IUPAC Nomenclature of Coordination Compounds

Master the systematic naming of coordination compounds with step-by-step rules, tricks, and JEE-focused examples.

8 min read

Introduction

Naming coordination compounds can feel like learning a new language - and in many ways, it is! The International Union of Pure and Applied Chemistry (IUPAC) created systematic rules so that every chemist worldwide can understand the exact structure of a complex just from its name.

Drug Names and Coordination Chemistry
Ever wondered why cancer drug cisplatin is called “cis-platin”? It’s because of coordination chemistry nomenclature! The “cis” tells us that two chloride ligands are on the SAME side of the platinum atom. Its geometric isomer, “transplatin,” has them on opposite sides - and is completely inactive against cancer! One small naming detail = life-saving vs. useless. That’s why IUPAC nomenclature matters!

The Golden Rules of Nomenclature

Rule 1: Order of Writing

Always write CATION before ANION (just like NaCl, not ClNa)

Examples:

  • [Co(NH₃)₆]Cl₃ → Hexaamminecobalt(III) chloride
  • K₄[Fe(CN)₆] → Potassium hexacyanidoferrate(II)

Rule 2: Inside the Coordination Sphere

Ligands first, then metal

Within ligands: Alphabetical order (ignore prefixes like di-, tri-)

graph LR
    A[Ligands] --> B[Metal]
    A --> A1[Alphabetical order]
    A --> A2[Ignore di-, tri-, tetra-]
    A --> A3[Anionic ligands get -o suffix]
    B --> B1[Metal name]
    B --> B2[Oxidation state in Roman numerals]

Rule 3: Ligand Naming

Neutral Ligands

Most keep their molecular names, except:

  • H₂O → aqua
  • NH₃ → ammine (note: double ’m')
  • CO → carbonyl
  • NO → nitrosyl

Anionic Ligands

Add -o suffix:

  • Cl⁻ → chlorido
  • CN⁻ → cyanido
  • OH⁻ → hydroxido
  • SO₄²⁻ → sulfato
  • NO₂⁻ → nitrito
Don't Confuse!
Ammine (NH₃ ligand) vs. Amine (organic compound R-NH₂) Nitrito (NO₂⁻ ligand) vs. Nitro (NO₂ group in organic chemistry)

Rule 4: Numerical Prefixes

NumberSimple ligandsComplex ligands
2di-bis-
3tri-tris-
4tetra-tetrakis-
5penta-pentakis-
6hexa-hexakis-

Use bis-, tris-, tetrakis- for:

  • Ligands with numbers already in them (ethylenediamine)
  • Ligands in parentheses (CH₃)₂NH

Rule 5: Metal Naming

In cationic or neutral complexes: Use the element name + oxidation state in Roman numerals

In anionic complexes: Add -ate suffix to the metal:

  • Iron → ferrate
  • Copper → cuprate
  • Chromium → chromate
  • Aluminum → aluminate

For some metals, use Latin names:

  • Iron → ferrate (not ironate)
  • Copper → cuprate (not copperate)
  • Silver → argentate
  • Gold → aurate
  • Tin → stannate
  • Lead → plumbate

Common Ligands Quick Reference

Neutral Ligands

LigandNameExample
NH₃ammine[Co(NH₃)₆]³⁺ = hexaamminecobalt(III)
H₂Oaqua[Cr(H₂O)₆]³⁺ = hexaaquachromium(III)
COcarbonyl[Ni(CO)₄] = tetracarbonylnickel(0)
NOnitrosyl[Fe(H₂O)₅(NO)]²⁺
py (pyridine)pyridine[Pt(py)₄]²⁺
en (ethylenediamine)ethylenediamine[Co(en)₃]³⁺

