CBSETUTOR.aiCBSETUTOR.ai
India's #1 AI Tutorformula-sheet · Chemistry · Chapter 5

Class 12 Chemistry Chapter 5 Coordination Compounds — Formulas & Key Points

Coordination Compounds forms a critical 5-mark question cluster in the CBSE Class 12 Chemistry board examination. This formula sheet compiles every nomenclature rule, bonding theory equation, isomerism criterion, and CFT calculation you need for rapid revision. Mastering these formulas ensures you can tackle IUPAC naming, predict geometry, calculate magnetic moments, and explain colour in transition metal complexes confidently within exam time constraints.

Your child's private AI tutor — trained on NCERT.
3-day free trial · ₹1 to start · Cancel anytime.
Start 3-day free trial →

Key takeaways

  • ✓Werner's primary valence equals oxidation state of metal; secondary valence equals coordination number.
  • ✓CFSE (Crystal Field Stabilization Energy) = (-0.4 × n_t2g + 0.6 × n_eg) × Δ₀ for octahedral complexes.
  • ✓Spin-only magnetic moment μ = √[n(n+2)] BM where n is number of unpaired electrons.
  • ✓EAN (Effective Atomic Number) = Z - oxidation state + 2 × coordination number; stable complexes often satisfy EAN = nearest noble gas atomic number.
  • ✓IUPAC naming order: [Cation] [Anion] with ligands alphabetically, metal oxidation state in Roman numerals.
  • ✓Geometrical isomerism exists in square planar (MA₂B₂) and octahedral (MA₄B₂, MA₃B₃) complexes; optical isomerism in chiral complexes without plane of symmetry.
  • ✓Spectrochemical series ranks ligands by field strength: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < NH₃ < en < CN⁻ < CO.

Core Definitions and Fundamental Terms

Understanding precise terminology is essential for answering definition-based questions worth 1-2 marks and for using correct language in 3-5 mark explanations. Every coordination compound consists of a central metal atom or ion surrounded by ligands. The coordination entity is the entire assembly enclosed in square brackets. Ligands donate electron pairs; the coordination number is the count of donor atoms directly bonded to the metal. Oxidation state is the charge the metal would carry if all ligands were removed along with their electron pairs. Coordination sphere denotes species within square brackets; counter ions lie outside.
  • Coordination compound: Compound containing a coordination entity (complex ion or neutral molecule) plus counter ions if charged.
  • Ligand: Ion or molecule with at least one donor atom possessing a lone pair of electrons (Lewis base).
  • Coordination number (CN): Total number of ligand donor atoms bonded to the central metal.
  • Denticity: Number of donor atoms in a single ligand that bind to the metal (monodentate = 1, bidentate = 2, polydentate ≥ 3).
  • Chelate: Ring structure formed when a polydentate ligand binds to a metal through two or more donor atoms.
  • Homoleptic complex: All ligands identical, e.g. [Co(NH₃)₆]³⁺. Heteroleptic: different ligands, e.g. [Co(NH₃)₄Cl₂]⁺.

Werner's Theory — Primary and Secondary Valences

Alfred Werner's 1913 Nobel Prize-winning coordination theory introduced two types of valences. Primary valence corresponds to the oxidation state of the metal and is ionizable; it equals the number of counter ions needed for neutrality. Secondary valence is non-ionizable, corresponds to the coordination number, and is always satisfied—often by ligands or sometimes by counter ions entering the coordination sphere. Werner proposed that secondary valences are directed in space, leading to definite geometries (octahedral, tetrahedral, square planar). This theory explained isomerism and the non-ionizable nature of certain groups in coordination compounds.
  • Primary valence = Oxidation state of metal; satisfied by negative ions (anions).
  • Secondary valence = Coordination number; satisfied by ligands or anions in the coordination sphere.
  • Secondary valences have fixed directional orientations, determining geometry.
  • Ionizable groups lie outside square brackets; non-ionizable groups inside.
  • Example: CoCl₃·6NH₃ has primary valence 3 (Co³⁺) and secondary valence 6 (six NH₃ ligands).

