Disulfur dinitride

Disulfur dinitride is the chemical compound with the formula S2N2.

Preparation and reactions
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,
 * S4N4 + 8 Ag → 4 Ag2S + 2 N2
 * S4N4 → 2 S2N2

An alternative uses the less explosive S4N3Cl.

S2N2 decomposes explosively above 30°C, and is shock sensitive. It readily sublimes, and is soluble in diethyl ether. Traces of water cause it to polymerize into S4N4. In the solid state it spontaneously polymerizes forming (SN)_{n}. It forms adducts with Lewis acids via a nitrogen atom, e.g. S2N2*BCl3, S2N2*2AlCl3, S2N2*SbCl5, S2N2*2SbCl5.

Structure and bonding
The S2N2 molecule is a four-membered ring, with alternating S and N atoms. The S2N2 molecule is a resonance hybrid of many contributing structures. In one of those structures, one S atom has valence 4 and the other S atom has valence 2, and both N atoms has valence 3. In the other structure both S atoms have valence 2 and both N atoms has valence 3, and one of the S atoms has a charge of +1, and one of the N atoms has a charge of −1. 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°. The S2N2 molecule is isoelectronic with the cyclic S4(2+) dication and has 6π electrons. The bonding has been investigated using a spin-coupled valence bond method and is described as having four framework sigma bonds, with the N atoms bearing a high negative charge and the S atoms a corresponding positive charge. Two π electrons from the sulfur atoms are coupled across the ring making the molecule overall a singlet diradical.