CBSE Class 12 Coordination Compounds — Naming and Isomerism
Coordination compounds reward students who work to a fixed procedure. Naming is a checklist of six rules applied in order. Isomerism is a list of six named types, each with one standard example. Neither needs cleverness — it needs the steps written down and practised until they are automatic. This guide gives you both, with the working shown.
The vocabulary you must be precise about
- Coordination entity — the part inside square brackets, e.g. [Co(NH₃)₆]³⁺. It survives dissolution as one unit.
- Ligand — an ion or molecule donating a lone pair to the metal. Monodentate donates through one atom (NH₃, Cl⁻, CN⁻); bidentate through two (ethane-1,2-diamine, oxalate); ambidentate can donate through either of two different atoms (NO₂⁻, SCN⁻).
- Coordination number — the number of donor atoms bonded to the metal, not the number of ligand particles. In [Co(en)₃]³⁺ there are three ligands but the coordination number is 3 × 2 = 6.
- Counter ion — the ion written outside the brackets. It is free in solution and will give the usual precipitation tests; a ligand inside the brackets will not.
The six naming rules, in order
2. Inside the complex, name the ligands alphabetically, then the metal.
3. Use di-, tri-, tetra- for simple ligands; use bis-, tris-, tetrakis- when the ligand name already contains a number or is a longer name in brackets.
4. Anionic ligands end in -o; neutral ligands keep their name, with the special cases aqua (H₂O), ammine (NH₃), carbonyl (CO), nitrosyl (NO).
5. Give the metal's oxidation state as a Roman numeral in brackets.
6. If the whole complex is an anion, the metal name takes the suffix -ate.
An edition difference you should know about. Current IUPAC practice, followed by recent NCERT printings, names anionic ligands as chlorido, cyanido, hydroxido, nitrito. Older textbooks and many question banks use chloro, cyano, hydroxo, nitro. Both refer to the same ligand and CBSE has accepted both forms; write the version your prescribed NCERT edition uses and stay consistent within one answer.
| Ligand | Current name | Older name |
|---|---|---|
| Cl⁻ | chlorido | chloro |
| CN⁻ | cyanido | cyano |
| OH⁻ | hydroxido | hydroxo |
| NO₂⁻ (through N) | nitrito-N | nitro |
| NO₂⁻ (through O) | nitrito-O | nitrito |
| H₂O | aqua (unchanged) | |
| NH₃ | ammine — two m's (unchanged) | |
Metals that change name in an anionic complex: iron → ferrate, copper → cuprate, silver → argentate, lead → plumbate, tin → stannate, gold → aurate. Others simply add -ate: cobaltate, nickelate, chromate, manganate, platinate.
Worked example 1 — [Co(NH₃)₆]Cl₃
Step 1 — oxidation state. Three chlorides outside carry −3 in total, so the
complex ion is [Co(NH₃)₆]³⁺. NH₃ is neutral, so
x + 6(0) = +3 → x = +3, cobalt(III).
Step 2 — the ligand. Six ammonia = hexaammine. Note the spelling: two m's, and "hexaammine" keeps both a's (hexa + ammine).
Step 3 — assemble. The complex is the cation, so it is named first, then the chloride.
Answer: hexaamminecobalt(III) chloride. Coordination number 6.
Worked example 2 — K₄[Fe(CN)₆]
Four K⁺ outside means the complex ion carries −4: [Fe(CN)₆]⁴⁻.
x + 6(−1) = −4 → x − 6 = −4 → x = +2, iron(II).
The complex is an anion, so rule 6 applies: iron becomes ferrate. The cation potassium is named first.
Answer: potassium hexacyanidoferrate(II) — or potassium hexacyanoferrate(II) in the older style.
Worked example 3 — [Cr(NH₃)₃(H₂O)₃]Cl₃ and [CoCl₂(en)₂]⁺
[Cr(NH₃)₃(H₂O)₃]Cl₃: x + 3(0) + 3(0) = +3 → x = +3.
Alphabetical order of ligand names: ammine before aqua (compare the third
letter, m before q). Prefixes are ignored when alphabetising.
Answer: triamminetriaquachromium(III) chloride.
[CoCl₂(en)₂]⁺ where en = ethane-1,2-diamine (ethylenediamine):
x + 2(−1) + 2(0) = +1 → x − 2 = +1 → x = +3.
"en" is a longer ligand name used in brackets, so rule 3 gives bis, never
"di". Alphabetically, chlorido (c) comes before ethane-1,2-diamine (e).
Answer: dichloridobis(ethane-1,2-diamine)cobalt(III) ion. Coordination
number 6, because each en supplies two donor nitrogen atoms.
Worked example 4 — the reverse direction
Write the formula of tetraamminedichloridocobalt(III) chloride.
Metal symbol first, then the ligands in alphabetical order inside the square brackets: Co, then ammine (NH₃), then chlorido (Cl).
Charge check: Co is +3, four NH₃ contribute 0, two Cl⁻ contribute −2, so the complex ion is 3 − 2 = +1. One chloride is needed outside to balance it.
Answer: [Co(NH₃)₄Cl₂]Cl. Always finish by checking that the total charge of the formula is zero — that catches most errors.
