New insight into the electronic structure of SiF₄: synergistic back-donation and the eighteen-electron rule

SiF₄ demonstrated high thermal stability in dry air or vacuum, and a Si-F bond length of 1.554 Å is close to the second period element C-C bond length (1.54 Å) of C₂H₆. To determine which factors confer this property of SiF₄, here we conduct a comparative study of a series of molecules SiHF_4- (n = 0, 1, 2, 3), SiX₄ (X = Cl, Br, I), CF₄ and TiF₄ in terms of bond length and energy, molecular orbitals, and adaptive natural density partitioning (AdNDP) analysis. The AdNDP analysis shows that there are five 5c-2c bonds in SiF₄, here named synergistic back-donation (SBD) bonds. These SBD bonds together with the Si-F σ bonds and the eighteen-electron rule are demonstrated as the main factors contributing to the short Si-F bond length and the high thermal stability of SiF₄ in dry air or vacuum. Moreover, the SBD bonds exist widely in other isoelectronic species of SiF₄ such as ClO₄^-, SO₄^2-, PO₄^3- and XeO₄.