Spin–orbit effects on the electronic spectroscopy of transition metal dihydrides H2M(CO)4 (M=Fe, Os)

Abstract

The influence of spin–orbit (SO) effects on the transition energies to the low-lying singlet and triplet electronic states of H2M(CO)4 (M = Fe, Os) are calculated by means of a two-step procedure in which CASSCF/MS-CASPT2 method using relativistic effective core potentials in the ab initio model potential (AIMP) approach was used to obtain the spin-free vertical transitions in a first step. Spin–orbit effects are taken into account in the second step by building a determinantal model space which includes a set of reference configurations able to represent all the desired states along with singly excited configurations selected with respect to the SO operator within the atomic mean-field approximation. The results of the first step calculation are exploited in order to include correlation effects via an effective Hamiltonian technique and diagonalize the full matrix on the determinantal basis. It is shown that while the SO has little influence on the absorption spectrum of H2Fe(CO)4 it does affect significantly the electronic spectroscopy of the third row transition metal complex H2Os(CO)4 leading to a large mixing between the low-lying states and to a splitting exceeding 3000 cm−1 for the lowest 3B2 state.