I was looking at the oxyacids of Phosphorous and it is given that (in Wikipedia article of "Phosphorous acid"), some of these acids (Eg: $H_3PO_2$ and $H_3PO_3$) exist in a dynamic equilibrium between their keto form and enol form.

This is a pure speculation from my side but I want you to think about a molecule with $P(OH)_5$ molecule with each hydroxide in a trigonal bi-pyramidal geometry.

I feel like there could be two reasons to think about it's possibility:

  1. Since 'P' have empty d-orbitals and there are lone pairs on the oxygen, it could form a type of back bond, thus making the P-OH bond a bit stronger.
  2. Since there are OH groups, there could be a intramolecular H-bonding that could further stabilise the molecule.

Maybe this can form in this can form in such a reaction: $$ \ce{H_3PO_4 + H_2O <=> H_5PO_5} $$ where the water molecule could on the P=O bond.

So, is it even possible for such a molecule to exist?

  • 1
    $\begingroup$ No, it doesn't exist: chemistry.stackexchange.com/q/135208/17368 $\endgroup$ Mar 1 at 9:40
  • $\begingroup$ So it would be called Holophosporic acid. Is there a reason for why it couldn't exist? $\endgroup$
    – BK01
    Mar 1 at 10:01
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    $\begingroup$ Does this answer your question? Why is H5PO5 not called as phosphoric acid? $\endgroup$ Mar 1 at 18:11
  • $\begingroup$ No. That question describes the nomenclature for this formula. I'm questioning whether this compound exists or not. And if it couldn't, why not? $\endgroup$
    – BK01
    Mar 7 at 12:13

1 Answer 1


The specific $\ce{P(OH)5}$ does not appear to be known, but high-pressure phosphates with pentacoordinate phosphorus have been observed.

Bykov et al.1 reported the first example, involving titanium(III) phosphate ($\ce{TiPO4}$), in 2016. Under ambient conditions this compound shows "classical" coordination in which the titanium shows polymerized octahedral coordination (blue) and the phosphorus tetrahedral (gold/orange), shown in the first figure below from the reference. Under 48 GPa pressure, both titanium and phosphorus increase their coordination numbers, giving the phosphorus coordination a polymerized trigonal-bipyramidal shape shown in the second figure (also from the reference above). The titanium has an irregular eightfold coordination. The 48 GPa pressure is too great to be realized within Earth but may be present in super-Earth exoplanets.

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  1. Bykov, Maxim; Bykova, Elena; Hanfland, Michael; Liermann, Hanns-Peter; Kremer, R.; Glaum, Robert; Dubrovinsky, Leonid; Van Smaalen, Sander. (2016). "High-Pressure Phase Transformations in TiPO4 : A Route to Pentacoordinated Phosphorus." Angewandte Chemie (International ed. in English). 55. 10.1002/anie.201608530.

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