(i) Ability of oxygen to form multiple bond / $\mathrm{p}\mathit{\pi }-$ $\mathrm{d}\mathit{\pi }$ bond.
(ii) Partially filled $\mathit{d}$-orbitals / due to comparable energies of $\mathit{n}\mathit{s}$ and $\left(\mathit{n}-1\right)\mathit{d}$-orbitals.
(iii) due to relative stabilities of the $\mathit{f},{\mathit{f}}^7$ and ${\mathit{f}}^4$ occupancies of the 5 -orbitals / Comparable energies of $7\mathit{s},6\mathit{d},5\mathit{f}$-orbitals.