IUPAC Nomenclature Rules — Formula Table

IUPAC naming ensures unambiguous communication. The order is: ligands first (alphabetically, ignoring prefixes di-, tri-, etc.), then metal with oxidation state in Roman numerals in parentheses. Anionic ligands end in '-o', neutral ligands use their molecular name (except aqua, ammine, carbonyl, nitrosyl). For complex anions, the metal name ends in '-ate'. Prefixes di-, tri-, tetra- for simple ligands; bis-, tris-, tetrakis- for complex ligands already containing numbers. Greek prefixes are not considered for alphabetical order. This systematic approach is tested in 2-3 mark nomenclature questions every year in CBSE boards.
  • Cationic complex: [metal name](oxidation state) counter anion, e.g. [Cr(NH₃)₆]Cl₃ = hexaamminechromium(III) chloride.
  • Anionic complex: counter cation [ligands + metal-ate](oxidation state), e.g. K₃[Fe(CN)₆] = potassium hexacyanoferrate(III).
  • Neutral complex: no space, e.g. [Pt(NH₃)₂Cl₂] = diamminedichloridoplatinum(II).
  • Ligand order: alphabetical by ligand name (ammine before chlorido, ignoring prefixes).
  • Anionic ligand suffixes: Cl⁻ = chlorido, CN⁻ = cyanido, OH⁻ = hydroxido, SO₄²⁻ = sulfato, NO₂⁻ = nitrito-N / nitrito-O, SCN⁻ = thiocyanato-S / thiocyanato-N.

Key Formulas — Oxidation State, Coordination Number, EAN, Magnetic Moment

Quantitative calculations in coordination chemistry rely on four core formulas. Oxidation state is found by charge balance. Coordination number is the sum of donor atoms. Effective Atomic Number (EAN) predicts stability when it equals a noble gas atomic number. Magnetic moment reveals the number of unpaired electrons and thus the electronic configuration and hybridization. These formulas appear in numerical problems worth 2-3 marks and in reasoning questions about stability, magnetism, and colour.

Crystal Field Theory (CFT) — Splitting Diagrams and CFSE

CFT explains colour, magnetism, and stability by treating ligands as point negative charges that split the degenerate d-orbitals of the metal ion into sets of different energies. In octahedral fields, d-orbitals split into lower-energy t₂g (dxy, dyz, dzx) and higher-energy eg (dx²-y², dz²) with energy gap Δ₀. In tetrahedral fields, splitting is inverted and smaller (Δt ≈ 4/9 Δ₀). Electrons fill according to Hund's rule and the spectrochemical series determines whether the complex is high-spin or low-spin. CFSE quantifies the stabilization energy gained from splitting.
  • Octahedral CFSE = [(-0.4 × n_t2g) + (0.6 × n_eg)] × Δ₀ + P (pairing energy if low-spin).
  • Tetrahedral CFSE = [(-0.6 × n_e) + (0.4 × n_t2)] × Δt.
  • High-spin: weak-field ligands (small Δ), maximum unpaired electrons, pairing energy > Δ.
  • Low-spin: strong-field ligands (large Δ), minimum unpaired electrons, pairing energy < Δ.
  • Δ₀ magnitude: depends on metal (higher oxidation state → larger Δ), ligand (spectrochemical series), and geometry.

Isomerism in Coordination Compounds — Types and Recognition

Isomerism is a favourite 3-5 mark CBSE question topic. Structural isomers have different bonding patterns: ionization isomers differ in counter ion vs coordinated ion; linkage isomers have ambidentate ligands bonded through different atoms; coordination isomers exchange ligands between cationic and anionic complexes; hydrate isomers differ in water inside vs outside the coordination sphere. Stereoisomers have the same bonds but different spatial arrangements: geometrical isomers (cis/trans in square planar MA₂B₂ or octahedral MA₄B₂; fac/mer in MA₃B₃) and optical isomers (non-superimposable mirror images in complexes lacking symmetry plane, common in tris-bidentate octahedral complexes).
  • Ionization: [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br — different ions precipitate with AgNO₃ or BaCl₂.
  • Linkage: [Co(NH₃)₅NO₂]²⁺ (nitrito-N) vs [Co(NH₃)₅ONO]²⁺ (nitrito-O) — ambidentate NO₂⁻ or SCN⁻/NCS⁻.
  • Coordination: [Cu(NH₃)₄][PtCl₄] vs [Pt(NH₃)₄][CuCl₄] — ligands and metals swap between cation and anion.
  • Geometrical: [Pt(NH₃)₂Cl₂] cis (same side) vs trans (opposite). Octahedral [Co(NH₃)₄Cl₂]⁺ cis/trans.
  • Optical: [Co(en)₃]³⁺ exists as d and l enantiomers (mirror images, optically active, no plane of symmetry).

