As PI5 is uncertainly stated as the combination of PI3 and I2. There are suggestions that phosphorus donates its 3s electron pair to molecular orbital of I2 or I2 donates its electron to 3d orbital of phosphorus. Are these suggestions correct?

  • 3
    $\begingroup$ That's an adduct $\ce{PI3.I2}$ formed in solution, and $\ce{PI3}$ co-crystallizes with $\ce{I2}$. Related: Explanation of steric hindrance and existence of PI5 $\endgroup$
    – andselisk
    May 22, 2023 at 11:47
  • $\begingroup$ I mean an adduct like H3N->BF3 is formed between an acid lewis and base lewis. So specifically, in the case of PI3 and I2, which is acid lewis and base lewis? $\endgroup$
    – Shira
    May 22, 2023 at 12:11

1 Answer 1


Assuming the structure $\ce{I3P - I - I}$ exists at all (Wikipedia does not mention this isomer and the claim is unsourced in the question), the currently accepted molecular orbital theory posits a three-center four-electron bond, essentially a standard covalent bond that joins three atoms by being delocalized over two linkages:

$\ce{I3P:\space I - I\space <-> I3\overset{+}{P} - I\space :I^-}$

In this model the Lewis acid is actually the bridging iodine in the form of $\ce{I^+}$, which is partly bounded to each of two Lewis bases $\ce{I^-}$ (in the left structure) and $\ce{PI3}$ (in the right structure).


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.