Anionic Ligands

LigandNameExample
Cl⁻chlorido[PtCl₆]²⁻ = hexachloridoplatinate(IV)
Br⁻bromido[CoBr₄]²⁻ = tetrabromi docobaltate(II)
CN⁻cyanido[Fe(CN)₆]⁴⁻ = hexacyanidoferrate(II)
OH⁻hydroxido[Al(OH)₄]⁻ = tetrahydroxidoaluminate(III)
O²⁻oxido[MnO₄]⁻ = tetraoxidomanganate(VII)
NO₂⁻nitrito-κN[Co(NH₃)₅(NO₂)]²⁺
ONO⁻nitrito-κO[Co(NH₃)₅(ONO)]²⁺
SCN⁻thiocyanato-κS[Fe(NCS)₆]³⁻
NCS⁻thiocyanato-κN[Co(NH₃)₅(NCS)]²⁺
Ambidentate Ligands

For ligands that can bind through different atoms (ambidentate):

  • Use κ (kappa) to specify binding atom
  • κN = bonded through nitrogen
  • κO = bonded through oxygen
  • κS = bonded through sulfur

Step-by-Step Naming Process

For Cationic/Neutral Complexes

Step 1: Write prefixes + ligand names (alphabetically) Step 2: Write metal name Step 3: Write oxidation state in Roman numerals in parentheses Step 4: Add counter ion name (if any)

Example: [Cr(NH₃)₄Cl₂]Cl

  1. Ligands: 4 NH₃ (tetraammine) + 2 Cl⁻ (dichloro)
  2. Alphabetical: “a” before “c” → tetraamminedichloro
  3. Metal: chromium
  4. Oxidation state: +3
  5. Counter ion: chloride

Answer: Tetraamminedichloridochromium(III) chloride

For Anionic Complexes

Same as above, but metal name gets -ate suffix

Example: K₃[Fe(CN)₆]

  1. Ligands: 6 CN⁻ (hexacyanido)
  2. Metal: iron → ferrate
  3. Oxidation state: +3
  4. Cation: potassium

Answer: Potassium hexacyanidoferrate(III)

Worked Examples

Example 1: Simple Cationic Complex

Formula: [Co(NH₃)₆]Cl₃

Step-by-step:

  • Ligands: 6 NH₃ → hexaammine
  • Metal: cobalt
  • Oxidation state: Let x be OS of Co
    • x + 6(0) = +3 (from 3 Cl⁻)
    • x = +3
  • Counter ion: chloride

Name: Hexaamminecobalt(III) chloride

Example 2: Mixed Ligands

Formula: [Pt(NH₃)₂Cl₂]

Step-by-step:

  • Ligands: 2 NH₃ (diammine) + 2 Cl⁻ (dichloro)
  • Alphabetical order: ammine before chloro
  • Metal: platinum
  • Oxidation state: 0 + 2(0) + 2(-1) = -2
    • For neutral complex: OS = +2
  • No counter ion

Name: Diamminedichloridoplatinum(II)

Example 3: Anionic Complex

Formula: K₄[Fe(CN)₆]

Step-by-step:

  • Ligands: 6 CN⁻ → hexacyanido
  • Metal: iron → ferrate (anionic complex)
  • Oxidation state: 4(-1) + x + 6(-1) = 0
    • x = +2
  • Cation: potassium

Name: Potassium hexacyanidoferrate(II)

Example 4: Complex with Bidentate Ligand

Formula: [Co(en)₃]Cl₃

Step-by-step:

  • Ligands: 3 en (ethylenediamine)
  • Use “tris” for complex ligands: tris(ethylenediamine)
  • Metal: cobalt
  • Oxidation state: +3

Name: Tris(ethylenediamine)cobalt(III) chloride

Example 5: Coordination Entity Only

Formula: [Ni(CO)₄]

Step-by-step:

  • Ligands: 4 CO → tetracarbonyl
  • Metal: nickel
  • Oxidation state: 0 (neutral ligands, no counter ion)

Name: Tetracarbonylnickel(0)

Example 6: Multiple Different Ligands

Formula: [Cr(H₂O)₄Cl₂]Cl

Step-by-step:

  • Ligands: 4 H₂O (tetraaqua) + 2 Cl⁻ (dichloro)
  • Alphabetical: aqua before chloro
  • Metal: chromium
  • Oxidation state: +3

