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Coordination Compounds

Study of compounds where a central metal atom is bonded to surrounding ligands. Covers nomenclature, types of isomerism, bonding theories (VBT, CFT), and applications in medicine, photography, and industry.

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Reading time~7 min
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Last updated2026-07-19
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🎯 Key Points

  • Coordination number 4 → tetrahedral (usually) or square planar; coordination number 6 → octahedral
  • Crystal Field Theory (CFT): ligands split d-orbitals into lower-energy (t2g) and higher-energy (eg) sets in an octahedral field
  • Strong field ligands (CN⁻, CO) cause low-spin (more pairing); weak field ligands (F⁻, H₂O) cause high-spin (less pairing, follows Hund's rule more)
  • IUPAC naming order: ligands alphabetically, then metal, then oxidation state in Roman numerals in brackets; anionic complex ends in "-ate"
  • Isomerism types: ionisation, hydrate, linkage, coordination (structural); geometric (cis-trans) and optical (chiral) (stereoisomerism)
  • Spectrochemical series (increasing field strength): I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < CN⁻ < CO

Coordination Compounds

Octahedralcoordination no. 6e.g. [Co(NH3)6]3+Tetrahedralcoordination no. 4e.g. [Ni(CO)4]Square Planarcoordination no. 4e.g. [Pt(NH3)2Cl2]

The three common coordination geometries: octahedral (6 ligands), tetrahedral (4 ligands), and square planar (4 ligands in one plane).

A coordination compound has a central metal atom or ion bonded to a fixed number of ligands (molecules or ions that donate electron pairs). Written with the coordination sphere in square brackets.

Example: [Cu(NH₃)₄]²⁺ (tetraamminecopper(II) ion)

Key Terms

  • Ligand: Molecule or ion that donates a lone pair to the metal
  • Coordination number: Total number of donor atoms bonded to the central metal
  • Chelate: Complex formed by a polydentate ligand

Types of Ligands

  • Monodentate: One donor atom; Cl⁻, NH₃, H₂O, CN⁻
  • Bidentate: Two donor atoms; en (ethylenediamine), oxalate (C₂O₄²⁻)
  • Polydentate: Many donor atoms; EDTA (6 donor atoms)

IUPAC Nomenclature

  • Name ligands alphabetically, then the central metal
  • Anionic ligands end in "-o": chloro, nitro, cyano, oxalato
  • Neutral ligands: aqua (H₂O), ammine (NH₃), carbonyl (CO)
  • Oxidation state of metal in Roman numerals: [Fe(CN)₆]³⁻ = hexacyanoferrate(III)

Isomerism

  • Geometrical: cis/trans in square planar and octahedral complexes
  • Optical: Non-superimposable mirror images (enantiomers)
  • Ionisation: [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br

Applications

  • Haemoglobin: Fe-porphyrin complex carrying O₂ in blood
  • Chlorophyll: Mg complex driving photosynthesis
  • Cisplatin [Pt(NH₃)₂Cl₂]: Cancer treatment drug
  • EDTA: Water softening, food preservation, chelation therapy

Werner's Coordination Theory

  • Metals show two types of valency: primary valency (ionisable, satisfied by anions, corresponds to oxidation state) and secondary valency (non-ionisable, satisfied by ligands, corresponds to coordination number)
  • Secondary valencies are directional and fixed in space, giving the complex a definite geometry (octahedral, tetrahedral, square planar)
  • Example: in CoCl₃·6NH₃, the 3 Cl⁻ are primary (precipitated by AgNO₃) and the 6 NH₃ satisfy the secondary valency of 6 → [Co(NH₃)₆]³⁺ 3Cl⁻
  • Werner explained the number of ions produced in solution using conductance and precipitation data, laying the foundation of modern coordination chemistry

Valence Bond Theory (VBT) and Hybridisation

  • Metal provides vacant hybrid orbitals that accept lone pairs from ligands (coordinate bonds)
  • Coordination number 6: d²sp³ (inner orbital, uses inner 3d) or sp³d² (outer orbital, uses outer 4d) → both octahedral
  • Coordination number 4: sp³ → tetrahedral; dsp² → square planar
  • Strong field ligands force pairing → inner-orbital (low-spin) complexes; weak field ligands → outer-orbital (high-spin) complexes
  • Magnetic moment μ = √[n(n+2)] BM, where n = number of unpaired electrons; measured μ reveals the number of unpaired electrons and hence the hybridisation
  • Limitations of VBT: cannot explain colour, does not predict relative stabilities or the exact magnitude of Δ, and fails for distortion/tetragonal geometries

Crystal Field Theory (CFT)

  • Metal-ligand bonding is treated as purely electrostatic; ligand approach splits the degenerate d-orbitals
  • Octahedral field: d-orbitals split into lower t₂g (dxy, dyz, dzx) and higher eg (dx²−y², dz²), separated by Δo
  • Tetrahedral field: splitting is inverted (e lower, t₂ higher) and smaller: Δt = (4/9)Δo, so tetrahedral complexes are almost always high-spin
  • Pairing occurs only if Δo > pairing energy P (strong field → low-spin); if Δo < P, electrons stay unpaired (weak field → high-spin)
  • CFSE = (−0.4 × nt2g + 0.6 × neg)Δo (+ pairing energy terms); more negative CFSE = greater stability
  • Colour arises from d-d transitions: a ligand raises an electron from t₂g to eg, absorbing a wavelength corresponding to Δo; the complex shows the complementary colour
  • Spectrochemical series (increasing Δ): I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO
Crystal field splitting diagram for an octahedral complex: the five degenerate d orbitals split into a lower triple set (dxy, dyz, dxz) and an upper double set (dx2-y2, dz2)

Crystal field splitting in an octahedral complex: the five originally degenerate d orbitals split into a lower-energy t2g set (dxy, dyz, dxz) and a higher-energy eg set (dx²−y², dz²). The size of the gap Δo governs the complex's colour and whether it is high-spin or low-spin. Image: YanA, CC BY-SA 4.0, via Wikimedia Commons.

