The structures of tetrasulphur tetranitride (a) and tetrathiazyl tetrafluoride (b) are given below. The statement that correctly describes the structures is :
Compound (b) satisfies Hückel rule and is more stable than (a)
The stem carries drawn structures with marked bond lengths; the four options are text and are reproduced in full above.
Compound (a) is S4N4, tetrasulphur tetranitride. It is not a planar aromatic ring at all but a cradle-shaped cage: the eight-membered S-N ring folds so that the two pairs of sulphur atoms bend towards each other, giving cross-ring S···S contacts of about 2.58 Å. The S-N distances are all similar (around 1.62-1.63 Å), showing delocalisation around the ring, but the folded geometry means the π system is not a planar Hückel system. S4N4 is also endothermic and shock sensitive — it detonates on grinding or heating.
Compound (b) is S4N4F4, tetrathiazyl tetrafluoride, obtained by fluorinating S4N4. Adding fluorine to each sulphur uses up the sulphur lone pairs and converts the cage into an eight-membered ring that adopts a puckered, crown-like conformation. The alternating bond lengths marked on the structure (about 1.54 Å and 1.66 Å) are the diagnostic point: unequal, alternating S-N distances mean localised single and double bonds rather than a delocalised system.
So neither compound is a genuine planar aromatic ring obeying the Hückel 4n+2 rule, and the bond-length alternation in (b) specifically rules out extensive delocalisation there.
Per the official final answer key the answer is option (C).
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