Chemical Bonding in Octahedral XeF₆ and SF₆

Quantum chemical density functional theory calculations have been carried out for octahedral XeF₆ and SF₆ at the BP86/TZ2P level with relativistic effects included by the ZORA approximation. The energy decomposition analysis of XeF₆ and SF₆ using neutral and charged fragments EF₅ + F and EF₅⁺ + F ^- as well as E + F₆ and E⁶⁺ + F ₆^6- indicates that the dominant E-F orbital interactions take place between σ-orbitals which have t_1u symmetry in the octahedral point group. The contribution of the a_1g orbitals is negligible in the 16 valence electron compound XeF₆. The a _1g contribution becomes larger in the 14 valence electron species SF₆ but it is less important than the t_1u term. The bonding between the neutral species comes mainly from covalent (orbital) interactions but the quasiclassical electrostatic attraction significantly contributes to the attractive interactions. The bonding which comes from the ΔE_orb term is compensated by the Pauli repulsion ΔE _pauli. The sum of ΔE_orb and ΔE_pauli is repulsive for XeF₆ and SF₆, which would not be stable molecules without quasiclassical electrostatic attraction.