Last Updated: April 2026
NEET 2027 | CHEMISTRY — COORDINATION COMPOUNDS
Complete guide to NCERT Class 12 Chapter 9 — coordination compounds, covering Werner’s theory, ligand types, IUPAC rules, isomerism, Crystal Field Theory, and biological applications.
Coordination Compounds (NCERT Class 12, Chapter 9) is one of the most conceptual chapters in NEET Chemistry. It bridges inorganic and organic chemistry through organometallic compounds and has strong connections to biochemistry (haemoglobin, chlorophyll). The chapter carries 4–6 questions per NEET paper on average.
| Topic | Questions/Year | Common Question Type |
|---|---|---|
| IUPAC Nomenclature | 1–2 | Name a complex / find formula from name |
| Isomerism | 1–2 | Identify type of isomerism |
| CFT and Magnetic Properties | 1–2 | Calculate magnetic moment, CFSE |
| Biological/Industrial Applications | 0–1 | Identify metal in biomolecules |
Introduction: What are Coordination Compounds?
Coordination compounds (or complex compounds) are compounds in which a central metal atom or ion is bonded to a number of ligands through coordinate bonds. The central metal atom/ion is a Lewis acid, while ligands are Lewis bases.
Examples: [Fe(CN)₆]⁴⁻, [Co(NH₃)₆]³⁺, [Pt(Cl)₂(NH₃)₂], K₄[Fe(CN)₆] (potassium ferrocyanide)
Werner’s Theory of Coordination Compounds
Alfred Werner (Nobel Prize 1913) proposed that metals exhibit two types of valency:
- Primary valence (ionisable valence): The charge of the metal ion; satisfied by negative ions. These ions can be precipitated.
- Secondary valence (coordination number): Number of ligands bonded to the metal; always satisfied. These are inside the coordination sphere.
Example: In [Co(NH₃)₆]Cl₃ — primary valence = 3 (3 Cl⁻ ions outside), secondary valence = 6 (6 NH₃ molecules).
Types of Ligands
| Type | Definition | Examples |
|---|---|---|
| Monodentate | One donor atom | Cl⁻, NH₃, H₂O, CN⁻, CO, NO |
| Bidentate | Two donor atoms | en (ethylenediamine), oxalate (C₂O₄²⁻) |
| Polydentate | Multiple donor atoms | EDTA (hexadentate — 4O + 2N), dmg (bidentate) |
| Ambidentate | Can donate via 2 different atoms | NO₂⁻ (N or O), SCN⁻ (S or N), CN⁻ (C or N) |
| Bridging (μ) | Links two metal centres | μ-Cl, μ-OH, μ-CO |
Chelate ligands: Polydentate ligands that form ring structures with the metal. EDTA is the most important chelating agent — forms 5 chelate rings with a metal ion.
IUPAC Nomenclature of Coordination Compounds
Rules (in order):
- Name the cation first, then the anion.
- Within the complex ion, name ligands first (in alphabetical order), then the metal.
- Ligand names: anionic ligands end in ‘-o’ (chloro, cyano, nitrito, oxalato); neutral ligands use IUPAC name EXCEPT aqua (H₂O), ammine (NH₃), carbonyl (CO), nitrosyl (NO).
- Multiplicative prefixes: di, tri, tetra (for simple ligands); bis, tris, tetrakis (for complex ligand names).
- Metal oxidation state in Roman numerals in parentheses.
- If complex ion is anionic, the metal name ends in ‘-ate’. E.g., ferrate, cobaltate, platinate.
[Co(NH₃)₅Cl]Cl₂: Pentaamminechloridicobalt(III) chloride
K₂[PtCl₄]: Potassium tetrachloridoplatinate(II)
[Cr(en)₂Cl₂]Cl: Dichlorobis(ethylenediamine)chromium(III) chloride
[Fe(CN)₆]³⁻: Hexacyanidoferrate(III) ion
[CoCl(NO₂)(NH₃)₄]NO₃: Tetraamminechloridonitrito-N-cobalt(III) nitrate
Crystal Field Theory (CFT)
CFT treats the interaction between metal d-orbitals and ligand electrons as purely electrostatic (ligands are point charges).
d-Orbital Splitting in Octahedral Field
In an octahedral complex, the 5 degenerate d orbitals split into:
- t₂g (lower energy): dxy, dyz, dxz — point between axes, less repulsion from ligands along axes
- eg (higher energy): dx²-y², dz² — point directly toward ligands, more repulsion
Energy splitting: Δo (crystal field splitting energy)
t₂g = −0.4Δo, eg = +0.6Δo per electron
High Spin vs Low Spin
- Strong field ligands (large Δo): electrons pair up → low spin (more paired). E.g., CN⁻, CO, NO, en
- Weak field ligands (small Δo): electrons fill all orbitals first (Hund’s rule) → high spin (more unpaired). E.g., I⁻, Br⁻, Cl⁻, F⁻, H₂O
Spectrochemical Series (weak → strong field):
I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < NO₃⁻ < F⁻ < OH⁻ < H₂O < NCS⁻ < NH₃ < en < bipyridyl < phen < CN⁻ < CO
CFSE Calculation
CFSE = (number of t₂g electrons × −0.4Δo) + (number of eg electrons × +0.6Δo) + pairing energy (P) penalty if applicable.
