Dinitrogen trioxide

Dinitrogen trioxide is the chemical compound with the formula N2O3. This pale blue liquid is one of binary nitrogen oxides. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below −21°C (−6°F):
 * NO +  NO2  $$\overrightarrow{\leftarrow}$$  N2O3

Dinitrogen trioxide is only isolable at low temperatures, i.e. in the liquid and solid phases. At higher temperatures the equilibrium favors the constituent gases. Kdiss = 193 kPa (25 °C).

Structure and bonding
Dinitrogen trioxide has an unusually long N−N bond at 186 pm. Whereas N−N bonds are more often similar to that in hydrazine (145 pm), some other oxides of nitrogen do possess long N−N bonds, including dinitrogen tetroxide (175 pm). The N2O3 molecule is planar and exhibits Cs symmetry. The dimensions displayed below come from microwave spectroscopy of low-temperature, gaseous N2O3:



It is the anhydride of the unstable nitrous acid (HNO2), and produces it when mixed into water. An alternative structure might be anticipated for the true anhydride, i.e. O=N-O-N=O, but this isomer is not observed. If the nitrous acid is not then used up quickly, it decomposes into nitric oxide and nitric acid. Nitrite salts are sometimes produced by adding N2O3 to solutions of bases:
 * N2O3 2 NaOH  →  2 NaN2O2  +  H2O

Nitrogen Trioxide = NO3