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

Abstract

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 SiHnF4-n (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-, SO42-, PO43- and XeO4.