Spectrochemical Series and Colour in Complexes

The spectrochemical series ranks ligands by increasing crystal field splitting power: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < dipy < phen < CN⁻ < CO. Weak-field ligands (left) cause small Δ and high-spin configurations; strong-field ligands (right) cause large Δ and low-spin. Colour arises from d-d electronic transitions: an electron absorbs visible light of energy equal to Δ and jumps from t₂g to eg. The colour observed is complementary to the absorbed wavelength. Larger Δ shifts absorption to higher energy (shorter wavelength). Complexes with d⁰ or d¹⁰ configurations are colourless because no d-d transitions are possible.
  • Complementary colours: absorbed (violet-blue 400-450 nm) → observed (yellow-orange); absorbed (red 650-700 nm) → observed (green).
  • Ligand strength: CN⁻ > NH₃ > H₂O > F⁻ > Cl⁻ in producing Δ₀.
  • Ti³⁺ (d¹) in [Ti(H₂O)₆]³⁺ absorbs green-yellow, appears purple.
  • Cu²⁺ (d⁹) complexes typically blue/green; Ni²⁺ (d⁸) green; Co²⁺ (d⁷ high-spin) pink.
  • Intensity: d-d transitions are Laporte-forbidden, hence colours are pale; charge-transfer transitions (e.g. in MnO₄⁻, CrO₄²⁻) are intense.

Hybridization, Geometry, and Magnetic Properties Table

Hybridization predicts geometry and is inferred from coordination number and magnetic data. Coordination number 6 typically gives octahedral (d²sp³ inner-orbital or sp³d² outer-orbital); CN = 4 gives tetrahedral (sp³) or square planar (dsp²); CN = 2 gives linear (sp). Inner-orbital (low-spin) complexes use (n-1)d orbitals and are often diamagnetic or have fewer unpaired electrons; outer-orbital (high-spin) use nd orbitals. Magnetic moment measured experimentally in BM (Bohr Magneton) confirms the number of unpaired electrons and thus the hybridization scheme. This interconnection is a common 5-mark reasoning question in CBSE Class 12 Chemistry boards.
  • Octahedral d²sp³ (inner, low-spin, strong-field): [Co(NH₃)₆]³⁺, [Fe(CN)₆]⁴⁻.
  • Octahedral sp³d² (outer, high-spin, weak-field): [FeF₆]³⁻, [CoF₆]³⁻.
  • Tetrahedral sp³: [NiCl₄]²⁻, [CoCl₄]²⁻ (always high-spin, Δt small).
  • Square planar dsp²: [Ni(CN)₄]²⁻, [PtCl₄]²⁻ (low-spin, diamagnetic, d⁸ metals).
  • Linear sp: [Ag(NH₃)₂]⁺, [CuCl₂]⁻.

Common Mistakes, Units, and Sign Conventions

Students frequently lose marks due to notation errors. Always enclose the coordination entity in square brackets; oxidation state must be in Roman numerals in parentheses immediately after the metal name. For magnetic moment, units are Bohr Magneton (BM); omitting BM costs marks. CFSE can be positive or negative: negative values indicate stabilization. When writing formulas, cationic complexes are written first, then anionic; neutral complexes stand alone. Denticity and coordination number are not the same: denticity is per ligand, coordination number is total donor atoms. Mixing up ionization isomers and hydrate isomers is common; test with AgNO₃ or BaCl₂ to distinguish.
  • Incorrect: Co(NH3)6Cl3 ✗ → Correct: [Co(NH₃)₆]Cl₃ ✓
  • Incorrect: Cobalt(3) ✗ → Correct: cobalt(III) ✓ (Roman numerals mandatory).
  • Incorrect: μ = 3.87 ✗ → Correct: μ = 3.87 BM ✓
  • Coordination number of EDTA⁴⁻ is 6 (hexadentate), not 4 (common error confusing charge with denticity).
  • For linkage isomerism: write [Co(NH₃)₅(NO₂)]²⁺ and [Co(NH₃)₅(ONO)]²⁺, not just NO₂ interchangeably.
  • CFSE in kJ/mol requires conversion: 1 Δ₀ ≈ 10Dq; for numerical questions, keep in Δ₀ or cm⁻¹ as specified.

