Silicon Tetrafluoride and Related Molecules

The calculated Si—F siugle-bond length (Sec. 7-2) is 1.69 A the observed Si—F distances are much smaller 1.54 A for SiFi, 1.56 A for SiIiF3 SiCIFs, and SiBrF, and 1.594 A for SiH8F. The extra shortening, 0.15 to 0.10 A, corresponds to 65 to 35 percent of double-bond character (Sec. 7-7). 

All other bonds between fluorine and second-row or heavier atoms 

8 It is interesting that the internuclear distance of the gas molecule SiF is 1.603 A presumably the bond is rather similar to the Si—F bonds in stable molecules of quadrivalent silicon. 

Bonds with Partial Double-Bond Character 

The structure H3C=F does not make a significant contribution to the normal state of the methyl fluoride molecule the carbon atom has only four stable orbitals, of which three are occupied in the H—C bonds, leaving only one available for bonding the fluorine atom. If, however, two or more fluorine atoms are substituted into the methane molecule, resonance with structures of the type 

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