Question

How do you write the orbital diagram for phosphate?

Answer 1

I interpreted this as the molecular orbital diagram... and it became rather complicated (I honestly don't think you want to know the minor details!).

Here's the scratchwork when I was getting the reducible representations `Gamma_(S(O))`, `Gamma_(p_y(O))`, `Gamma_(p_x(O))`, and `Gamma_(p_z(O))`, and the decomposed irreducible representations (`A_1 + T_2`, `E + T_1 + T_2`), for the four oxygen atoms transforming as a group:

The whole point of that was to see how the oxygen orbital energies split up (and which were two-fold or three-fold degenerate). It was also to figure out which orbitals on phosphorus interact with which orbitals on the oxygen atoms.

The resultant MO diagram was:

Takeaways:

• This is only qualitative, so take it with a grain of salt.
• Phosphate has all electrons paired.
• The nonbonding orbitals are nonbonding either due to mismatched symmetries, overly different inner-atom/outer-atom orbital energies, or interactions between both high-lying and low-lying orbitals.
• There are `22` nonbonding electrons, which accounts for all `11` lone pairs of electrons that belong to the oxygen atoms. The remaining `2` nonbonding electrons account for the `bb(pi)` bond that is delocalized.
• The `sigma` bond order that this MO diagram seems to account for is `1` for all four `bb(sigma)` bonds, as there are four clearly-bonding MOs (`a_1 + t_2`), one for each bond.