REINTERPRETATION OF THE LENGTHS OF BONDS TO FLUORINE IN TERMS OF AN ALMOST IONIC MODEL

We have calculated the electron density distributions, electron densit ies 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 examine d the factors that determine the lengths of these bonds. We have shown that all the molecules except NF3, Of(2), and F-2 have considerable i onic character. The bond lengths of the fluorides reach a minimum valu e at BF3 in period 2 and at SiF4 in period 3 when the product of the c harges on the central atom and a fluorine reaches a maximum, consisten t 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 electro negativities of A and F. We propose a new value for the covalent radiu s of fluorine and point out the limitations of its use.