I start with computing an accurate wavefunction, using the DFT-based ωB97XD/6-311++G(d,p). Can VSEPR work with only eight electrons in this instance? And what are the coordinates of the so-called two “lone pairs”: is the angle subtended at the Cl by them really trigonal (~120)? If it’s not real, then we cannot be dealing with five electron pairs. Well, if you take a look at earlier blogs, you may have observed that this expanded octet is not real (IMHO). My issue is with the above explanation, of five electron pairs all associated in some way with the central atom. Regular readers of this blog may have noted that I often like to question the text books. The above at least is the standard “text-book” picture. The remaining two Cl-F bond pairs occupy the di-axial positions (rule 7 above). The other two trigonal positions are occupied by two sets of electron lone pairs (yellow below) at ≥ 120° (rule 5, but much more and the repulsions between the lone pair and the trigonal Cl-F bond would become too great, rule 6 above). One of the trigonal positions is occupied by the pair deriving from a Cl-F bond (F=white, Cl=red below).
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