Disulfur dinitride

Passing gaseous S₄N₄ over silver metal wool at 250–300 °C at low pressure (1 mmHg) yields cyclic S₂N₂. The silver reacts with the sulfur produced by the thermal decomposition of the S₄N₄ to form Ag₂S, and the resulting Ag₂S catalyzes the conversion of the remaining S₄N₄ into the four-membered ring S₂N₂,^[1]

S₄N₄ + 8 Ag → 4 Ag₂S + 2 N₂

S₄N₄ → 2 S₂N₂

An alternative uses the less explosive S₄N₃Cl.^[2]

S₂N₂ decomposes explosively above 30 °C, and is shock sensitive.^[1] It readily sublimes, and is soluble in diethyl ether. Traces of water^[2] or other nucleophiles cause it to polymerize into S₄N₄.^[3] In the solid state it spontaneously polymerizes forming (SN)_n,^[1] catalyzed by radical impurities.^[3] It forms adducts with Lewis acids via a nitrogen atom, e.g. S₂N₂·BCl₃, S₂N₂·2AlCl₃, S₂N₂·SbCl₅, S₂N₂·2SbCl₅.^[2][4]

The S₂N₂ molecule is a four-membered ring, with alternating S and N atoms. The molecule is almost square and planar. The S–N bond lengths are 165.1 pm and 165.7 pm and the bond angles are very close to 90°.^[1]

The S₂N₂ molecule is isoelectronic with the cyclic S2+4 dication,^[2] but the oft-quoted description of S₂N₂ bearing 6 π electrons is inaccurate. According to spin-coupled valence bond analysis, the molecule is instead better interpreted as two conjoined sulfur diimides: each sulfur atom bears a single +1 formal charge and one lone pair and each nitrogen atom a single -1 formal charge and two lone pairs. The remaining two electrons, each located on sulfur, are coupled across the ring as a singlet diradical. Loosely, the HOMO and LUMO of S₂N₂ are each nonbonding orbitals corresponding to these two electrons.^[5]

• Sulfur nitride
• Tetrasulfur tetranitride
• Polythiazyl
• Square planar molecular geometry