Reinterpretation of the Lengths of Bonds to Fluorine in Terms of an Almost Ionic Model.

We have calculated the electron density distributions, electron densities at the bond critical point, and atomic charges in the period 2 and 3 fluorides and a number of their cations and anions. On the basis of this information and an analysis of X-F bond lengths, we have examined the factors that determine the lengths of these bonds. We have shown that all the molecules except NF(3), OF(2), and F(2) have considerable ionic character. The bond lengths of the fluorides reach a minimum value at BF(3) in period 2 and at SiF(4) in period 3 when the product of the charges on the central atom and a fluorine reaches a maximum, consistent with a predominately ionic model for these fluorides. The length of a given A-F bond decreases with decreasing coordination number, and we show that it is determined primarily by packing considerations. This provides an alternative to the previously proposed back-bonding model explanation, for which our work provides no convincing evidence. There is also no evidence to support the Schomaker-Stevenson equation which has been widely used to correct A-F bond lengths calculated from the sum of the covalent radii of A and F for the difference in the electronegativities of A and F. We propose a new value for the covalent radius of fluorine and point out the limitations of its use.