Coordination Compounds Class 12 Chemistry Notes

• Coordination entity: the whole metal-plus-ligand unit, written in square brackets [Fe(CN)₆]⁴⁻.
• Ligand: any molecule/ion that donates a lone pair — NH₃, Cl⁻, H₂O, CN⁻, en, EDTA.
• Monodentate vs polydentate: en (bidentate) and EDTA (hexadentate); a ring formed with the metal is a chelate.
• Coordination number: number of donor atoms, not ligands, attached to the metal.
• Counter ions: ions outside the brackets that neutralise the complex; they are ionisable in solution.

MCQ Questions

Werner solved the puzzle of constant-valence coordination compounds by proposing two valencies for the metal: a primary (ionisable) valency satisfied by anions and a secondary valency (the fixed coordination number) satisfied by ligands, arranged in space around the metal.

• Primary valency = oxidation state; it is non-directional and satisfied by counter ions.
• Secondary valency = coordination number; it is directional and fixes the geometry — 4 gives tetrahedral/square planar, 6 gives octahedral.
• Werner's isomers: CoCl₃·5NH₃ (1 Cl⁻ free), CoCl₃·4NH₃ (2 Cl⁻ free) etc. — the number of free Cl⁻ ions corresponds to the primary valency.
• Coordination compounds preserve their secondary valency in solution; only the primary valency ionises.

MCQ Questions

IUPAC naming follows a fixed order: write ligands alphabetically (cation before anion in a complex), then the metal. Prefixes number identical ligands: di/tri for simple (chloro, ammine) but bis/tris/tetrakis for ligands with built-in numbers (en, EDTA).

• Order: alphabetical by ligand name (ignoring di/tri prefixes), then metal, then oxidation state in Roman numerals.
• Cationic/neutral complexes: metal name unchanged. Anionic complexes: metal ends in −ate (ferrate, cobaltate, cuperate).
• Examples: [Co(NH₃)₆]Cl₃ = hexaamminecobalt(III) chloride; K₃[Fe(CN)₆] = potassium hexacyanidoferrate(III).
• Counter ion named as usual (chloride, sulphate).

MCQ Questions

• cis-[Pt(NH₃)₂Cl₂] (cis-platin) is an anti-cancer drug; trans-platin is not.
• Square planar MA₂B₂: two geometric isomers. Octahedral MA₃B₃: fac and mer.
• Optical isomers rotate plane-polarised light in opposite senses; a 1:1 mixture is racemic.

MCQ Questions

Valence bond theory explains complex geometry by hybridisation of the metal's d, s and p orbitals. The metal accepts ligand lone pairs into vacant hybrid orbitals, and the number of such empty hybrid orbitals fixes the coordination number and shape.

• CN 4 → sp³ (tetrahedral, e.g. [Ni(CO)₄]) or dsp² (square planar, e.g. [Ni(CN)₄]²⁻).
• CN 6 → d²sp³ inner orbital (low spin) or sp³d² outer orbital (high spin).
• Inner-orbital complexes pair electrons using the inner (n−1)d orbitals; outer-orbital use nd orbitals.
• [Co(NH₃)₆]³⁺: Co³⁺ d⁶ + strong NH₃ → d²sp³, all electrons paired, diamagnetic.
• [CoF₆]³⁻: Co³⁺ d⁶ + weak F⁻ → sp³d², four unpaired electrons, paramagnetic.

MCQ Questions

Crystal field theory views metal–ligand bonding as purely electrostatic: the ligand field splits the d orbitals into lower-energy t₂g (dxy, dyz, dzx) and higher-energy eg (dx²−y², dz²) sets in octahedral complexes. The splitting energy Δₒ decides electron arrangement.

• Spectrochemical series (weak→strong): I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < py < NH₃ < en < NO₂⁻ < CN⁻ < CO.
• Weak field → high spin (e.g. [CoF₆]³⁻); strong field → low spin (e.g. [Co(CN)₆]³⁻).
• Colour: d–d transition between t₂g and eg absorbs visible light.
• Magnetic moment from unpaired electrons: μ = √(n(n+2)) BM.

• cis-platin — anti-cancer drug.
• EDTA — chelation therapy for lead/mercury poisoning, water softening.
• Chlorophyll — Mg porphyrin complex; haemoglobin — Fe porphyrin complex; vitamin B12 — cobalt complex.
• Metal carbonyls, dyes, photographic fixing, dyeing and tanning industries.

MCQ Questions

Exercises (NCERT Q1–Q3)

1. 1
   Explain Werner's theory of coordination compounds with the help of a suitable example.
2. 2
   What is the coordination number and oxidation state of cobalt in [Co(NH3)5Cl]Cl2?
3. 3
   Draw the geometrical isomers for [Pt(NH3)2Cl2].

Answers at a glance

1. 1
   Werner's theory distinguishes primary (ionisable) and secondary (non‑ionisable, coordination) valencies; e.g., [Co(NH3)6]Cl3 shows 6 NH3 in coordination sphere and 3 ionisable Cl.
2. 2
   Coordination number = 6; oxidation state of Co = +3.
3. 3
   Cis‑ and trans‑isomers exist for [Pt(NH3)2Cl2].

MCQ Questions

1 Mark Questions

1. 1
   Define ligand and give an example.
2. 2
   What is coordination number?
3. 3
   Write the formula of tetraamminezinc(II) sulphate.

2 Mark Questions

1. 1
   Using VBT, explain why [Ni(CN)4]2‑ is diamagnetic.
2. 2
   What is chelate effect? Give an example.

3 Mark Questions

1. 1
   Explain crystal field splitting in an octahedral field. How does the magnitude of Δo depend on the ligand?
2. 2
   Discuss the geometrical isomerism in [Co(NH3)4Cl2]+.

5 Mark Questions

1. 1
   Compare VBT and CFT with respect to their ability to explain magnetic properties, colour and isomerism in coordination compounds.
2. 2
   A coordination compound CrCl3·6H2O is mixed with AgNO3 solution. It gives a precipitate of AgCl. On adding excess NH3, the precipitate dissolves. Identify the complex ion present and write its formula.