Orthonitrate

Orthonitrate is a tetrahedral oxoanion of nitrogen with the formula NO3−
4. It was first identified in 1977 and is currently known in only two compounds, sodium orthonitrate (Na₃NO₄) and potassium orthonitrate (K₃NO₄). These can be prepared by fusion of the nitrate and metal oxide under high temperatures and ideally high pressures (several GPa).

NaNO₃ + Na₂O → Na₃NO₄ (300 °C for 3 days)

The resulting orthonitrates are white solids which are extremely moisture and CO₂ sensitive, decomposing within minutes to carbonate and hydroxide upon exposure to air.

Na₃NO₄ + CO₂ → NaNO₃ + Na₂CO₃Na₃NO₄ + H₂O → NaNO₃ + 2NaOH

The orthonitrate ion is tetrahedral with N–O bond lengths of 139 pm, which is unexpectedly short, indicating that polar interactions are shortening the bond as dπ orbitals are too high in energy to be involved. This short bond length parallels that of oxoanions containing third-row elements like PO₄³⁻ and SO₄²⁻, for which pπ-dπ bonding was previously proposed as the explanation for the short bond length and thus, argues against the importance of this explanation for these heavier anions. (See the article on hypervalence for a discussion of bonding models)