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 angstrom is close to the second period element C-C bond length (1.54 angstrom) 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 sigma 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.

Automated summary

In this study, a comparative analysis was conducted on SiF4 and other molecules in terms of bond length and energy, molecular orbitals, and adaptive natural density partitioning (AdNDP) analysis, revealing that the high thermal stability of SiF4 and the short Si-F bond length are mainly attributed to the presence of five 5c-2c bonds, called synergistic back-donation (SBD) bonds, along with the Si-F sigma bonds and the eighteen-electron rule.