Memory Tricks, Mnemonics, and Exam Tips

Mnemonics help retain the spectrochemical series and ligand names. For spectrochemical series weak to strong: 'I Bought Some Chocolate Santas For One Cold Happy New England Day' (I, Br, S, Cl, S, F, OH, C₂O₄²⁻, H₂O, NH₃, en, dipy). For anionic ligand suffixes: 'All Anions Add -o' (chloro, cyano, hydroxo). Remember Werner's two valences: Primary = Positive charge (oxidation state), Secondary = Surrounding ligands (coordination number). For CFSE sign: Negative = Net stabilization (desirable). Geometrical isomerism checklist: MA₂B₂ (square planar or octahedral), MA₄B₂ (octahedral), MA₃B₃ (fac/mer). Before the exam, revise the one-glance box below and practice writing five complex formulas with correct brackets and nomenclature.
  • Ambidentate ligands: NO₂⁻/ONO⁻, SCN⁻/NCS⁻, CN⁻/NC⁻ (memorize pairs).
  • Neutral ligand special names: H₂O = aqua, NH₃ = ammine (note double m), CO = carbonyl, NO = nitrosyl.
  • For optical isomerism: draw mirror image; if non-superimposable and no plane of symmetry → optically active.
  • High-spin vs low-spin quick test: calculate μ for both configurations, match with given or calculate from Δ and pairing energy.
  • In 2025 CBSE board: expect one 3-mark nomenclature, one 3-mark isomerism, one 5-mark VBT/CFT, one 2-mark numerical (EAN or μ).

Three Solved Mini-Examples for Rapid Practice

Example 1 – Nomenclature: Name [Cr(en)₂Cl₂]Cl. Solution: Cationic complex with Cr³⁺ (charge balance: x + 2(0) + 2(-1) = +1 → x = +3). Ligands: two Cl (dichloro) and two en (bis(ethylenediamine), alphabetically chloro before ethylene). Full name: dichlorobis(ethylenediamine)chromium(III) chloride. Example 2 – Magnetic moment: [CoF₆]³⁻ is high-spin. Co³⁺ is d⁶. Weak-field F⁻ → t₂g⁴ eg² → 4 unpaired electrons. μ = √[4(4+2)] = √24 = 4.90 BM. Example 3 – EAN: [Fe(CO)₅]. Fe is 0 oxidation state (neutral CO). Z = 26, oxidation = 0, CN = 5. EAN = 26 - 0 + 2(5) = 36 (Kr, stable). These three patterns cover nomenclature, magnetism, and stability — the core numerical question types.

One-Glance Last-Minute Revision Box

Werner: Primary valence = oxidation state, Secondary = CN. | EAN = Z - ox.state + 2×CN (stable if = noble gas). | μ (spin-only) = √[n(n+2)] BM. | Octahedral CFSE = (-0.4×t₂g + 0.6×eg)Δ₀. | IUPAC: ligands alphabetical (ignore di-, tri-), metal(oxidation state), anion suffix -ate. | Isomerism: ionization (counter ion swap), linkage (ambidentate), coordination (ligand swap cation/anion), hydrate (H₂O in/out), geometrical (cis/trans, fac/mer), optical (mirror images). | Spectrochemical: I < Br < Cl < F < H₂O < NH₃ < en < CN. | High-spin = weak field, max unpaired; Low-spin = strong field, min unpaired. | Colour = complementary to absorbed λ, depends on Δ. | Geometries: CN=6 octahedral (d²sp³ or sp³d²), CN=4 tetrahedral (sp³) or square planar (dsp²), CN=2 linear (sp). | Common ligands: en (bidentate), EDTA (hexadentate), NH₃ (monodentate neutral), Cl⁻ (monodentate anionic). | Practice: write [Co(NH₃)₅Cl]²⁺, [Ni(CN)₄]²⁻, K₃[Fe(CN)₆], [Pt(NH₃)₂Cl₂] with correct brackets and nomenclature. This box is your 5-minute pre-exam confidence booster.

