GATE Coordination Chemistry — Isomerism and Nomenclature
Coordination chemistry in GATE rewards two skills that have nothing to do with reaction mechanisms: telling isomer types apart, and writing (or reading) a correct IUPAC name. Both are fully mechanical once you know the rules, which makes them some of the safest marks in the Inorganic section — provided you have actually memorised the rules rather than half-remembered them. This guide covers every category of isomerism a coordination compound can show, then the complete naming procedure, each with worked examples.
The isomerism family tree
├── Structural (constitutional) isomerism — different connectivity
│ ├── Ionization isomerism
│ ├── Hydrate (solvate) isomerism
│ ├── Linkage isomerism
│ ├── Coordination isomerism
│ └── Polymerization isomerism
└── Stereoisomerism — same connectivity, different spatial arrangement
├── Geometrical isomerism (cis–trans, fac–mer)
└── Optical isomerism (chiral pairs)
Structural isomerism, one type at a time
Ionization isomerism — two compounds with the same overall formula give different ions in solution because a ligand and the counter-ion have swapped roles. [Co(NH₃)₅Br]SO₄ ionises to give SO₄²⁻ in solution and gives a white precipitate with BaCl₂ (test for sulfate), while [Co(NH₃)₅SO₄]Br ionises to give Br⁻ and gives a pale yellow precipitate with AgNO₃ instead. Same atoms, different ion released.
Hydrate (solvate) isomerism — water is either a ligand bound to the metal or lattice (crystallisation) water sitting outside the coordination sphere, and which role it plays changes the compound's properties. The classic set, all with formula CrCl₃·6H₂O:
| Formula | Colour | Cl⁻ precipitated instantly by AgNO₃ |
|---|---|---|
| [Cr(H₂O)₆]Cl₃ | Violet | 3 (all three) |
| [Cr(H₂O)₅Cl]Cl₂·H₂O | Grey-green | 2 |
| [Cr(H₂O)₄Cl₂]Cl·2H₂O | Dark green | 1 |
Only chloride ions outside the coordination sphere react instantly with AgNO₃ — a Cl⁻ that is a ligand does not precipitate until the complex is broken down. That reactivity difference is exactly how these three were told apart experimentally, long before spectroscopy.
Linkage isomerism — an ambidentate ligand (one with two different donor atoms) binds through either atom. The nitrite ion NO₂⁻ can bind through N (nitro, M–NO₂) or through O (nitrito, M–ONO); the thiocyanate ion SCN⁻ can bind through S (thiocyanato, M–SCN) or through N (isothiocyanato, M–NCS). [Co(NH₃)₅(NO₂)]Cl₂ and [Co(NH₃)₅(ONO)]Cl₂ are linkage isomers of each other.
Coordination isomerism — occurs only when both the cation and the anion of a salt are complex ions, and the ligands are distributed differently between the two metal centres. [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆] have exactly the same overall atoms but the ammine and cyanide ligands have swapped which metal they surround.
Polymerization isomerism — compounds with the same empirical (simplest-ratio) formula but different actual molecular formulas, related as integer multiples of each other. Both [Pt(NH₃)₂Cl₂] and [Pt(NH₃)₄][PtCl₄] reduce to the empirical formula Pt(NH₃)₂Cl₂, but the second is literally twice the atoms of the first, split into a cation and an anion. Some sources classify this as a special case of coordination isomerism; either label is accepted if you can explain the formula relationship.
Stereoisomerism
Geometrical isomerism arises in square planar and octahedral complexes when ligands can occupy adjacent (cis) or opposite (trans) positions.
Worked example 1 — square planar [Ma₂b₂]. A square planar complex with two identical ligands a and two identical ligands b (e.g. [Pt(NH₃)₂Cl₂]) has exactly two geometrical isomers: cis (the two a ligands adjacent, 90° apart) and trans (the two a ligands opposite, 180° apart). This pair is famous in real chemistry — cis-[Pt(NH₃)₂Cl₂] is the anticancer drug cisplatin; the trans isomer is clinically inactive, showing how much geometry alone can matter.
Worked example 2 — octahedral [Ma₃b₃]. A complex like [Cr(NH₃)₃Cl₃] can arrange its three a ligands and three b ligands two ways: facial (fac — the three identical ligands occupy one triangular face of the octahedron, mutually cis) or meridional (mer — the three identical ligands span a plane through the centre, with two mutually trans and one cis to both). These are the only two arrangements possible for this ligand pattern.
Optical isomerism exists when a complex is chiral — non-superimposable on its mirror image. This needs the complex to lack any improper symmetry element (a mirror plane σ, a centre of inversion i, or an improper rotation axis Sₙ).
Worked example 3 — [Co(en)₂Cl₂]⁺. This octahedral complex (en = ethylenediamine, a neutral bidentate ligand) has two geometrical isomers, and only one of them is optically active. The cis isomer has no mirror plane or inversion centre — it is chiral and exists as a pair of non-superimposable mirror-image enantiomers, labelled Δ and Λ. The trans isomer has a centre of inversion (and a mirror plane), so it is achiral and superimposable on its own mirror image. The general rule this illustrates: for an [M(AA)₂X₂] type octahedral complex, the cis form is chiral and the trans form is not — check for a mirror plane before declaring a species optically active, never assume from the formula alone.