Example: [Fe(CN)₆]⁴⁻ — Fe²⁺ is d⁶, strong field (CN⁻), low spin → t₂g⁶ eg⁰
CFSE = 6 × (−0.4Δo) + 0 = −2.4Δo
Tetrahedral and Square Planar Complexes
| Property | Octahedral | Tetrahedral | Square Planar |
|---|---|---|---|
| CN | 6 | 4 | 4 |
| Splitting | Δo (t₂g & eg) | Δt = 4/9 Δo (e & t₂) | Δsp > Δo |
| Spin state | High or low spin | Almost always high spin | Always low spin (d⁸) |
| Common for | Most transition metals | Zn²⁺, Ni²⁺ (weak field) | Pt²⁺, Pd²⁺, Au³⁺, Ni²⁺ (strong field) |
| Colour | Coloured (if d-d transition) | Coloured | Coloured |
Isomerism in Coordination Compounds
Structural Isomerism
- Ionisation isomerism: Different ions in/outside the coordination sphere. E.g., [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br
- Hydrate (solvate) isomerism: Water inside or outside coordination sphere. E.g., [Cr(H₂O)₆]Cl₃ (violet) vs [Cr(H₂O)₅Cl]Cl₂·H₂O (grey-green)
- Linkage isomerism: Due to ambidentate ligands. E.g., [Co(NH₃)₅(NO₂)]²⁺ (nitro, N-bonded) vs [Co(NH₃)₅(ONO)]²⁺ (nitrito, O-bonded)
- Coordination isomerism: Exchange of ligands between cation and anion complexes. E.g., [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]
Stereoisomerism
- Geometric (cis-trans) isomerism: In square planar (MA₂B₂ type) and octahedral complexes. Cisplatin = cis-[PtCl₂(NH₃)₂] is an anticancer drug; trans isomer (transplatin) is inactive.
- Optical isomerism: Non-superimposable mirror images (enantiomers). Occurs in octahedral complexes like [Co(en)₃]³⁺. Detected by optical rotation.
Stability of Coordination Compounds
Stability constant (Kf): Equilibrium constant for complex formation. Higher Kf = more stable complex.
M + nL ⇌ [MLn]; Kf = [MLn] / [M][L]ⁿ
Chelate effect: Chelating (polydentate) ligands form more stable complexes than monodentate ligands of comparable donor ability. This is primarily an entropy effect — ring formation increases disorder when monodentate ligands are displaced.
Inert vs Labile complexes:
- Labile: Fast ligand exchange. E.g., [Fe(H₂O)₆]²⁺
- Inert: Slow ligand exchange (kinetically stable). E.g., [Co(NH₃)₆]³⁺, [Cr(NH₃)₆]³⁺
Organometallic Compounds and EAN Rule
Organometallic compounds contain M–C bonds. The Effective Atomic Number (EAN) rule states that stable organometallic complexes have the same electron count as the next noble gas.
EAN = (atomic number of metal) − (oxidation state) + 2 × (number of ligands donating electron pairs)
Examples:
Ferrocene [Fe(C₅H₅)₂]: Fe²⁺ + 2 × Cp⁻ ligands. EAN = 26 − 2 + 10 + 10 = 34? (each Cp⁻ donates 6 electrons). 18-electron rule satisfied = stable.
Zeiss’s salt [PtCl₃(C₂H₄)]⁻: First organometallic compound with π-bonding.
Biological Importance of Coordination Compounds
| Biomolecule | Metal Ion | Ligand System | Function |
|---|---|---|---|
| Haemoglobin | Fe²⁺ | Porphyrin ring (protoporphyrin IX) | Oxygen transport in blood |
| Chlorophyll | Mg²⁺ | Porphyrin ring | Photosynthesis — light absorption |
| Vitamin B₁₂ | Co³⁺ | Corrin ring (like porphyrin) | Nerve function, DNA synthesis |
| Cisplatin | Pt²⁺ | 2 Cl⁻ + 2 NH₃ (cis) | Anticancer drug — cross-links DNA |
| Carbonic anhydrase | Zn²⁺ | 3 histidine residues | CO₂ hydration in blood |
Industrial Applications
- Cyanide process (gold/silver extraction): Au + 2CN⁻ + ½O₂ + H₂O → [Au(CN)₂]⁻ + OH⁻
- Electroplating: Complexes provide controlled metal ion concentration (e.g., Ag plating uses [Ag(CN)₂]⁻)
- EDTA in water treatment: Chelates heavy metal ions (Pb²⁺, Hg²⁺) — used in hard water treatment and as preservative
- Wilkinson’s catalyst: [RhCl(PPh₃)₃] — used in hydrogenation of alkenes
- IUPAC nomenclature: Practice writing names for 10+ complexes. Most marks are lost here due to silly errors (wrong suffix, wrong order, forgetting oxidation state).
