New insight into the electronic structure of SiF 4 : synergistic back-donation and the eighteen-electron rule.

SiF 4 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 C 2 H 6 . To determine which factors confer this property of SiF 4 , here we conduct a comparative study of a series of molecules SiH n F 4- n ( n = 0, 1, 2, 3), SiX 4 (X = Cl, Br, I), CF 4 and TiF 4 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 SiF 4 , 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 SiF 4 in dry air or vacuum. Moreover, the SBD bonds exist widely in other isoelectronic species of SiF 4 such as ClO 4 - , SO 4 2- , PO 4 3- and XeO 4 .