The IUPAC naming rules, in order
- Name the cation first, then the anion, as separate words — regardless of which one is the complex ion.
- Inside a complex ion, name the ligands first (alphabetically), then the metal.
- Ligand names are alphabetised by the ligand's own name, ignoring multiplying prefixes. "Dichloro" is alphabetised under C (chloro), not D.
- Multiplying prefixes: di-, tri-, tetra-… for simple ligand names; bis-, tris-, tetrakis-… for ligand names that are themselves compound words or already contain a prefix (ethylenediamine, oxalato) — using "di" there would read as a different ligand entirely ("diethylenediamine" looks like a distinct name).
- Anionic ligands end in -o (older, still standard in most Indian textbooks: chloro, bromo, cyano, hydroxo, oxalato, nitrato, sulfato; 2005 IUPAC recommendation: chlorido, bromido, cyanido, hydroxido). Neutral ligands mostly keep their molecular name, except aqua (H₂O), ammine (NH₃, note the double m — do not confuse with organic "amine"), carbonyl (CO) and nitrosyl (NO).
- The metal's oxidation state is written as a Roman numeral in parentheses immediately after the metal name, with no space: cobalt(III), not cobalt (III).
- If the complex ion is an anion, the metal name takes the suffix -ate, and several metals switch to a Latin root: iron → ferrate, copper → cuprate, silver → argentate, gold → aurate, tin → stannate, lead → plumbate. Cobalt, chromium, nickel and manganese keep their English names and simply add -ate (cobaltate, chromate, nickelate, manganate).
- Bridging ligands (shared between two metal centres) are prefixed with μ- and, for more than one bridge of the same kind, di-μ-; they are alphabetised alongside the other ligands using their own name.
Worked example 4 — formula to name. Name [Co(NH₃)₅Cl]Cl₂.
The outer two Cl⁻ mean the complex ion is [Co(NH₃)₅Cl]²⁺. Ligand charges: 5 NH₃ (neutral) = 0,
1 Cl⁻ (ligand) = −1. So Co + 0 + (−1) = +2 → Co is +3.
Ligands alphabetically: ammine before chloro (a before c). Cation is a complex, so no -ate
suffix.
Pentaamminechlorocobalt(III) chloride — pentaamminechloridocobalt(III)
chloride in the 2005-recommended spelling.
Worked example 5 — formula to name, anionic complex. Name K₄[Fe(CN)₆].
Four K⁺ balance the complex, so it is [Fe(CN)₆]⁴⁻. Six CN⁻ contribute −6, so Fe + (−6) = −4 →
Fe is +2.
Complex is an anion, so iron becomes ferrate.
Potassium hexacyanoferrate(II) — this is the familiar compound
"potassium ferrocyanide" by its older common name.
Worked example 6 — name to formula. Write the formula for potassium
diaquadioxalatochromate(III).
Work outward from the metal: chromium is (III), so +3. Oxalato (C₂O₄²⁻, bidentate) appears
twice: 2 × (−2) = −4. Aqua (H₂O, neutral) appears twice: 2 × 0 = 0.
Complex ion charge = +3 + (−4) + 0 = −1, so it is an anion needing one K⁺ to balance.
K[Cr(C₂O₄)₂(H₂O)₂]
Worked example 7 — a bridging ligand. Name
[(NH₃)₅Cr(μ-OH)Cr(NH₃)₅]Cl₅.
Five Cl⁻ balance a +5 complex cation. The single bridging hydroxide contributes −1 total
(shared between the two chromium centres, counted once in the overall charge). Ten NH₃
contribute 0. So 2 × Cr + (−1) = +5 → each Cr is +3.
Because the two identical [pentaamminechromium(III)] fragments are joined by the bridge, the
complex ligand-name needs "bis" and square brackets:
μ-hydroxido-bis[pentaamminechromium(III)] chloride
Common mistakes that cost marks
- Alphabetising by the multiplying prefix. "Dichloro" is filed under C, not D — the prefix is invisible for ordering purposes.
- Using di/tri for a compound ligand name. Two ethylenediamine ligands is "bis(ethylenediamine)", never "diethylenediamine" — the latter names a completely different molecule.
- Forgetting the -ate suffix (and the Latin root) when the complex is an anion. K₃[Fe(CN)₆] is potassium hexacyanoferrate(III), not "potassium hexacyanoiron(III)".
- Confusing linkage isomerism with ionization isomerism. Linkage isomers keep exactly the same ligand set and only the donor atom of one ambidentate ligand changes; ionization isomers swap which species is a ligand and which is the counter-ion.
- Assuming cis is always chiral or trans is always achiral without checking symmetry. The rule holds for [M(AA)₂X₂]-type complexes specifically because the trans form has an inversion centre — always look for a mirror plane or inversion centre rather than pattern-matching on "cis" and "trans" as words.
Where this appears in GATE Chemistry
| Question style | What is actually being tested |
|---|---|
| "Which pair are linkage/ionization/hydrate isomers?" | Correctly classifying the type of isomerism from two given formulas |
| "How many geometrical/optical isomers does [complex] have?" | Counting cis/trans or fac/mer arrangements and checking each for a symmetry element |
| "Write the IUPAC name of [formula]" | Oxidation-state arithmetic, alphabetical ligand order, correct suffix |
| "Write the formula for [IUPAC name]" | The same rules run in reverse, tracking overall charge to get the counter-ion right |
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