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.
1
Ligands in coordination sphere: 5 NH₃ (neutral, ammine) + 1 Cl⁻ (chlorido)
2
Charge: x + 5(0) + 1(−1) = +2 → Co is +3. Wait: the complex ion is [Co(NH₃)₅Cl]²⁺ (with 2Cl⁻ outside).
3
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].
1
Charge balance: 4(+1) + [Fe + 6(CN⁻)] = 0
2
4 + Fe + 6(−1) = 0 → Fe − 2 = −4 → Fe = +2
3
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].
1
Square planar MA₂B₂: two geometric isomers possible — cis and trans.
2
Cis: both NH₃ on same side. Trans: NH₃ on opposite sides.
3
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.
1
Fe in +2 state: electron configuration [Ar]3d⁶. 6 d electrons to fill.
2
CN⁻ is a strong field ligand (high in spectrochemical series) → large Δo.
3
Low-spin d⁶: all 6 electrons fill t₂g³ first, then pair up → t₂g⁶ eₘ⁰
4
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.
1
CO is indeed neutral (charge = 0) as a free ligand — the student is correct on this point.
2
The trap is in the next step: students then incorrectly ignore CO when calculating metal oxidation state, which is actually correct here.
3
For Ni(CO)₄: charge = 0 (neutral complex). CO = 0 each. So Ni = 0 (zerovalent). Ni⁰, not Ni²⁺.
4
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~8 Isomerism (geometric, optical)~6 Magnetic property (spin state)~5 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).