Orthonitrate

Orthonitrate is a tetrahedral anion of nitrogen with the formula . It was first identified in 1977 and is currently known in only two compounds, sodium orthonitrate (Na3NO4) and potassium orthonitrate (K3NO4). The corresponding oxoacid, orthonitric acid (), is hypothetical and has never been observed. Sodium and potassium orthonitrate can be prepared by fusion of the nitrate and metal oxide under high temperatures and ideally high pressures (several GPa).

: (300 °C for 3 days)

The resulting orthonitrates are white solids which are extremely sensitive to moisture and CO2, decomposing within minutes to hydroxides, carbonates, and nitrates upon exposure to air. : :

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. This short bond length parallels that of hypervalent oxyanions containing third-row elements like and , for which pπ–dπ bonding was previously proposed as the explanation for the short bond length. Since 3d orbitals of nitrogen are much too high in energy to be involved in the case of orthonitrate, the shortness of the N–O bond in orthonitrate indicates that pπ–dπ bonding is likely not the most important explanation for the bond lengths of these heavier anions either.