The Remarkable [ReH9]2- Dianion: Molecular Structure and Vibrational Frequencies

The equilibrium geometries and vibrational frequencies of the extraordinary [ReH9](2-) dianion (D-3h symmetry) are investigated using Hartree-Fock (HF) theory, coupled cluster theory with single and double excitations (CCSD), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The new generation of energy-consistent relativistic pseudopotentials and correlation consistent basis sets [cc-pVXZ-PP (Re) and cc-pVXZ (H) (X = D, T, Q)] are used. Anharmonicity was considered using second-order vibrational perturbation theory. The predicted geometries and vibrational frequencies generally agree with experimental findings. In order to stabilize the [ReH9](2-) dianion, the M2ReH9 (M = Na, K) sandwich complexes (D-3h symmetry) are studied at the CCSD(T)/VTZ (VTZ = cc-pVTZ-PP (Re) and cc-pVTZ (H, Na, K)) level of theory. Compared to the [ReH9](2-) dianion, the predicted vibrational frequencies involving Re-H stretching modes are improved, indicating the importance of considering counterions in electronically dense systems. The natural bond orbital analysis shows that each H atom only bonds with the Re center, and the 5d orbitals of Re and is orbitals of H are major factors for the covalent Re-H bonding.