Tetra-hydrides of the third-row transition elements: spin–orbit coupling effects on geometrical deformation in WH4 and OsH4

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The dissociation energies of MH4 (M = La, Hf–Hg) were computed using full optimized reaction space (FORS) multi-configuration self-consistent field (MCSCF) and second-order multi-reference Moller–Plesset perturbation methods with the SBKJC basis sets augmented by a set of polarization functions (SBKJC(f,p)). It was shown that of the molecules examined, only four tetra-hydrides HfH4, TaH4, WH4, and OsH4 with Td symmetry are lower in energy than the corresponding dissociation limits. For WH4 and OsH4, the potential energy surfaces from the D4h to the Td structure were explored from both theoretical calculations and symmetry arguments based on the pseudo-Jahn- Teller effect. As for WH4, it is found that the ground state could be 3Eg, 3A2g, or 3B2g at the D4h structure. The present calculations suggest that the ground state is 3Eg, and that this state is stabilized by the eu deformation into a C2v structure (3B1) and then sequentially to the most stable Td structure (3A2). If the molecular system is promoted to the lowest 3B2g state, the D4h structure can directly deform into the most stable Td structure along the b2u vibrational mode. For OsH4, the ground state (5B1g) at the D4h structure deforms into a D2d structure and the resulting 5B2 state strongly interacts with the lowest 3E and 1A1 states due to the spin-orbit couplings (SOCs). As a result, it was shown that the relativistic potential energy of the lowest spin-mixed state (ground state) monotonically decreases along the D2d deformation path from the D4h to the Td structure.