I analysed the bonding in chlorine trifluoride a few years back in terms of VSEPR theory. I noticed that several searches on this topic which led people to this post also included a query about the differences between it and the bromine analogue. For those who posed this question, here is an equivalent analysis.
The calculation is done at the same level as before (ωB97XD/6-311++D(d,p)) for consistency (DOI: 10.14469/hpc/2160)
- Basins 8 and 9 have electron populations of 2.33e (2.07e for the chlorine analogue) with an angle subtended at Br of 159°. The greater electron population and hence electron pair repulsion has the effect of increasing the angle compared to Cl (154°). The coordination is even more square pyramidal than with Cl.
- Basin 7 has a population of 0.73e, this time less than Cl (0.87e).
- Basins 11 and 12 are 0.82e. With Cl, this single basin was replaced by a pair of split basins, each pair summing to 0.91e (the same effect happens with F-F). The angle 4-2-3 is 172° (174° for Cl) which suggests a slightly increased 2-electron-3-centre interaction between e.g. atoms 1-4 or 1-3 compared to Cl.
- The total basin count surrounding the Br is therefore 7.03e, compared to 6.84e with Cl, which suggests Br is slightly more electronegative in this context than Cl.
Bromine has a habit of springing surprises, but not so much in this example.