Coordination Compounds

Concept Core

Ligands, coordination number, IUPAC naming, isomerism, and bonding theories.

Key Terminology

Term	Definition	Example
Central metal ion	Transition metal; Lewis acid	Fe²⁺, Co³⁺, Pt²⁺
Ligand	Electron donor; Lewis base	NH₃, Cl⁻, en, EDTA
Coordination number	Number of bonds from ligands to metal	6 in [Co(NH₃)₆]³⁺
Coordination sphere	Central metal + ligands in [ ]	[Co(NH₃)₆]Cl₃
Chelate	Polydentate ligand forms ring with metal	en, oxalate, EDTA

Types of Ligands (Denticity)

Type	Donor Atoms	Examples
Monodentate	1	Cl⁻, Br⁻, NH₃, H₂O, CN⁻, CO
Bidentate	2	en (ethylenediamine), ox²⁻, bipyridyl
Polydentate	4–6	EDTA⁴⁻ (hexadentate)
Ambidentate	1 (two possible atoms)	SCN⁻ (S or N), NO₂⁻ (N or O)

IUPAC Nomenclature Rules

Order: anionic ligands → neutral ligands → metal (oxidation state)

Ligand prefixes: di, tri, tetra, penta, hexa for simple ligands; bis, tris, tetrakis for complex ligands.

If complex is cationic: name normally. If anionic: add -ate suffix to metal name (e.g., ferrate, cobaltate).

Examples: [Co(NH₃)₆]Cl₃ = hexaamminecobalt(III) chloride

Types of Isomerism

Structural isomers:

• Ionisation: [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br

• Linkage: [Co(NO₂)(NH₃)₅] vs [Co(ONO)(NH₃)₅] (ambidentate ligand)

• Coordination: [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]

Stereoisomers:

• Geometric (cis-trans): square planar MA₂B₂ and octahedral MA₄B₂

• Optical: non-superimposable mirror images — common in octahedral with bidentate ligands

Crystal Field Theory (CFT)

Ligands as point charges split d-orbitals:

Octahedral: d orbitals split into t₂g (lower, 3 orbitals) and eₘ (higher, 2 orbitals). Crystal Field Splitting Energy = Δo.

Strong field ligands (large Δo): CO > CN⁻ > NO₂⁻ > en > NH₃ > H₂O > F⁻ > Cl⁻ > Br⁻ > I⁻ (spectrochemical series)

Strong field → low spin (electrons pair up). Weak field → high spin.

EAN Rule & Metal Carbonyls

Effective Atomic Number (EAN): stable complexes often have 18 electrons around the metal (18-electron rule).

Metal carbonyls: Ni(CO)₄ (tetrahedral, Ni⁰), Fe(CO)₅ (trigonal bipyramidal), Cr(CO)₆ (octahedral). CO is a π-acceptor ligand — causes back-bonding.

Fe oxidation state test: In K₄[Fe(CN)₆]: Fe is +2 (ferrocyanide). In K₃[Fe(CN)₆]: Fe is +3 (ferricyanide).

Formula Vault

IUPAC rules, CFT splitting, and ligand charge facts.

Charge of Complex

n(metal) + Σcharge(ligands) = overall

Use to find oxidation state

Crystal Field Splitting

Octahedral: t₂g³ eₘ⁰ → t₂g⁶ eₘ⁰ etc.

Δo = CFT splitting energy

CFSE (Octahedral)

t₂g: −0.4Δo per e⁻; eₘ: +0.6Δo per e⁻

Net stabilisation energy

Spectrochemical Series

CO > CN⁻ > ... > F⁻ > I⁻

Strong field → large Δo → low spin

Common Ligand Charges

Cl⁻,Br⁻,I⁻,OH⁻,CN⁻: −1
ox²⁻,SO₄²⁻: −2
NH₃,H₂O,CO: 0

Neutral or anionic

Naming Cation First

Cation before anion
Ligands alphabetically
Metal last with (OS)

IUPAC nomenclature order

Hybridisation of Metal

CN=4: sp³ or dsp²
CN=6: sp³d² or d²sp³

VBT prediction

Effective Atomic Number

EAN = Z − oxidation state + 2×(ligand e⁻)

Stable if EAN = noble gas

Worked Examples

5 problems — IUPAC naming, oxidation state, isomerism, CFT, and magnetic properties.

EasyName the complex [Co(NH₃)₅Cl]Cl₂▾

Give the IUPAC name of [Co(NH₃)₅Cl]Cl₂.

Ligands in coordination sphere: 5 NH₃ (neutral, ammine) + 1 Cl⁻ (chlorido)

Charge: x + 5(0) + 1(−1) = +2 → Co is +3. Wait: the complex ion is [Co(NH₃)₅Cl]²⁺ (with 2Cl⁻ outside).

Name: pentaamminechloridocobalt(III) chloride

✓ Pentaamminechloridocobalt(III) chloride

EasyFind oxidation state of Fe in K₄[Fe(CN)₆]▾

Find the oxidation state of Fe in K₄[Fe(CN)₆].

Charge balance: 4(+1) + [Fe + 6(CN⁻)] = 0

4 + Fe + 6(−1) = 0 → Fe − 2 = −4 → Fe = +2

Fe is in +2 oxidation state (ferrocyanide)

✓ Fe = +2 (potassium hexacyanoferrate(II))

MediumHow many geometric isomers does [Pt(NH₃)₂Cl₂] have?▾

Find the number of geometric isomers of square planar [Pt(NH₃)₂Cl₂].

Square planar MA₂B₂: two geometric isomers possible — cis and trans.

Cis: both NH₃ on same side. Trans: NH₃ on opposite sides.

Cis-platin (cancer drug) is the cis isomer.

