Chemistry of Sodium Nitroprusside

Na₂[Fe(CN)₅NO]·2H₂O – Structure, Properties & Analytical Uses

Basic Information

Property	Value
Systematic name	Disodium pentacyanonitrosylferrate(II) dihydrate
Formula	Na2[Fe(CN)5NO]·2H2O
Molar mass	297.95 g/mol
Appearance	Deep ruby-red transparent crystals
Solubility	Highly soluble in water; slightly soluble in ethanol

Structure & Bonding

[Fe(CN)₅NO]²⁻ – Octahedral complex

Ruby-red crystals of Na₂[Fe(CN)₅NO]·2H₂O

• Octahedral complex with five CN⁻ and one NO ligand.
• NO is coordinated as NO⁺ (nitrosyl cation) → linear Fe–N–O (Bond angle ~176.2°.).
• Formal oxidation state of Fe = +2
  (5 CN⁻ = –5, NO⁺ = +1 → Fe = +2)
• Low-spin d⁶ complex → diamagnetic.
• Intense red colour due to MLCT (metal-to-ligand charge transfer) involving NO⁺.

Note: The bonding in sodium nitroprusside involves both coordinate covalent bonds from the cyanide and nitrosyl ligands to iron and significant π-backbonding from the iron to the ligands, particularly the NO ligand, stabilizing the linear nitrosyl coordination and the unique electronic structure of the complex.

Preparation

Laboratory method:

Sodium Nitroprusside can be prepared in the laboratory by treating potassium ferrocyanide with nitric acid followed by neutralization with sodium carbonate. This method yields the ruby-red crystals of the dihydrate form, Na₂[Fe(CN)₅NO]·2H₂O.

K₄[Fe(CN)₆] + 6 HNO₃ → H₂[Fe(CN)₅NO] + CO₂ + NH₄NO₃ + 4 KNO₃
H₂[Fe(CN)₅NO] + Na₂CO₃ → Na₂[Fe(CN)₅NO] + CO₂ + H₂O

Procedure:

• Dissolve 20 g of K₄[Fe(CN)₆] in ~100 mL distilled water in a beaker and heat to 60-70°C with stirring.
• Slowly add concentrated HNO₃ dropwise while maintaining temperature at 65-75°C; continue stirring for 2-4 hours until reaction completes (test by no blue precipitate with ferrous sulfate).
• Cool the mixture, then add Na₂CO₃ solution gradually until neutral (pH ~7), precipitating the product.
• Filter the ruby-red crystals, wash with cold water and ethanol, and dry in vacuum desiccator away from light.

Important Chemical Properties

1. Aqueous solution is unstable in light → decomposes to Prussian blue (photodegradation).
2. Reacts with H₂S → deep violet colour (S²⁻ test).
3. With NaOH → red colour → brown precipitate of Fe(OH)₃.
4. With ketones (acetone) → violet colour in alkaline medium (Legal's test).
5. Releases NO in vivo → powerful vasodilator (drug: Nipride, Nitropress).

Analytical Uses (Qualitative Tests)

Test	Reagent	Observation	Ion/Compound Detected
Sulphide (S²⁻) test	Freshly prepared sodium nitroprusside	Intense violet/purple colour	S²⁻ (e.g., Na2S, (NH4)2S)
Legal’s test for acetone	Nitroprusside + NaOH + acetone	Wine-red/violet colour	Acetone (in urine for diabetic ketoacidosis)
Test for reducing agents	Alkaline nitroprusside	Blue → green → yellow	SO3²⁻, S2O3²⁻, etc.

Sulphide Test Reaction (Most Important)

[Fe(CN)₅NO]²⁻ + S²⁻ → [Fe(CN)₅NOS]⁴⁻
(Intense violet complex – exact structure debated, possibly with Fe–S bonding)

Medical Use – Sodium Nitroprusside Injection

• Potent intravenous antihypertensive (emergency).
• Rapidly releases NO → direct vasodilation.
• Very short half-life (~2 minutes).
• Metabolized to cyanide → risk of cyanide poisoning if prolonged use.
• Also used in cardiac surgery and acute heart failure.

Summary Table – Quick Revision

Feature	Details
Colour of crystal	Ruby red
Oxidation state of Fe	+2
NO ligand	NO⁺ (nitrosyl cation)
Test for sulphide	Violet colour
Test for acetone	Wine-red colour (Legal's test)
Medical use	Acute hypertension (NO donor)
Stability	Decomposes in light → store in amber bottle

Sodium nitroprusside – one of the most beautiful and versatile coordination compounds in the chemistry laboratory and hospital.