Introduction
Coordination compounds are one of the most scoring chapters in NEET Inorganic Chemistry. Werner's coordination theory, IUPAC naming, isomerism, bonding theories, and colour/magnetism typically yield 2–3 direct NEET questions.
Key Terminology
- Central metal ion: Transition metal accepting electron pairs (Lewis acid)
- Ligands: Ions/molecules donating electron pairs (Lewis base)
- Coordination number: Number of coordinate bonds to central metal
- Coordination sphere: Central metal + ligands (written in square brackets)
Types of Ligands
- Monodentate: Cl⁻, NH₃, H₂O, CN⁻, CO (one donor atom)
- Bidentate: en (ethylenediamine), oxalate C₂O₄²⁻
- Polydentate: EDTA (hexadentate, 6 donor atoms)
- Ambidentate: Can bond via two different atoms — NO₂⁻ (N or O), SCN⁻ (S or N)
IUPAC Naming Rules
- Cation named before anion
- Inside bracket: ligands (alphabetical, with prefixes di, tri…) then metal with oxidation state in Roman numerals
- Anionic complex ends in -ate; use Latin names for Fe (ferrate), Cu (cuprate), Pb (plumbate)
Example: [Co(NH₃)₄Cl₂]Cl → tetraamminedichloridocobalt(III) chloride
Isomerism
Structural isomers:
- Linkage: [Co(ONO)(NH₃)₅]²⁺ vs [Co(NO₂)(NH₃)₅]²⁺
- Ionisation: [Co(SO₄)(NH₃)₅]Br vs [Co(Br)(NH₃)₅]SO₄
- Coordination: [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]
Stereoisomers:
- Geometric (cis-trans): square planar MA₂B₂ or octahedral MA₄B₂
- Optical: chiral complexes with no plane of symmetry (e.g., [Co(en)₃]³⁺)
Crystal Field Theory (CFT)
In an octahedral field, d-orbitals split into t₂g (lower, dx²-y², dz²) and eg (higher). Δo = Crystal Field Splitting Energy.
- Strong field ligands (CN⁻, CO, en): large Δo → low spin, fewer unpaired e⁻, diamagnetic/less paramagnetic
- Weak field ligands (F⁻, Cl⁻, H₂O): small Δo → high spin, more unpaired e⁻, paramagnetic
Spectrochemical series (partial): I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < en < CN⁻ < CO
Colour in Coordination Compounds
Colour arises from d-d transition. The complementary colour of absorbed light is observed. Complexes with d⁰ (no d electrons, e.g. TiO₂) or d¹⁰ (Cu⁺) are colourless.
Valence Bond Theory
Hybridisation determines geometry: sp³ (tetrahedral), dsp² (square planar, inner orbital), sp³d² (octahedral outer), d²sp³ (octahedral inner).