Frequently asked questions

What is the difference between coordination number and oxidation state in coordination compounds?+
Coordination number is the total number of ligand donor atoms directly bonded to the central metal atom, indicating geometry. Oxidation state is the charge on the metal ion if all ligands are removed with their electrons, found by charge balance. For example, in [Fe(CN)₆]⁴⁻, CN = 6 and oxidation state = +2.
How do I quickly determine if a complex is high-spin or low-spin?+
Check the ligand in the spectrochemical series: weak-field ligands (Cl⁻, F⁻, H₂O) produce high-spin (maximum unpaired electrons); strong-field ligands (CN⁻, NH₃, en) produce low-spin (minimum unpaired electrons). Calculate μ for both and match experiment, or compare Δ with pairing energy: Δ < P → high-spin; Δ > P → low-spin.
Why is [Ni(CN)₄]²⁻ square planar but [NiCl₄]²⁻ tetrahedral despite both being CN = 4?+
CN⁻ is a strong-field ligand causing electron pairing in d⁸ Ni²⁺, leaving one 3d orbital vacant for dsp² hybridization (square planar, diamagnetic). Cl⁻ is weak-field, no pairing occurs, so all four 3d orbitals remain unpaired and sp³ hybridization occurs (tetrahedral, paramagnetic with 2 unpaired electrons).
What is the correct IUPAC name for K₃[Fe(CN)₆]?+
Potassium hexacyanoferrate(III). Counter cation first (potassium), then ligand prefix (hexa) + ligand name (cyanido → cyano in practice) + metal-ate suffix (ferrate) + oxidation state in Roman numerals (III). Note: in anionic complexes, metal name ends in -ate.
How does Crystal Field Theory explain the colour of transition metal complexes?+
CFT explains colour by d-orbital splitting. When visible light irradiates the complex, an electron absorbs energy equal to Δ and jumps from lower t₂g to higher eg level (d-d transition). The absorbed wavelength is removed from white light; the complementary colour is observed. Larger Δ shifts absorption to shorter wavelength (higher energy), changing observed colour.
What are ionization isomers and how do you test for them in the lab?+
Ionization isomers have different ions inside vs outside the coordination sphere, e.g. [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br. They give different ions in solution. Test with AgNO₃: the first isomer precipitates BaSO₄ with BaCl₂ (SO₄²⁻ outside), the second precipitates AgBr (Br⁻ outside). This distinguishes which ion is the counter ion.
What is EAN and how is it used to predict stability of a coordination compound?+
Effective Atomic Number (EAN) = Z (atomic number) - oxidation state + 2 × coordination number. It represents the total electrons around the metal. Complexes are especially stable when EAN equals the atomic number of the next noble gas (18, 36, 54, 86). For example, [Fe(CO)₅]: EAN = 26 - 0 + 10 = 36 (Kr), highly stable.
Why are tetrahedral complexes generally high-spin even with moderately strong ligands?+
In tetrahedral geometry, d-orbital splitting (Δt) is only about 4/9 of octahedral splitting (Δ₀) due to fewer ligands and less direct orbital overlap. This small Δt is usually less than the pairing energy, so electrons occupy higher orbitals rather than pairing, resulting in high-spin configurations for all common ligands except extremely strong fields.
How do I identify geometrical isomers in octahedral complexes?+
For MA₄B₂ (four A, two B), cis has both B adjacent (90°), trans has B opposite (180°). For MA₃B₃, fac (facial) has three identical ligands on one triangular face, mer (meridional) has three in a plane bisecting the octahedron. Draw the structure and check positions; if arrangements differ, they are geometrical isomers.
Which platform helps Class 12 students master Coordination Compounds numericals and nomenclature efficiently?+
CBSETUTOR.ai offers 24×7 AI-powered tutoring with photo-upload problem solving for CBSE Class 12 Chemistry. At a flat ₹999/month for all classes 6-12, students get instant step-by-step solutions for IUPAC naming, CFT calculations, isomerism identification, and magnetic moment numericals. A 3-day free trial lets you test the platform before the boards.

Related resources

CBSE Class 12 Chemistry Chapter 5 Coordination Compounds Worksheet with AnswersImportant Questions: CBSE Class 12 Chemistry Chapter 5 Coordination CompoundsImportant Questions: CBSE Class 12 Chemistry Chapter 4 The d- and f-Block ElementsCBSE Class 12 Chemistry Chapter 4 The d- and f-Block Elements Worksheet with AnswersAI Tutor for Class 12: The Smart Alternative to TuitionAI Tutor for Class 12 Accountancy: Learn Faster with Instant HelpOnline Class 9 Chemistry Tutor in Mumbai — NCERT-Aligned Board Exam CoachingClass 9 Mathematics Chapter 2 Polynomials — Formulas & Key Points

Keep learning — related guides

Class 12Chemistry
Class 12 Chemistry Tutor in Rushikonda, Visakhapatnam
Class 12Chemistry
Class 12 Chemistry Chapter 9 Amines — Formulas & Key Points
Class 12Chemistry
Class 12 Chemistry Chapter 10 Biomolecules — Formulas & Key Points
Class 12Chemistry
CBSE Class 12 Chemistry Chapter 10 Biomolecules Worksheet with Answers
Class 12Chemistry
Important Questions: CBSE Class 12 Chemistry Chapter 8 Aldehydes, Ketones and Carboxylic Acids
Class 12Chemistry
Important Questions: CBSE Class 12 Chemistry Chapter 7 Alcohols, Phenols and Ethers

Ready to give your Class 12 child the tutor that never sleeps?

CBSETUTOR.ai covers every chapter in the Class 12 NCERT syllabus — Maths, Science, Social Science, English, Hindi and more. 24×7. Patient. Unlimited. 3-day free trial.

Start your child's 3-day free trial →