Coordination Compounds

Why This Chapter Matters

Coordination compounds give 5-7 marks in CBSE Class 12. Nomenclature, Werner's theory, isomerism, Crystal Field Theory, and stability are all tested — one full question.

Core Concepts

1. Key Terms

Central metal atom/ion: Transition metal that accepts electron pairs.

Ligands: Lewis bases that donate lone pairs to central metal. Can be:

•Monodentate: one donor atom (Cl⁻, OH⁻, NH₃, H₂O, CN⁻, NO₂⁻, CO)

•Bidentate: two donor atoms (en = ethylenediamine, ox²⁻ = oxalate)

•Polydentate: multiple donor atoms (EDTA = hexadentate)

Chelate: Complex formed with polydentate ligand (ring structure). More stable (chelate effect).

Coordination number: Number of coordinate bonds from ligands to central atom.

Coordination sphere: Central atom + ligands (written in [ ]).

2. Nomenclature Rules

1.Cation named first, then anion.

2.In complex: ligands alphabetically, then metal.

3.Anionic ligands: suffix -o (Cl⁻→chloro, CN⁻→cyano, NO₂⁻→nitro, OH⁻→hydroxo, O²⁻→oxo).

4.Neutral ligands: use name (NH₃→ammine, H₂O→aqua, CO→carbonyl, NO→nitrosyl).

5.Oxidation state of metal: Roman numerals in parentheses.

6.If complex is anion: metal gets suffix -ate (cobalt→cobaltate, iron→ferrate, copper→cuprate).

7.Bridging ligand: prefix μ-.

Examples:

[Co(NH₃)₆]³⁺ = hexaamminecobalt(III) ion

[Fe(CN)₆]⁴⁻ = hexacyanoferrate(II) ion

[PtCl₂(NH₃)₂] = diamminedichloridoplatinum(II)

K₄[Fe(CN)₆] = potassium hexacyanoferrate(II)

3. Werner's Theory

Primary valency: oxidation state of metal (ionisable, satisfied by anions outside coordination sphere).

Secondary valency: coordination number (non-ionisable, satisfied by ligands inside [ ]).

Octahedral and square planar geometries for 6 and 4 ligands respectively.

4. Isomerism

Structural isomers:

Ionisation isomers: [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br.

Linkage isomers: NO₂ can bind through N (nitro) or O (nitrito).

Coordination position: ligands distributed differently.

Solvate (hydrate): [Cr(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl]Cl₂·H₂O.

Stereoisomers:

Geometrical (cis-trans): MA₂B₂ type square planar or octahedral.

cis: same ligands adjacent. trans: same ligands opposite.

Example: cis-[PtCl₂(NH₃)₂] (cisplatin — anticancer drug) vs trans-[PtCl₂(NH₃)₂].

Optical isomers: non-superimposable mirror images. Example: [Co(en)₃]³⁺.

5. Crystal Field Theory (CFT)

Metal d-orbitals split in presence of ligands (crystal field).

Octahedral field: d orbitals split into t₂g (lower, 3 orbitals) and eₘ (higher, 2 orbitals).

Δₒ (crystal field splitting energy): energy difference between t₂g and eₘ.

Strong field ligands (large Δₒ, low spin): CO, CN⁻, NO₂⁻, en, NH₃.

Weak field ligands (small Δₒ, high spin): I⁻, Br⁻, Cl⁻, F⁻, OH⁻, H₂O.

Spectrochemical series: CO>CN⁻>NO₂⁻>en>NH₃>H₂O>OH⁻>F⁻>Cl⁻>Br⁻>I⁻

High spin vs Low spin:

If Δₒ > pairing energy → electrons pair in t₂g first (low spin, fewer unpaired e⁻).

If Δₒ < pairing energy → electrons go to eₘ before pairing (high spin, more unpaired e⁻).

Colour in coordination compounds: absorption of visible light causes d-d transitions. Complementary colour is observed.

PYQs (CBSE)

CBSE 2023: IUPAC name of [CrCl₂(NH₃)₄]⁺?

Ligands alphabetically: ammine (NH₃), chlorido (Cl). 4 ammine, 2 chlorido, Cr(III) (charge: +3-2+4×0=+1 confirms +3 for Cr if Cl⁻ = -1 each: +3-2=+1 ✓). Name: tetraamminedichloridochromium(III) ion.

CBSE 2022: What is chelate effect? Why are chelate complexes more stable?

Chelate effect: extra stability of complexes formed by polydentate (chelating) ligands compared to monodentate ligands.

Reason: Formation of ring structures increases entropy (more disorder). Also, loss of chelating ligand requires breaking multiple bonds simultaneously → less likely.

Revision Notes

LIGANDS:
Monodentate: Cl⁻,OH⁻,NH₃,H₂O,CN⁻,CO
Bidentate: en (ethylenediamine), ox²⁻
Hexadentate: EDTA

NOMENCLATURE:
Anionic ligands: -o suffix | Neutral: own name (ammine,aqua,carbonyl)
Metal in anion complex: -ate suffix
Oxidation state: Roman numerals

ISOMERISM:
Structural: ionisation, linkage, solvate, coordination position
Stereoisomers: geometrical (cis/trans), optical (mirror images)

CFT:
t₂g (lower) + eg (higher) in octahedral field
Strong ligands: CO,CN⁻,NH₃ → low spin (Δₒ large)
Weak ligands: Cl⁻,Br⁻,I⁻,H₂O → high spin (Δₒ small)
Colour: d-d transition absorbs visible light