Max. p pi-p pi back bonding exist in:
1. BCl3
2.BF3
3.BBr3
4.BI3

Dear Student,

Please find below the solution to the asked query:

Among the given boron trihalides, maximum $\mathrm{p}\pi -\mathrm{p}\pi$ back bonding exists in BF₃. This is because as the size of halogen atom increases down the group, the overlapping of the vacant 2p orbital of boron cannot take place easily and efficiently with the p-orbitals of high energy levels.

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