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I had read that for the trihalides of boron, boron trifluoride is the weakest Lewis acid due to backbonding. The order I had seen was -

$\ce{BI3 > BBr3 > BCl3 > BF3}\$ $$\ce{}$

$- order of acidic strength Why is the same not true for phosphorus trihalides as backbonding exists in them too?

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In phosphorus trihalides backbonding effect is not as big as boron trihalides. Firstly boron trihalides are trigonal planar and have an empty-and-small p orbital (violates the octet rule), so it is favourable that the p orbitals of the boron and the halogen atom will overlap; meanwhile phosphorus trihalides are trigonal pyramidal which is less favourable for orbital overlap, and the phosphorus atom already has a complete octet which includes a lone pair. Lone pair-lone pair repulsion would make backbonding even less favourable. The Lewis acidity should be largest for PF3, since the electronegative fluorine atoms would reduce electron density of the phosphorus atom and make it more hungry for electrons. Moreover the fluorine's small size allows more space for extra bonding. Hope that helps and forgive me for grammar and spelling errors - I am not a native English speaker.

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