- Magnetic moment: μ = √[n(n+2)] BM where n = number of unpaired electrons. First determine d-configuration and spin state.
- Isomerism: Know which complexes show cis-trans (MA₂B₂ square planar, MA₄B₂ octahedral) and which show optical (MA₂B₂C₂ octahedral, tris-chelate complexes).
- Biological applications: High-frequency NEET topic — memorize Hb(Fe), Chl(Mg), B12(Co), Cisplatin(Pt).
- CFSE calculation is formulaic — practice with d³, d⁵, d⁶ configurations for octahedral complexes.
- Biological metals: “HeMo Chloro CoBalt Cis-Platin” — Haemoglobin-Fe, Chlorophyll-Mg, Cobalamin-Co, Cisplatin-Pt
- Spectrochemical series (strong field ligands): “Can No Bother (en) People Now?” — CN⁻, NO, bipyridyl, en, phen, NH₃
- IUPAC nomenclature neutral ligands exceptions: “Aqua Ammine Carbonyl Nitrosyl” — these 4 special names to memorize
- Anionic complex metal suffix -ate: ferrate (Fe), cobaltate (Co), platinate (Pt), chromate (Cr), argentate (Ag), aurate (Au)
- Cisplatin vs Transplatin: Cis = Cancer drug; Trans = Thrown out (inactive)
Frequently Asked Questions
What is the difference between primary valence and secondary valence in Werner’s theory?
In Werner’s theory, primary valence (also called ionisable valence) refers to the oxidation state of the metal, satisfied by negative ions outside the coordination sphere that can be precipitated. Secondary valence (coordination number) is the number of ligands directly bonded to the metal inside the coordination sphere — these cannot be precipitated. For example, in [Co(NH₃)₅Cl]Cl₂, the primary valence is 3 (3 Cl⁻ total, 2 outside + 1 inside), while secondary valence (coordination number) is 6 (5 NH₃ + 1 Cl inside).
Why is cisplatin an anticancer drug but transplatin is not?
Cisplatin (cis-[PtCl₂(NH₃)₂]) is effective as an anticancer drug because its cis geometry allows it to form intrastrand cross-links with DNA by binding to two adjacent guanine bases. This cross-linking distorts the DNA helix, preventing replication and transcription of cancer cells. Transplatin cannot form such intrastrand cross-links due to its trans geometry — the two Cl⁻ ligands are too far apart to bridge adjacent DNA bases. It also gets deactivated by cellular proteins before reaching DNA.
What is the chelate effect and why are EDTA complexes so stable?
The chelate effect is the extra thermodynamic stability gained when polydentate (chelating) ligands bind to a metal compared to the equivalent number of monodentate ligands. EDTA (ethylenediaminetetraacetic acid) is hexadentate — it uses 4 oxygen and 2 nitrogen atoms to bind a single metal ion, forming 5 chelate rings. The stability is mainly entropic: when EDTA replaces 6 monodentate water molecules, the number of free particles in solution increases from 1+1 to 1+6 (net +5 entropy gain), making the reaction thermodynamically very favourable (ΔG = ΔH − TΔS, where TΔS is very positive).
How do you calculate the magnetic moment of a coordination complex?
Use the spin-only formula: μ = √[n(n+2)] Bohr Magnetons (BM), where n = number of unpaired electrons. Steps: (1) Find the oxidation state of the metal from the complex formula. (2) Write the d-electron configuration of the metal ion. (3) Determine if the complex is high spin (weak field ligands) or low spin (strong field ligands). (4) Fill electrons accordingly and count unpaired electrons. (5) Apply formula. Example: [Fe(H₂O)₆]²⁺ — Fe is +2, d⁶, H₂O is weak field → high spin → 4 unpaired electrons → μ = √(4×6) = √24 ≈ 4.90 BM.
What are the different types of isomerism shown by coordination compounds?
Coordination compounds show two main types: (1) Structural isomerism — includes ionisation isomerism (different ions inside/outside coordination sphere), hydrate isomerism (water inside vs outside), linkage isomerism (ambidentate ligand bound through different atoms), and coordination isomerism (exchange of ligands between cationic and anionic complexes). (2) Stereoisomerism — includes geometric (cis-trans) isomerism (different spatial arrangement, e.g., cisplatin) and optical isomerism (non-superimposable mirror images/enantiomers, e.g., [Co(en)₃]³⁺).
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