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:
- 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.
- 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?