Name: Tetraaquadichloridochromium(III) chloride

Reverse Process: Writing Formulas from Names

Example 1

Name: Hexaamminenickel(II) chloride

Step-by-step:

  1. “Hexaammine” → 6 NH₃
  2. “Nickel(II)” → Ni²⁺ central metal
  3. “chloride” → Cl⁻ counter ion (outside coordination sphere)
  4. Balance charge: [Ni(NH₃)₆]²⁺ needs 2 Cl⁻

Formula: [Ni(NH₃)₆]Cl₂

Example 2

Name: Potassium hexacyanidoferrate(II)

Step-by-step:

  1. “Potassium” → K⁺ counter ion
  2. “Hexacyanido” → 6 CN⁻
  3. “Ferrate(II)” → Fe²⁺ in anionic complex
  4. Complex charge: Fe²⁺ + 6(-1) = -4
  5. Need 4 K⁺

Formula: K₄[Fe(CN)₆]

Memory Tricks

The “ALAN” Rule

Anionic ligands get -ido/-o Ligands are listed alphabetically Ate suffix for anionic complexes Numerical prefixes ignored in alphabetization

Common Confusions Chart

WriteDON’T WriteWhy
ammineamineNH₃ ligand needs double ’m'
chloridochloroModern IUPAC uses -ido
aquawaterSpecific name for H₂O ligand
hexaaquahexawaterUse proper ligand name
ferrateironateUse Latin name
bis(en)dien“bis” for complex ligands

Oxidation State Trick

For neutral complex [M(ligands)]:

  1. Sum charges of all ligands
  2. OS of metal = -(sum)

For charged complex:

  1. Let OS = x
  2. x + (sum of ligand charges) = overall charge

Common Mistakes

Mistake 1: Wrong Alphabetical Order

Wrong: Hexaamminedichloridocobalt(III) chloride Right: Hexaamminedichloridocobalt(III) chloride

Wait - both look the same! That’s because it’s already correct. The mistake would be: Wrong: Dichlorohexaamminecobalt(III) chloride Right: Hexaamminedichloridocobalt(III) chloride

Alphabetize by ligand name (ammine, chlorido), NOT by prefix (di-, hexa-)!

Mistake 2: Forgetting -ate for Anionic Complexes

Wrong: Potassium hexacyanidoiron(II) Right: Potassium hexacyanidoferrate(II)

Anionic complexes MUST use -ate suffix (and often Latin names)!

Mistake 3: Including Cation in Complex Name

Wrong: [Sodium hexachloridoplatinate(IV)] Right: Sodium hexachloridoplatinate(IV)

The cation name is separate, NOT part of the complex name!

Mistake 4: Wrong Counter Ion Placement

Wrong: [Co(NH₃)₆Cl₃] Right: [Co(NH₃)₆]Cl₃

Counter ions stay OUTSIDE the square brackets!

Practice Problems

Level 1: Basic Naming

Q1. Name the following coordination compounds: a) [Cu(NH₃)₄]SO₄ b) [Ag(NH₃)₂]Cl c) K₃[Al(C₂O₄)₃] d) [Co(NH₃)₅Cl]Cl₂ e) [Pt(NH₃)₂Cl(NO₂)]

Q2. Write formulas for: a) Hexaammineplatinum(IV) chloride b) Potassium trioxalatoaluminate(III) c) Pentaamminechloridocobalt(III) chloride

Level 2: Complex Scenarios

Q3. Name the following: a) [Cr(H₂O)₅Cl]Cl₂·H₂O b) [Co(en)₂(NO₂)₂]Cl c) [Pt(py)₄][PtCl₄] d) K₂[PtCl₆] e) [Fe(H₂O)₅(NCS)]SO₄

Q4. Write formulas for: a) Pentaamminenitrito-κN-cobalt(III) sulfate b) Tris(ethylenediamine)chromium(III) chloride c) Sodium tetrahydroxidozincate(II)

Q5. For [Cr(NH₃)₄Cl₂]NO₃: a) Name the compound b) What is the oxidation state of Cr? c) What is the coordination number? d) Identify the counter ion

Level 3: JEE Advanced

Q6. A coordination compound has molecular formula CoCl₃·4NH₃. Two isomers exist:

  • Isomer A gives 1 mole AgCl with AgNO₃
  • Isomer B gives 2 moles AgCl with AgNO₃

Write structural formulas and IUPAC names for both isomers.