Isomerism — the six types with one example each
Isomers have the same molecular formula but a different arrangement. Coordination chemistry splits them into structural isomers (different bonds) and stereoisomers (same bonds, different arrangement in space).
Structural isomerism — four types
| Type | What differs | Standard example |
|---|---|---|
| Ionisation | Which ion is inside the brackets and which is the counter ion | [Co(NH₃)₅SO₄]Br and [Co(NH₃)₅Br]SO₄ |
| Hydrate (solvate) | Whether water is a ligand or is water of crystallisation | [Cr(H₂O)₆]Cl₃, [Cr(H₂O)₅Cl]Cl₂·H₂O and [Cr(H₂O)₄Cl₂]Cl·2H₂O — all CrCl₃·6H₂O, and described in textbooks as violet, grey-green and dark green |
| Linkage | Which atom of an ambidentate ligand is bonded | [Co(NH₃)₅(NO₂)]Cl₂ — the yellow form is bonded through nitrogen, the red form through oxygen |
| Coordination | How the ligands are shared between a complex cation and a complex anion | [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆] |
How ionisation isomers are told apart in the lab — a favourite exam follow-up. [Co(NH₃)₅SO₄]Br releases free Br⁻, so it gives a pale yellow precipitate of AgBr with silver nitrate but no precipitate with barium chloride. [Co(NH₃)₅Br]SO₄ releases free SO₄²⁻, so it gives a white precipitate of BaSO₄ with barium chloride but none with silver nitrate.
Stereoisomerism — geometrical and optical
Geometrical (cis–trans). Two identical ligands are either adjacent (cis, 90° apart in an octahedron) or opposite (trans, 180° apart).
- Square planar MA₂B₂ — two isomers. [Pt(NH₃)₂Cl₂] is the classic pair; the cis form is the anticancer drug cisplatin and the trans form is not used that way, which is a real illustration that geometry, not composition, decides behaviour.
- Octahedral MA₄B₂ — two isomers, e.g. [Co(NH₃)₄Cl₂]⁺ as cis and trans.
- Octahedral MA₃B₃ — two isomers described as facial (fac), where the three identical ligands occupy one triangular face, and meridional (mer), where they lie around a meridian. [Co(NH₃)₃(NO₂)₃] is the standard example.
- Tetrahedral complexes do not show geometrical isomerism — in a tetrahedron every position is adjacent to every other, so there is no "opposite".
Optical isomerism. A complex is optically active if it has no plane of symmetry, so that it and its mirror image cannot be superimposed. The two forms are called d (dextro) and l (laevo) and rotate plane-polarised light in opposite directions.
- [Co(en)₃]³⁺ exists as a d and an l form.
- In [CoCl₂(en)₂]⁺ only the cis isomer is optically active. The trans isomer has a plane of symmetry, so it is superimposable on its mirror image and is optically inactive.
- cis-[Co(NH₃)₄Cl₂]⁺, by contrast, does have a plane of symmetry and is not optically active — so "cis" does not automatically mean chiral. Check for the symmetry plane every time.
Mistakes that cost marks
- Counting ligands instead of donor atoms. [Co(en)₃]³⁺ has coordination number 6, not 3; [Cr(C₂O₄)₃]³⁻ is also 6.
- Writing "diethylenediamine". Rule 3 requires bis(ethane-1,2-diamine), because "di" inside the ligand name would be ambiguous.
- Alphabetising by the prefix. "Tetraammine" is filed under A for ammine, not under T.
- Forgetting the overall charge when finding the oxidation state. In [CoCl₂(en)₂]⁺ the right-hand side is +1, not 0.
- Ammine and amine. Ammine (two m's) is coordinated NH₃; amine (one m) is an organic compound. One letter, one mark.
- Claiming cis–trans isomerism for a tetrahedral complex.
- Assuming every cis isomer is optically active. It is optical activity that requires the absence of a symmetry plane, and many cis isomers have one.
- Confusing ionisation with coordination isomerism. Ionisation isomers swap a ligand with a counter ion; coordination isomers swap ligands between two complex ions.
Where this appears in your exam
| Question type | What is being tested |
|---|---|
| "Write the IUPAC name of …" | The six rules applied in order, with the oxidation state worked out |
| "Write the formula of …" | The reverse procedure plus a charge balance check |
| "Draw / state the number of isomers of …" | Recognising the MA₄B₂ or MA₃B₃ pattern |
| "Which isomer is optically active and why?" | The plane-of-symmetry test |
| "How would you distinguish the two isomers?" | AgNO₃ and BaCl₂ precipitation tests |
| "Give the coordination number and oxidation state" | Donor atoms, and charge arithmetic |
Coordination chemistry is a compact chapter that repays practice, but check the current syllabus and the naming convention used in your prescribed edition from the official CBSE notification and the latest NCERT textbook.
Check the metal before you name the complex. The Interactive Periodic Table gives you each metal's symbol, group, block and electron configuration, so you can confirm which oxidation states are reasonable and spot an arithmetic slip before it reaches your answer sheet.
Open the Interactive Periodic Table →Want naming and isomerism drilled with graded practice rather than read once and forgotten? ABC Chemistry runs Class 11–12 chemistry coaching from its Gurugram centre and online classes across India — details at abcchemistry.in. Families in Delhi, Noida or Gurgaon who prefer one-to-one teaching at home can also arrange home tuition.