaces corroborates a complex sequential two-photon photoisomerization mechanism involving nonadiabatic processes in agreement with experimental observations and previous density functional theory calculations.