A Density Matrix Renormalization Group Study of the Low‐Lying Excited States of a Molybdenum Carbonyl‐Nitrosyl Complex

A density matrix renormalization group‐self consistent field (DMRG‐SCF) study has been carried out to calculate the low‐lying excited states of CpMo(CO)(2)NO, a molybdenum complex containing NO and CO ligands. In order to automatically select an appropriate active space, a novel procedure employing the maximum single‐orbital entropy for several states has been introduced and shown to be efficient and easy‐to‐implement when several electronic states are simultaneously considered. The analysis of the resulting natural transition orbitals and charge‐transfer numbers shows that the lowest five excited electronic states are excitation into metal‐NO antibonding orbitals, which offer the possibility for nitric oxide (NO) photorelease after excitation with visible light. Higher excited states are metal‐centered excitations with contributions of metal‐CO antibonding orbitals, which may serve as a gateway for carbon monoxide (CO) delivery. Time‐dependent density functional theory calculations done for comparison, show that the state characters agree remarkably well with those from DMRG‐SCF, while excitation energies are 0.4–1.0 eV red‐shifted with respect to the DMRG‐SCF ones.