1. Chapter at a Glance
Coordination compounds contain a central metal atom or ion bonded to surrounding molecules or ions called ligands. They are everywhere in biology and industry: haemoglobin (Fe), chlorophyll (Mg), vitamin B₁₂ (Co) and cisplatin (Pt) are all coordination compounds. This chapter covers Werner's theory, key terms, IUPAC naming, isomerism, valence bond theory (VBT) and crystal field theory (CFT). NEET asks about two questions every year.
- Oxidation state, coordination number and IUPAC name of a complex.
- Number of ions from a complex (conductivity, AgNO₃ precipitation).
- Geometrical and optical isomerism; counting isomers.
- Hybridisation, geometry and magnetic nature by VBT.
- Crystal field splitting, spectrochemical series, high- vs low-spin complexes.
2. Werner's Theory and Key Terms
- Werner proposed that a metal has two kinds of valence: primary (ionisable, equal to the oxidation state, satisfied by anions) and secondary (non-ionisable, equal to the coordination number, satisfied by ligands with a fixed geometry).
- Example: CoCl₃·6NH₃ is [Co(NH₃)₆]Cl₃, giving 4 ions and 3 AgCl per formula unit; CoCl₃·4NH₃ is [CoCl₂(NH₃)₄]Cl, giving 2 ions and 1 AgCl.
- Coordination entity: the metal and its attached ligands, written in square brackets.
- Coordination number (CN): number of donor atoms bonded to the metal.
- Ligands: unidentate (Cl⁻, NH₃, H₂O, CN⁻), bidentate (ethane-1,2-diamine "en", oxalate C₂O₄²⁻), polydentate (EDTA⁴⁻, hexadentate). A ligand that binds through two or more atoms forms a ring and is called a chelate; chelates are more stable.
- Ambidentate ligands can bind through either of two atoms: NO₂⁻ (N or O), SCN⁻ (S or N), CN⁻.
- Homoleptic complexes have one kind of ligand ([Co(NH₃)₆]³⁺); heteroleptic have more ([CoCl₂(NH₃)₄]⁺).
3. IUPAC Nomenclature
- Name the cation first, then the anion (as in ordinary salts).
- Inside the coordination entity, name the ligands in alphabetical order, then the metal.
- Anionic ligands end in -o: chlorido, cyanido, hydroxido, oxalato, nitrito-N (for –NO₂) and nitrito-O (for –ONO). Neutral ligands keep their names, except aqua (H₂O), ammine (NH₃), carbonyl (CO) and nitrosyl (NO).
- Use di-, tri-, tetra- for simple ligands and bis-, tris-, tetrakis- for ligands whose names already contain a number (ethane-1,2-diamine).
- Give the oxidation state of the metal in Roman numerals. If the complex is an anion, the metal name ends in -ate (cobaltate, ferrate, cuprate, argentate).
| Formula | IUPAC name | NEET Note |
|---|---|---|
| [Co(NH₃)₆]Cl₃ | Hexaamminecobalt(III) chloride | CN 6, ON +3 |
| K₄[Fe(CN)₆] | Potassium hexacyanidoferrate(II) | Anionic complex: -ate |
| [Pt(NH₃)₂Cl₂] | Diamminedichloridoplatinum(II) | Square planar; cis form is cisplatin |
| [CoCl₂(en)₂]⁺ | Dichloridobis(ethane-1,2-diamine)cobalt(III) | CN 6 (en is bidentate) |
| [Ni(CO)₄] | Tetracarbonylnickel(0) | Tetrahedral, diamagnetic |
4. Isomerism in Coordination Compounds
| Type | Description | Example |
|---|---|---|
| Ionisation | Ligand inside swaps with an ion outside | [Co(NH₃)₅SO₄]Br and [Co(NH₃)₅Br]SO₄ |
| Hydrate (solvate) | Water inside vs outside the sphere | [Cr(H₂O)₆]Cl₃ (violet) and [CrCl(H₂O)₅]Cl₂·H₂O (green) |
| Linkage | Ambidentate ligand binds through different atoms | –NO₂ (nitro) vs –ONO (nitrito) |
| Coordination | Ligands exchange between cationic and anionic complexes | [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆] |
| Geometrical (cis/trans, fac/mer) | Different spatial arrangements | [Pt(NH₃)₂Cl₂]; [Co(NH₃)₃Cl₃] (fac, mer) |
| Optical | Non-superimposable mirror images | [Co(en)₃]³⁺; cis-[CoCl₂(en)₂]⁺ |
- Tetrahedral complexes (MA₂B₂) show no geometrical isomerism; square planar MA₂B₂ shows cis and trans.
- Octahedral MA₄B₂ gives cis and trans; MA₃B₃ gives fac and mer.
- trans-[CoCl₂(en)₂]⁺ has a mirror plane and is optically inactive; the cis form is optically active (a pair of enantiomers).
5. Valence Bond Theory
- The metal provides vacant hybrid orbitals, and each ligand donates a lone pair into one.
