IUPAC Nomenclature, Ligands and Coordination Number

• Define coordination entity, coordination number, and ligand
• Classify ligands by denticity: mono, bi, polydentate, and chelate
• Apply IUPAC rules to name coordination compounds systematically
• Write formulas from IUPAC names and vice versa
• Name anionic complex ions using -ate suffix with correct metal roots
• Apply the EAN rule to predict stability of complexes

Level 1 — Foundations

Basic Definitions

• Central metal atom/ion: Accepts electron pairs from ligands (Lewis acid)
• Ligand: Electron pair donor bonded to central metal (Lewis base)
• Coordination entity: The entire complex unit, inside square brackets: [Co(NH₃)₆]³⁺
• Coordination number (CN): Total number of donor atoms bonded to central metal (NOT number of ligands if bidentate)

Ligand Types by Denticity

Chelate: A polydentate ligand bonded to the same metal through multiple donor atoms, forming a ring. Chelates are generally more stable than analogous monodentate complexes (chelate effect — entropy driven).

Level 2 — JEE Depth

IUPAC Naming Rules (Step-by-Step)

1. Cation before anion (same as ionic compounds): name the cation complex first, then the anion.
2. Inside square brackets: name ligands alphabetically, then the metal with oxidation state.
3. Anionic ligands: change suffix to -o
   • F⁻ → fluoro (or fluorido), Cl⁻ → chlorido, Br⁻ → bromido, I⁻ → iodido
   • OH⁻ → hydroxido, CN⁻ → cyano (cyanido), NO₂⁻ → nitrito (O-bonded) or nitro (N-bonded)
   • O²⁻ → oxo, O₂²⁻ → peroxido
4. Neutral ligands: keep name, except these special cases:
   • H₂O → aqua, NH₃ → ammine (double m), CO → carbonyl, NO → nitrosyl
5. Number prefixes: di, tri, tetra, penta, hexa for simple ligands; bis, tris, tetrakis, pentakis, hexakis for complex ligands (those containing di/tri already, or for polydentate)
6. Complex anion: metal name uses Latin root + suffix -ate
   • Fe → ferrate, Cu → cuprate, Pb → plumbate, Au → aurate, Ag → argentate, Sn → stannate
   • Others use English name + -ate: Co → cobaltate, Cr → chromate, Zn → zincate

Common Ligands Table

Effective Atomic Number (EAN) Rule
EAN = (electrons on metal ion) + (electrons donated by all ligands)
Stable complexes often achieve EAN = 36 (Kr), 54 (Xe), or 86 (Rn) — noble gas configuration
Example: [Co(NH₃)₆]³⁺: Co³⁺ has 24 e⁻; 6 NH₃ donate 6×2=12 e⁻; EAN = 24+12 = 36 (Kr config) ✓
Note: EAN is an older rule; it works well for carbonyls but has many exceptions in other complexes.

JEE Traps

• NH₃ is "ammine" (two m's) in complexes, not "amine"
• Alphabetical order ignores prefixes: tetraammine comes before chlorido? No — the alpha order is on the ligand name: "a" for ammine, "c" for chlorido → ammine first
• Coordination number ≠ number of ligands when bidentate/polydentate ligands are present: [Co(en)₃]³⁺ has 3 ligands but CN = 6
• Anionic complex uses Latin metal name (-ate): [Fe(CN)₆]⁴⁻ is hexacyanoferrate(II), not hexacyanoiron(II)

Example 1: Naming [Co(NH₃)₄(Cl)₂]⁺

Step 1: Identify components
  Central metal: Co (cobalt)
  Ligands: 4 NH₃ (ammine, neutral) and 2 Cl⁻ (chlorido, anionic)
  Charge on complex: +1

Step 2: Find oxidation state of Co
  Let Co = x
  x + 4(0) + 2(−1) = +1
  x − 2 = +1
  x = +3 → Co(III)

Step 3: Name ligands alphabetically
  ammine (a) comes before chlorido (c)
  4 NH₃ → tetraammine (use 'tetra' prefix for 4; 'ammine' is simple)
  2 Cl⁻ → dichlorido

Step 4: Name the complex
  Complex is cationic (or neutral in this case it's +1)
  Metal name stays as cobalt (not -ate since it's cationic/neutral)
  → tetraamminedichloridocobalt(III) ion

  Full name: tetraamminedichloridocobalt(III) chloride (if the counter-ion is Cl⁻)
  But the question asks to name [Co(NH₃)₄Cl₂]⁺
  → tetraamminedichloridocobalt(III) ion

Answer: tetraamminedichloridocobalt(III)

Example 2: Writing Formula for tetraamminecopper(II) sulphate

Step 1: Parse the name
  tetraammine: 4 NH₃ ligands
  copper(II): Cu²⁺, oxidation state +2
  sulphate: SO₄²⁻ counter-ion (outside coordination sphere)

Step 2: Coordination sphere charge
  Cu²⁺ + 4 NH₃(0) → complex cation has charge +2

Step 3: Counter-ion needed
  Charge of cation = +2; sulphate SO₄²⁻ has charge −2
  One sulphate balances one copper(II) complex → ratio 1:1

Step 4: Write formula
  Complex cation: [Cu(NH₃)₄]²⁺
  Full formula: [Cu(NH₃)₄]SO₄

Answer: [Cu(NH₃)₄]SO₄
(This is Schweizer's reagent — dissolves cellulose, used historically in silk/rayon production)

• Q1: What is the coordination number of cobalt in [Co(en)₂Cl₂]⁺?
• Q2: Name the complex [Cr(H₂O)₄Cl₂]Cl.
• Q3: Write the formula of potassium hexacyanidoferrate(III).
• Q4: EDTA is hexadentate. If [Co(EDTA)]⁻ has CN = 6, what is the oxidation state of Co?
• Stretch: Q5: [Pt(NH₃)₄][PtCl₄] — name both the cation and anion parts. What type of isomerism does this compound show with [Pt(NH₃)₂Cl₂]?

NCERT Chapter 9 link: Chapter 9 "Coordination Compounds" Class 12 — Sections 9.2–9.4 cover Werner's theory, definitions, nomenclature rules, and worked examples. NCERT Table 9.2 lists common ligands and their names.

Exam connections: JEE Mains: name a given formula, write formula from name, find oxidation state, identify CN. JEE Advanced: linkage isomerism naming (nitro vs nitrito), naming chelate complexes with EDTA or en, EAN calculation for carbonyls, comparing stability of chelate vs monodentate complexes.

Study strategy: Practise naming in both directions (formula→name, name→formula) as separate drills. Memorise the 5 special neutral ligands (aqua, ammine, carbonyl, nitrosyl + phosphine). The Latin roots (ferrate, cuprate, plumbate, aurate, argentate) appear in both nomenclature and isomerism questions.

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