Oxo ligand (${\mathrm{O}}^{2-}$)

A coordinated oxide, formally ${\mathrm{O}}^{2-}$. A terminal $\mathrm{M}=\mathrm{O}$ unit is conventionally counted as an $X_2$ ligand giving four electrons in ionic electron counting. Additional oxygen-to-metal pi donation is an orbital-bonding issue and must not be silently mixed with another counting convention.