- Strong-field ligands (CN⁻, CO, en, NH₃ for Co³⁺) can pair up the metal's d electrons, freeing inner d orbitals: inner-orbital (d²sp³) complexes, usually low-spin.
- Weak-field ligands (F⁻, Cl⁻, H₂O) do not force pairing, so outer d orbitals are used: outer-orbital (sp³d²) complexes, usually high-spin.
| Complex | Hybridisation | Geometry | Magnetism |
|---|---|---|---|
| [Co(NH₃)₆]³⁺ | d²sp³ (inner) | Octahedral | Diamagnetic |
| [CoF₆]³⁻ | sp³d² (outer) | Octahedral | Paramagnetic (4 unpaired) |
| [Ni(CN)₄]²⁻ | dsp² | Square planar | Diamagnetic |
| [NiCl₄]²⁻ | sp³ | Tetrahedral | Paramagnetic (2 unpaired) |
| [Ni(CO)₄] | sp³ | Tetrahedral | Diamagnetic |
| [Fe(CN)₆]³⁻ | d²sp³ | Octahedral | Paramagnetic (1 unpaired) |
Limitations of VBT: it does not explain colour, it cannot predict high- or low-spin behaviour quantitatively, and it does not distinguish between weak and strong ligands.
6. Crystal Field Theory
- CFT treats ligands as point charges. In an octahedral field, the five d orbitals split into a lower t₂g set (3 orbitals) and a higher eg set (2 orbitals), separated by Δ₀.
- In a tetrahedral field the order reverses (e below t₂) and the splitting is smaller: Δt ≈ (4/9)Δ₀. Tetrahedral complexes are therefore almost always high-spin.
- If Δ₀ > P (pairing energy), electrons pair in t₂g first: low-spin. If Δ₀ < P, they occupy eg before pairing: high-spin.
- Spectrochemical series (increasing field strength): I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < CN⁻ < CO.
- Colour: an electron absorbs light to jump from t₂g to eg; we see the complementary colour. A stronger ligand gives a larger Δ₀ and absorbs shorter wavelengths.
7. Metal Carbonyls and Importance of Complexes
- In metal carbonyls ([Ni(CO)₄], [Fe(CO)₅], [Cr(CO)₆]), CO donates a σ lone pair to the metal, and the metal donates d electrons back into empty π* orbitals of CO (synergic bonding). This strengthens the M–C bond and weakens the C–O bond.
- Stability of complexes is measured by the formation (stability) constant β. Chelates are more stable than similar non-chelated complexes (chelate effect).
- Applications: EDTA titration for water hardness (Ca²⁺, Mg²⁺); cisplatin in cancer therapy; purification of nickel (Mond process via Ni(CO)₄); extraction of gold and silver as [Au(CN)₂]⁻ and [Ag(CN)₂]⁻; photography (Ag⁺ with thiosulphate); EDTA in lead poisoning.
8. Formula Master Table
| Quantity | Formula / rule |
|---|---|
| Oxidation state | Charge of complex − Σ(ligand charges) |
| Coordination number | Σ(denticity of each ligand) |
| Ions in solution | 1 complex ion + counter-ions |
| AgCl precipitated | = number of Cl⁻ outside the bracket |
| CFSE (octahedral) | (−0.4 nt2g + 0.6 neg)Δ₀ (+ pairing energy terms) |
| Tetrahedral splitting | Δt = (4/9)Δ₀ |
| Low vs high spin | Δ₀ > P ⇒ low spin |
| Magnetic moment | μ = √(n(n + 2)) BM |
9. Master Summary Table
| Concept | Key idea | NEET Note |
|---|---|---|
| Werner | Primary vs secondary valence | Count ions and AgCl |
| Chelate | Ring-forming ligand | More stable |
| Ambidentate | Two donor atoms | Linkage isomerism |
| Optical isomerism | No mirror plane | [Co(en)₃]³⁺, cis-[CoCl₂(en)₂]⁺ |
| VBT | Hybridisation | CN⁻ ⇒ inner-orbital, low spin |
| CFT | t₂g/eg splitting | Explains colour and magnetism |
| Spectrochemical series | I⁻ weakest, CO strongest | Strong field ⇒ larger Δ₀ |
| Carbonyls | Synergic bonding | C–O bond weakened |
10. Mind Map
- Werner
- Ligands, CN
- Chelates
- Alphabetical ligands
- -ate for anions
- Roman numerals
- Structural
- Geometrical
- Optical
- VBT
- CFT
- Carbonyls
11. Tips and Tricks
- For Co³⁺ (d⁶): almost every ligand except F⁻ gives a low-spin, diamagnetic complex.
- Ni²⁺ with CN⁻ is square planar and diamagnetic; with Cl⁻ it is tetrahedral and paramagnetic.
- Number of geometrical isomers: MA₄B₂ = 2, MA₃B₃ = 2, [M(AA)₂B₂] = 2 (cis is optically active).
- Conductivity order for complexes with the same metal: more counter-ions ⇒ higher conductivity.