✓ 2 isomers — cis-[Pt(NH₃)₂Cl₂] and trans-[Pt(NH₃)₂Cl₂]

EAPCET LevelPredict high-spin or low-spin for [Fe(CN)₆]⁴⁻▾

Predict whether [Fe(CN)₆]⁴⁻ is high-spin or low-spin. Find the number of unpaired electrons.

Fe in +2 state: electron configuration [Ar]3d⁶. 6 d electrons to fill.

CN⁻ is a strong field ligand (high in spectrochemical series) → large Δo.

Low-spin d⁶: all 6 electrons fill t₂g³ first, then pair up → t₂g⁶ eₘ⁰

Paired configuration: t₂g⁶ = 6 electrons all paired → 0 unpaired electrons. Diamagnetic.

✓ Low-spin, 0 unpaired electrons, diamagnetic

Trap QuestionCO is a positively charged ligand since it's neutral — False?▾

A student says: 'CO is neutral as a free molecule, so it has no effect on the charge of the complex.' Evaluate.

CO is indeed neutral (charge = 0) as a free ligand — the student is correct on this point.

The trap is in the next step: students then incorrectly ignore CO when calculating metal oxidation state, which is actually correct here.

For Ni(CO)₄: charge = 0 (neutral complex). CO = 0 each. So Ni = 0 (zerovalent). Ni⁰, not Ni²⁺.

CO is a π-acceptor ligand — it accepts electron density from metal d-orbitals. This stabilises low oxidation states of metals.

✓ CO is neutral (charge = 0) ✓ — Ni(CO)₄ has Ni in 0 oxidation state; CO stabilises low OS via π backbonding

Mistake DNA

4 coordination chemistry errors that EAPCET distractors exploit.

🔢

Wrong Oxidation State Due to Incorrect Ligand Charge

Students assign wrong charge to common ligands, leading to wrong metal oxidation state.

❌ Wrong

K₃[Fe(CN)₆]: CN = 0 (neutral) ✗ → Fe = −3 (nonsense)

✓ Correct

CN⁻ = −1 each ✓ 3(+1) + Fe + 6(−1) = 0 Fe = +3 ✓

Memorise ligand charges: Cl⁻, Br⁻, I⁻, OH⁻, CN⁻ = −1. SO₄²⁻, ox²⁻, CO₃²⁻ = −2. NH₃, H₂O, CO, en = 0.

🔤

IUPAC: Listing Ligands in Wrong Order

IUPAC requires ligands listed alphabetically (ignoring di/tri prefixes).

❌ Wrong

[CoCl₂(en)(NH₃)₂]Cl: en listed after NH₃ (alphabetical ignored) ✗

✓ Correct

Alphabetical: ammine, dichloro, ethylenediamine ✓ (ignore 'di' prefix for alphabetical order) ✓

Alphabetical order uses the actual ligand name, not the prefix. 'Diammine' comes after 'chlorido' because you compare 'ammine' vs 'chlorido', ignoring 'di'.

🧲

All Transition Metal Complexes are Paramagnetic

Paramagnetic (unpaired electrons) or diamagnetic depends on ligand field strength and d electron count.

❌ Wrong

[Fe(CN)₆]⁴⁻: d⁶, must have unpaired e⁻ → paramagnetic ✗

✓ Correct

CN⁻ = strong field → low spin ✓ d⁶ low spin → t₂g⁶ eₘ⁰ ✓ 0 unpaired → diamagnetic ✓

Strong-field ligands force electron pairing → low spin → fewer/no unpaired e⁻ → possibly diamagnetic. Weak-field ligands → high spin → more unpaired e⁻ → paramagnetic.

🔗

Coordination Number = Number of Ligands (Not Bonds)

Coordination number = number of DONOR ATOMS bonded to the metal, not the number of ligand molecules.

❌ Wrong

[Co(en)₃]³⁺ has 3 ligands: CN = 3 ✗ (en is bidentate!)

✓ Correct

en is bidentate (2 N donors) ✓ 3 en × 2 = 6 donor atoms CN = 6 ✓

Coordination number counts donor atom bonds. Bidentate ligand en provides 2 bonds each. Three en ligands → 6 donor atoms → CN = 6.

Chapter Intelligence

Coordination compounds requires systematic IUPAC knowledge and CFT — practise naming and oxidation states.

EAPCET Weightage (2019–2024)

Oxidation state of metal ion

IUPAC nomenclature

Isomerism (geometric, optical)

Magnetic property (spin state)

Crystal field theory

VBT hybridisation

High-Yield PYQ Patterns

Find OS of metal in complexIUPAC name of given complexGeometric isomers of MA₂B₂High spin vs low spin predictionMagnetic moment from unpaired e⁻Number of ions from complex in solutionCoordination number counting

Exam Strategy

Oxidation state: set up equation (sum of all charges = overall charge of complex). Ligand charges: neutral (NH₃, H₂O, CO, en=0), anionic (Cl⁻, CN⁻, OH⁻=−1; SO₄²⁻=−2).
IUPAC naming: ligands alphabetically → metal name → (oxidation state in Roman numerals). If complex anion, add -ate to metal (iron→ferrate, cobalt→cobaltate).
Magnetic properties: check if ligand is strong-field (CN⁻, CO, NO₂⁻) or weak-field (Cl⁻, Br⁻, H₂O). Strong → low spin → fewer unpaired e⁻.
Geometric isomers: square planar MA₂B₂ → cis/trans. Octahedral MA₄B₂ → cis/trans. MA₂B₂C₂ → more isomers.
This chapter connects to Atomic Structure (d-orbital splitting) and Transition Elements (d-block properties).

Concept Core

Formula Vault

Worked Examples

Mistake DNA

Chapter Intelligence

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