Geometrical and electronic structure of the molybdenum and tungsten halides MX3 and MX4 (M = Mo, W; X = F, Cl): Jahn-Teller effect and spin-orbit coupling

Abstract The ground and lower-lying excited electronic states of MX 3 and MX 4 (M = Mo, W; X = F, Cl) molecules were systematically studied by the complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (XMCQDPT2) methods. Scalar-relativistic effects and spin-orbit coupling were taken into account employing the third-order Douglas-Kroll-Hess (DKH) Hamiltonian and full Breit-Pauli operator. The ground orbital state of MoF 3 and MoCl 3 complexes is quadruplet 4 A 2 ′ state in the equilibrium configuration with D 3h symmetry. If the spin-orbit interaction is taken into account the calculations result in two 4 E 1/2 and 4 E 3/2 spin-orbit states with 11–14 cm −1 energy differences instead of one 4 A 2 ′ state. Spin-orbit interaction quenches the Jahn-Teller effect in 2 E″ ground orbital state of WF 3 complex in D 3h configuration, and C 2v configuration with ( 4 E 1/2  +  4 E 3/2 ) spin-mixed state corresponds to the minimum on the PES. The WCl 3 complex reveals strong interaction of the spin-orbit states of different multiplicity. For WCl 3 complex the configuration with D 3h symmetry corresponds to the minimum on the PES for four lowest spin-mixed electronic states. The MoF 4 , MoCl 4 and WCl 4 complexes possess the tetrahedral equilibrium configuration with 3 A 2 orbital electronic state. If the spin-orbit interaction is taken into account the ground state changes to the threefold degenerated 3 T 2 spin-orbit state with weak Jahn-Teller effect. In the case of WF 4 complex the strong Jahn-Teller effect has been discovered in the first excited 1 E singlet electronic state of the tetrahedral structure, and D 2d configuration with 1 A 1 electronic state possesses the lowest energy. The comparison with available experimental data has been performed. The tendencies in molecular parameters of complexes studied were analyzed.