Chemistry of Nitrides of Sulphur

Sulphur Nitrides (S₄N₄, S₂N₂, (SN)_x)

High-Yield for JEE Advanced, IIT-JAM, GATE, CSIR-NET | Repeatedly Asked in Previous Years

1. Important Sulphur Nitrides

Compound	Common Name	State at Room Temp	Stability
S4N4	Tetrasulphur tetranitride	Orange-red crystalline solid	Most stable & most important
S2N2	Disulphur dinitride	Colorless crystalline solid	Highly explosive
(SN)x	Polymeric sulphur nitride	Golden-yellow lustrous solid	Metallic conductor, Superconductor at 0.26 K

2. Preparation of S₄N₄ (Most Important Reaction)

Standard Method (Asked in JEE Adv, CSIR-NET):
6 S₂Cl₂ + 16 NH₃ (l) → S₄N₄ + S₈ + 12 NH₄Cl
(or more commonly written as)
6 S₂Cl₂ + 4 NH₃ → S₄N₄ + 8 S + 12 HCl (in presence of base to absorb HCl)

Alternative Methods

• 4 NH₄Cl + 6 S₂Cl₂ → S₄N₄ + 8 S + 16 HCl
• 10 S + 4 NH₃(liquid) → S₄N₄ + 6 H₂S

3. Physical Properties of S₄N₄

Property	Observation
Colour & state	Orange-red crystals
Solubility	Insoluble in water but Soluble in organic solvents
Thermal decomposition	S4N4 → 2 S2N2 → 2 (SN)x (on heating above 130°C)
Standard enthalpy of formation	+460 kJ/mol
Heat capacity (solid)	210 J/mol·K at 298 K
Thermal conductivity	0.35 W/m·K

Chemical Properties

1. Reaction with BF₃ and SbCl₅
   S₄N₄ forms adduct with BF₃ and SbCl₅.
   S₄N₄ + BF₃ → S₄N₄.BF₃
   S₄N₄ + SbCl₅ → S₄N₄.SbCl₅
2. Hydrolysis
   S₄N₄ hydrolysed with dilute and concentration alkali solutions.
   2 S₄N₄ + 6 OH⁻(dil.) + 9 H₂O → S₂O₃²⁻ + 2 S₃O₆²⁻ + 8 NH₃
   S₄N₄ + 6 OH⁻(conc.) + 3 H₂O → S₂O₃²⁻ + 2 SO₃²⁻ + 4 NH₃
3. Decomposition
   S₄N₄ is thermodynamically unstable and is a shock- and friction-sensitive primary explosive, decomposing into stable gaseous nitrogen and elemental sulfur upon detonation or heating above its melting point.
   S₄N₄ → 2 N₂ + 4 S   (ΔG = -320 kJ/mol)
4. Fluorination
   S₄N₄ + 4 AgF₂ → (NS)₄F₄ + 4 AgF
5. Reducion by SnCl₂
   S₄N₄ reduced to tetrasulphur tetraimide with SnCl₂ in ethanol.
   S₄N₄ → S₄(NH)₄
6. Oxidation by Chlorine
   S₄N₄ oxidized by chlorine to trithiazyle trichloride.
   S₄N₄ + Cl₂ → (NS)₃Cl₃

4. Structure & Bonding of S₄N₄ (Extremely Important)

Cradle or basket-shaped structure

• 8-membered ring (S–N–S–N–S–N–S–N) with two S–S cross links
• All S–N bonds are equal → extensive delocalisation (36 π electrons in ring)
• Each S has 10 electrons in valence shell → hypervalent
• Contains dπ–pπ bonding and partial double bond character
• Bond length: S–N ≈ 1.62 Å (between single bond (≈1.74 Å) and double bond (≈1.55 Å))
• Bond energies: S-N ≈ 270 kJ/mol. (Intermediate between typical S-S (226 kJ/mol) and N-N (167 kJ/mol) single bonds but significantly lower than S≡N triple bonds (532 kJ/mol))
• bond order: between 1.2 and 1.3 for all S-N connections

5. S₂N₂ → (SN)_x (Most Favourite Reaction in CSIR-NET)

S₄N₄ on silver wool at 130°C → 2 S₂N₂ (colourless crystals)
S₂N₂ on further heating or standing → (SN)_x (golden polymeric solid)

6. Polymeric Sulphur Nitride (SN)_x – Superconductor

• Golden-yellow, lustrous, metallic appearance
• First known polymeric superconductor (Tc = 0.26 K)
• Highly conducting even at room temperature (better than mercury)
• Structure: Zig-zag chains of alternating S and N atoms with partial π-bonding


Structure of Polymeric Sulphur Nitride (SN)x

• Contains dπ–pπ overlap giving metallic conductivity

7. Uses & Applications (Asked in GATE/CSIR-NET)

• S₄N₄ → Precursor for (SN)ₓ polymer
• (SN)_x → Potential material in molecular electronics, superconductors
• Used in preparation of other S–N compounds
• Study of inorganic ring systems and electron-delocalised structures

8. Previous Year Exam Questions Summary

Exam & Year	Topic Asked
JEE Advanced 2018	Structure of S4N4
CSIR-NET Dec 2019	Preparation of S₄N₄ from S₂Cl₂ + NH₃
GATE Chemistry 2021	Superconducting property of (SN)ₓ
IIT-JAM 2022	Thermal decomposition: S4N4 → S2N2 → (SN)ₓ
CSIR-NET June 2023	Bonding in S4N4 and (SN)x (dπ–pπ)

One-Liner Revision:
S₄N₄ (orange explosive) → heat → S₂N₂ → polymerises to (SN)_x (golden superconductor at 0.26 K)
S₄N₄ has cradle structure with equal S–N bonds due to delocalisation.

Must Read: Sulphur Nitride MCQs asked in CSIR-NET, GATE, IIT-JAM NEET, IIT-JEE