![]() The total electron count for the ELF basins surrounding the central Cl is 6.84, not 10 as was implied in the simple argument set out above.This is in fact an anomeric effect! An NBO analysis reveals E(2) between Lp 16 and σ* 2-1 to be 6.3 kcal/mol, a relatively weak but still a real anomeric interaction. 16 is oriented anti-periplanar to the 2-1 bond. Other features include eg the orientation of the “lone pairs” on fluorines 3 and 4, in which e.g.Indeed, any small 2-electron-3-centre 1-4 or 1-3 contributions (Wiberg index 0.07) might cause these bonds to actually move together resulting in the "T" shape in which the angle 4-2-3 is actually < 180° (174). So the three Cl-F bonds are < 2-electron bonds and so their mutual repulsion might be expected to be less than rule 7 above. Basins 11+19 and 10+15 (similar basin splitting is observed for F 2 ) each total 0.91e again less than a pair (Wiberg index 0.63).Basin 7 is a Cl-F bond, with a population of 0.87e, rather less than a “pair” (the Wiberg bond index is 0.82).In which case, the geometry is not that of a trigonal bipyramid but is closer to that of a square pyramid. Indeed, one might almost describe 8 and 9 as being di-axial. The 8-2-9 angle of 154° results from rule 5 above lone pairs repel greatly. The electron basins are shown here as red spheres 8 and 9 are the “lone pairs”, as it happens a very reasonable description since the populations for these are 2.07e.The electron count and the coordinates of the localised basins will be obtained using ELF (Electron localisation function). 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). the next highest is between one lone pair and a bond pairĪs applied to chlorine trifluoride, it results in a trigonal bipyramidal geometry for the shape-determining five electron pairs.The highest repulsion is between any two “lone electron pairs”, resulting in these moving apart as far as possible.Three fluorine atoms contribute: 1 each.The standard application of VSEPR theory to this molecule is as follows: ClF 3 is a good illustration of this theory. Valence shell electron pair repulsion theory is a simple way of rationalising the shapes of many compounds in which a main group element is surrounded by ligands.
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