Stability of Coordination Compounds

  • Stability in solution is expressed by the stability (formation) constant K; a larger K means a more stable complex
  • Formation proceeds stepwise (K₁, K₂, …) and the overall constant β = K₁ × K₂ × … × Kn
  • Higher metal charge, smaller size, and stronger-field ligands increase stability
  • Chelate effect: complexes with polydentate (chelating) ligands like EDTA or en are far more stable than those with comparable monodentate ligands, due to a favourable entropy change

🚀 JEE Advanced Edge

Crystal Field Splitting Energy (CFSE) and spin state: In an octahedral field, d-orbitals split into t2g (lower, 3 orbitals) and eg (higher, 2 orbitals), separated by Δo. If Δo is large (strong field ligand), electrons pair up in t2g before occupying eg (low-spin, fewer unpaired electrons). If Δo is small (weak field ligand), electrons fill all 5 orbitals singly first per Hund's rule before pairing (high-spin). This directly determines the magnetic behaviour and colour of the complex.

Why [Ni(CN)₄]²⁻ is square planar but [NiCl₄]²⁻ is tetrahedral: CN⁻ is a strong field ligand that forces Ni²⁺'s d-electrons to pair up completely, freeing a d-orbital for dsp² hybridisation (square planar, diamagnetic). Cl⁻ is a weak field ligand that doesn't force pairing, so Ni²⁺ retains unpaired electrons and adopts sp³ hybridisation (tetrahedral, paramagnetic) instead.

Worked problem: Calculate the EAN (Effective Atomic Number) of Co in [Co(NH₃)₆]³⁺ and verify the 18-electron rule. Approach: Co³⁺ has atomic number 27, so Co³⁺ has 27−3=24 electrons. Each of the 6 NH₃ ligands donates 2 electrons: 6×2=12. EAN = 24+12 = 36, matching the electron count of Kr (the nearest noble gas) — many stable, especially low-spin, complexes follow this 18-electron-equivalent stability pattern (EAN equal to the next noble gas).

2 Revise ~2 min before the exam

📐 Formula Sheet

  • Coordination number: number of ligand donor atoms bonded to the central metal
  • Oxidation state of metal: overall charge − (sum of ligand charges)
  • EAN: (metal electrons − oxidation state) + (2 × coordination number)
  • Werner: primary valence = oxidation state (ionisable); secondary valence = coordination number (fixed geometry)
  • Crystal field splitting: octahedral splits d into t₂g (lower) and eg (upper), gap Δo
  • Spin state: strong-field ligand (large Δ) ⇒ low spin, paired; weak-field ⇒ high spin
  • Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < CN⁻ < CO (weak → strong)
  • Magnetic moment: μ = √(n(n + 2)) BM, where n = number of unpaired electrons
3 Practice apply it

✍️ Worked Examples

Example 1 — Oxidation state of the central metal
Q: Find the oxidation state of iron in [Fe(CN)₆]³⁻.
Step 1 — Each cyanide (CN⁻) carries −1, and there are six: total −6.
Step 2 — Let Fe be x; the complex ion is −3: x + (−6) = −3.
Step 3 — Solve: x = +3.
Answer: +3. Note: naming it would give hexacyanidoferrate(III).

Example 2 — Magnetic moment
Q: Calculate the spin-only magnetic moment of a d⁵ high-spin complex.
Step 1 — High spin d⁵ means all five d electrons are unpaired, so n = 5.
Step 2 — Apply μ = √(n(n + 2)): = √(5 × 7) = √35.
Step 3 — Compute: ≈ 5.92 BM.
Answer: ≈ 5.92 BM. Note: a strong-field (low-spin) d⁵ would instead have just one unpaired electron, giving ≈ 1.73 BM.

Example 3 — Strong vs weak field
Q: Why is [Fe(CN)₆]³⁻ low-spin while [FeF₆]³⁻ is high-spin?
Step 1 — Both have Fe³⁺ (d⁵), so the ligand decides the spin state.
Step 2 — CN⁻ is a strong-field ligand: it creates a large Δo, so electrons pair up in t₂g (low spin).
Step 3 — F⁻ is a weak-field ligand: small Δo, so electrons spread out singly (high spin).
Answer: the ligand strength (from the spectrochemical series) sets the spin state. Key idea: big Δ favours pairing; small Δ favours singly-occupied orbitals.

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Frequently Asked Questions — Coordination Compounds

What are the key concepts in Coordination Compounds?
Study of compounds where a central metal atom is bonded to surrounding ligands. Covers nomenclature, types of isomerism, bonding theories (VBT, CFT), and applications in medicine, photography, and industry.
Is Coordination Compounds important for NEET & JEE?
Yes. Coordination Compounds is part of the Chemistry Class 12 NCERT syllabus and is directly tested in NEET and JEE examinations. StudyHub provides structured notes, diagrams, and practice questions covering all exam-level subtopics.
How can I practice Coordination Compounds questions on StudyHub?
Open StudyHub and select Chemistry → Coordination Compounds. Choose Easy, Medium, or Hard difficulty. Hard-tier questions are at NEET & JEE level with full step-by-step explanations.

References

  1. NCERT Class 12 Chemistry Textbook — Chapter: Coordination Compounds
  2. CBSE Curriculum — Chemistry (Class 12)
  3. NTA NEET UG Official Syllabus — subject-wise topic list
  4. NTA JEE Main Official Syllabus — subject-wise topic list

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