- Counting "en" as one donor atom (it is bidentate: CN rises by 2).
- Forgetting the "-ate" ending for anionic complexes.
- Expecting geometrical isomers for tetrahedral complexes.
- Using the prefix di- for ethane-1,2-diamine (use bis).
12. Important One-liners
- EDTA is a hexadentate ligand.
- Chlorophyll contains Mg; haemoglobin contains Fe; vitamin B₁₂ contains Co.
- Cisplatin, cis-[Pt(NH₃)₂Cl₂], is used in cancer treatment.
- CO is the strongest ligand in the spectrochemical series.
- Tetrahedral complexes are generally high-spin.
- Nickel is purified by the Mond process through Ni(CO)₄.
- In [Ni(CO)₄], the oxidation state of Ni is zero.
13. NEET-pattern Questions with Detailed Answers
These questions are original and based on concepts that NEET UG asks again and again.
The oxidation state and coordination number of Fe in K₃[Fe(C₂O₄)₃] are:
- +3, 3
- +3, 6
- +2, 6
- +6, 3
Show solution
The complex ion has a charge of −3. Each oxalate is −2, so x − 6 = −3 and x = +3. Oxalate is bidentate, so three of them give CN = 6. (A) forgets that oxalate binds through two atoms.
Answer: (B)
Which is an ambidentate ligand?
- NH₃
- H₂O
- SCN⁻
- en
Show solution
SCN⁻ can bind through S (thiocyanato-S) or through N (thiocyanato-N). NH₃ and H₂O each have only one donor atom. en is bidentate, meaning it binds through two atoms at the same time, which is different from ambidentate.
Answer: (C)
One mole of CoCl₃·5NH₃ gives two moles of AgCl with excess AgNO₃. Its correct formula is:
- [Co(NH₃)₅]Cl₃
- [CoCl(NH₃)₅]Cl₂
- [CoCl₂(NH₃)₅]Cl
- [CoCl₃(NH₃)₃]·2NH₃
Show solution
Two Cl⁻ are outside the bracket, so they are ionisable. The third Cl is a ligand, which completes CN = 6 with the five NH₃. (A) would have CN = 5 and would give 3 AgCl.
Answer: (B)
[Ni(CN)₄]²⁻ is:
- Tetrahedral and paramagnetic
- Square planar and diamagnetic
- Tetrahedral and diamagnetic
- Octahedral and paramagnetic
Show solution
Ni²⁺ is d⁸. The strong-field ligand CN⁻ pairs up the electrons, leaving one 3d orbital empty for dsp² hybridisation. This gives a square-planar complex with no unpaired electrons. [NiCl₄]²⁻, with a weak-field ligand, is tetrahedral and paramagnetic instead.
Answer: (B)
Which complex can show optical isomerism?
- trans-[CoCl₂(en)₂]⁺
- [Co(NH₃)₆]³⁺
- [Co(en)₃]³⁺
- [Pt(NH₃)₂Cl₂]
Show solution
[Co(en)₃]³⁺ has no plane of symmetry, so its mirror images cannot be superimposed. The trans isomer has a mirror plane, [Co(NH₃)₆]³⁺ is highly symmetric, and square-planar [Pt(NH₃)₂Cl₂] lies in a plane of symmetry.
Answer: (C)
The IUPAC name of K₃[Fe(CN)₆] is:
- Potassium hexacyanidoferrate(II)
- Potassium hexacyanidoferrate(III)
- Potassium hexacyanidoiron(III)
- Tripotassium ferricyanide(II)
Show solution
The complex ion has a charge of −3, and six cyanides give −6, so Fe is +3. An anionic complex takes the name "ferrate". (A) has the wrong oxidation state, and (C) misses the -ate ending.
Answer: (B)
The spin-only magnetic moment of [CoF₆]³⁻ is:
- 0 BM
- 2.83 BM
- 4.90 BM
- 5.92 BM
Show solution
Co³⁺ is d⁶. F⁻ is a weak-field ligand, so the complex is high-spin: t₂g⁴eg² with 4 unpaired electrons, giving μ = √24 ≈ 4.90 BM. The value 0 would apply to low-spin [Co(NH₃)₆]³⁺.
Answer: (C)
Which ligand causes the largest crystal field splitting?
- H₂O
- NH₃
- Cl⁻
- CN⁻
Show solution
In the spectrochemical series, CN⁻ lies near the strong end (just below CO), above NH₃, H₂O and Cl⁻. A stronger ligand gives a larger Δ₀.
Answer: (D)
14. Quick Revision Summary
- Werner: primary (ionisable) vs secondary (CN) valence; count ions and AgCl.
- Name: cation first; ligands alphabetical; -ate for anionic complexes; Roman numeral ON.
- Isomerism: ionisation, hydrate, linkage, coordination; cis/trans, fac/mer; optical.
- VBT: strong ligand ⇒ inner-orbital d²sp³/dsp², low spin.
- CFT: t₂g below eg (octahedral); Δt = 4/9 Δ₀; I⁻ weakest, CO strongest.