N-S

Tetrasulfur tetranitride is an inorganic compound with the formula S₄N₄. This gold-poppy coloured solid is the most important binary sulfur nitride, which are compounds that contain only the elements sulfur and nitrogen. more...

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It is a precursor to many S-N compounds and has attracted wide interest for its unusual structure and bonding.

Nitrogen and sulfur have similar electronegativities, which is the ability to attract electrons. When atoms are so evenly matched, they often form extensive families of covalently bonded structures. Indeed, a large number of S-N and S-NH compounds are known with S₄N₄ as their parent.

Structure

S₄N₄ adopts an unusual “extreme cradle” structure, with D_2d point group symmetry. It can be viewed as a derivative of a hypothetical eight-membered ring of alternating sulfur and nitrogen atoms. The pairs of sulfur atoms across the ring are separated by 2.586 Å, resulting in a cage-like structure as determined by single crystal X-Ray diffraction. The nature of the "transannular" S---S interactions remains a matter of investigation because it is significantly shorter than the sum of the van der Waal's distances. but has been explained in the context of molecular orbital theory. The bonding in S₄N₄ is considered to be delocalized, which is indicated by the fact that the bond distances between neighboring sulfur and nitrogen atoms are almost the same.

Properties

S₄N₄ is stable to air. It is, however, unstable in the thermodynamic sense with a positive heat of formation of 460 kJ/mol). This endothermic heat of formation anticipates its inherent instability, and originates in the difference in energy of S₄N₄ compared to its highly stable decomposition products:

S₄N₄ → 2 N₂ + 0.5 S₈

It is not really very unusual for complex molecules to be unstable in a thermodynamic sense yet stable kinetically; this situation describes many compounds. This combination of kinetic stability and thermodynamic instability is, however, uncommon for very simple compositions, such as sulfur nitride. Because one of its decomposition products is a gas, S₄N₄ is an explosive. Purer samples tend to be more explosive. Small samples can be detonated by striking with a hammer.

Read more at Wikipedia.org