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I need to compare the Lewis basic strength of trihalides of phosphorus

My thoughts : Since fluorine is more electronegative, the percentage of $\mathrm s$-character in $\ce{P-F}$ bonds will be lower, which will in turn increase the percentage $\mathrm s$-character of the orbital which contains the lone pair of phosphorus atom .

So the lone pair will be closer to the nucleus and so its donation will be more difficult, and thus I came to the conclusion that the order should be

$$\ce{PF3}<\ce{PCl3}<\ce{PBr3}<\ce{PI3}$$

But I have read that $\ce{PF3}$ behaves as a better ligand in coordinate compounds.

Also, I have seen different orders at different places.

Could someone help me figure out the correct order and also the actual reason for that order, and also about whether lewis basic strength and ligand strength go parallel to each other or whether there can be discrepancies in order?

orthocresol
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ARROW
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    You need to distinguish between $\sigma$-donating and $\pi$-accepting properties. As the electronegativity of halide substituents increases, the $\sigma$ donor strength of the phosphorus atom will decrease. However, the $\pi$-accepting character will increase. The amount of back-donation from M will also depend on the $\pi$ acidity of the metal. It is often said that PF$_3$ behaves like CO in terms of ligand strength. In order to assess the donating properties of PX$_3$, compare one particular series of complexes [L$_n$M(PX$_3$)] and alter just X. Have a look QALE or Tolman's parameters. – tstone-1 Sep 09 '20 at 10:56
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    For formatting, See here and here. For a more detailed MathJax guide, look here, minor other details – Safdar Faisal Sep 09 '20 at 10:57
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    Indeed, ligand binding of PX3 is heavily influenced by pi backbonding. – orthocresol Sep 09 '20 at 10:58
  • @tstone So the basic strength order I've mentioned is correct , right ? – ARROW Sep 09 '20 at 14:43
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    @ARROW If the question is solely about $\sigma$ donor strength: yes. However, if a ligand's strength (= Lewis basicity?) is to be classified according to the sum of $\sigma$ and $\pi$ donor properties, ask the question: At which system are we looking at? As I said, you need to look at one specific complex [L$_n$M(PX$_3$)] and not compare across systems. For X$_3$P$\cdot$BBr$_3$, the order given above is correct. In a textbook case, the poor $\sigma$ donor character of PF$_3$ will be overcompensated by its $\pi$-accepting properties, thus rendering PF$_3$ the strongest ligand. – tstone-1 Sep 10 '20 at 07:05

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