Q7. Identify the incorrect IUPAC name: a) [Co(NH₃)₅(ONO)]Cl₂ - Pentaamminenitrito-κO-cobalt(III) chloride b) K₃[Fe(CN)₆] - Potassium hexacyanidoiron(III) c) [Pt(NH₃)₃Cl₃]Cl - Triamminetrichloridoplatinum(IV) chloride d) [Cr(en)₃]Cl₃ - Tris(ethylenediamine)chromium(III) chloride

Q8. Write IUPAC names for: a) [Co(NH₃)₅(H₂O)][NO₃]₃ b) [Pt(NH₃)₄Cl₂][PtCl₄] c) Na₃[Co(NO₂)₆]

Q9. A complex compound has the formula PtCl₂·2NH₃. It can exist in two isomeric forms. One is used as an anticancer drug. Write: a) Both possible structural formulas b) IUPAC names for both c) Which one is the anticancer drug and why?

Quick Check
What is wrong with this name: “Diammoniumdichloroplatinum(II)”? (Hint: Three mistakes!)

Solutions to Selected Problems

Q1. a) Tetraamminecopper(II) sulfate b) Diammine silver(I) chloride c) Potassium trioxalatoaluminate(III) d) Pentaamminechloridocobalt(III) chloride e) Diamminechloridonitr oplatinum(II)

Q2. a) [Pt(NH₃)₆]Cl₄ b) K₃[Al(C₂O₄)₃] c) [Co(NH₃)₅Cl]Cl₂

Q5. a) Tetraamminedichloridochromium(III) nitrate b) +3 c) 6 d) NO₃⁻

Q6.

  • Isomer A: [Co(NH₃)₄Cl₂]Cl - Tetraamminedichloridocobalt(III) chloride
  • Isomer B: [Co(NH₃)₄Cl]Cl₂ - (This formula doesn’t give CN=6; question might have error)

Q7. b) is incorrect. Should be “Potassium hexacyanidoferrate(III)” (use -ate for anionic complex)

Q9. a) cis-[Pt(NH₃)₂Cl₂] and trans-[Pt(NH₃)₂Cl₂] b) cis-Diamminedichloridoplatinum(II) and trans-Diamminedichloridoplatinum(II) c) cis-isomer (cisplatin) is the anticancer drug because the geometry allows it to bind DNA effectively

Special Cases and Advanced Topics

Bridging Ligands

When a ligand bridges two metal centers, use μ (mu):

Example: [(NH₃)₅Cr-OH-Cr(NH₃)₅]Cl₅ Name: μ-Hydroxidobis[pentaamminechromium(III)] chloride

Polynuclear Complexes

Use bis, tris, etc., for the entire complex units:

Example: [{Co(NH₃)₅}₂(μ-OH)]Cl₅ Name: μ-Hydroxidobis[pentaamminecobalt(III)] chloride

Why Nomenclature Matters

  1. Unambiguous communication: Scientists worldwide understand exact structure
  2. Drug development: Cisplatin vs transplatin - only one works!
  3. Safety: Some isomers are toxic, others safe
  4. Research: Precise naming enables reproducible science
  5. JEE exams: 3-5 marks guaranteed in every paper!
Real-World Application
In 2007, a pharmaceutical company nearly lost $50 million because two research teams were working on the SAME compound but using different non-systematic names. They didn’t realize it until months later. IUPAC nomenclature prevents such disasters!

Within Coordination Compounds

Cross-Chapter Connections

Interactive Demo: Visualize Coordination Complex Structures

See 3D structures of coordination compounds and understand ligand arrangements.