Sulfur forms a variety of covalent binary nitrides, but the two most interesting
ones are tetrasulfur tetranitride, S4N4, and disulfur dinitride, S2N2, because
they are precursors to an unusual polymer called polythiazyl, (SN)x. This
polymeric sulfur nitride is unusual because, even though it is composed solely
of two nonmetals, it exhibits some properties normally associated only with
metals. The best preparation of S4N4 involves bubbling NH3 into a heated
(50° C) solution of S2Cl2 dissolved in CCl4 or C6H6.
6S2Cl2 + 16NH3 -> S4N4 + S8 + 12NH4Cl
Tetrasulfur tetranitride forms thermochromic crystals, which are crystals that
change colour with temperature. They are red at temperatures above 100° C,
orange at 25° C, and colourless at -190° C. The crystals are stable in air but
will explode in response to shock or friction. The compound has a cage
structure with a plane of four nitrogen atoms with two sulfur atoms above and
below the plane (see Figure 15). When S4N4 vapour is pumped through silver
wool at 250°–300° C and low pressure (less than 1.0 mm Hg), an unstable
dimer, S2N2, can be condensed. This compound has an essentially square
structure with alternating sulfur and nitrogen atoms. Like S4N4, it is sensitive
to shock and can explode when heated to temperatures higher than 30° C.
At 25° C, S2N2 slowly polymerizes through a ring opening mechanism to
polythiazyl, (SN)x. This rather amazing material has a bronze colour, a metallic
lustre, and the electrical and thermal conductivity of a metal. It becomes a
superconductor at 0.26 kelvin (K; see superconductivity). |