What is isomerism?
Isomers are compounds with the same chemical formula but a different arrangement of atoms. Because the arrangement is different, their properties (colour, solubility, reactions, effect on light) can be different.
In coordination compounds there are two big families:
- Structural isomerism: the bonds are different (which atom is joined to which).
- Stereoisomerism: the bonds are the same, only the positions in space are different.
Structural isomerism
1. Ionisation isomerism
An ion inside the bracket swaps places with a counter ion outside. [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄ give different ions in water (Br⁻ vs SO₄²⁻), so they give different test results.
2. Linkage isomerism
Only with ambidentate ligands. NO₂⁻ can bind by N (nitrito-N) or by O (nitrito-O); SCN⁻ by S (thiocyanato-S) or by N (thiocyanato-N). Example: [Co(NH₃)₅(NO₂)]Cl₂ (yellow) and [Co(NH₃)₅(ONO)]Cl₂ (red).
3. Coordination isomerism
When both cation and anion are complexes, ligands swap between the two metals. [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].
4. Solvate (hydrate) isomerism
Like ionisation isomerism but with the solvent. When water is the solvent it is called hydrate isomerism. [Cr(H₂O)₆]Cl₃ is violet; [Cr(H₂O)₅Cl]Cl₂·H₂O is grey-green. One water has moved from inside to outside, and one Cl from outside to inside.
Geometrical isomerism
Possible when ligands can be next to each other (cis, 90°) or opposite (trans, 180°).
- Square planar MA₂B₂ like [Pt(NH₃)₂Cl₂]: cis and trans. MABXL types can have 3 isomers.
- Octahedral MA₄B₂ like [Co(NH₃)₄Cl₂]⁺: cis and trans. [M(AA)₂B₂] like [Co(en)₂Cl₂]⁺: cis and trans.
- Octahedral MA₃B₃ like [Co(NH₃)₃(NO₂)₃]: fac (facial — the three B on one face) and mer (meridional — the three B around a meridian).
- Tetrahedral complexes show no geometrical isomerism, because every corner is next to every other corner.
Optical isomerism
Two isomers that are mirror images of each other but cannot be placed exactly on top of each other are enantiomers. Such a molecule is called chiral. One turns plane-polarised light to the right (dextro, d), the other to the left (laevo, l), by the same amount.
- Common in octahedral complexes with didentate ligands: [Co(en)₃]³⁺, [PtCl₂(en)₂]²⁺.
- cis-[Co(en)₂Cl₂]⁺ is optically active (its mirror image is different). trans-[Co(en)₂Cl₂]⁺ is not, because it has a plane of symmetry.
- So [Co(en)₂Cl₂]⁺ has 3 stereoisomers in all: trans, d-cis and l-cis.
Test: if a molecule has a mirror plane cutting it into two matching halves, it is not chiral.
Try it: the hand and glove test
Hold your left hand in front of a mirror. The image looks like a right hand. Now place your right hand on your left hand, palm down on palm down. The thumbs point opposite ways — they never match. In the 3D (last step), move the slider through all angles and watch: the mirror-image [Co(en)₃]³⁺ never matches the original. Then make cis and trans [Pt(NH₃)₂Cl₂] with 4 coins (2 of one kind) on a square: how many different patterns can you make? Only 2.
Key formulas and definitions
- Structural types: ionisation · linkage · coordination · solvate (hydrate)
- Stereo types: geometrical (cis/trans, fac/mer) · optical (d/l)
- Square planar MA₂B₂ → 2 geometrical isomers; tetrahedral → none
- Octahedral MA₄B₂ → cis + trans; MA₃B₃ → fac + mer
- [M(AA)₂B₂]: trans (inactive) + cis pair (d, l) = 3 stereoisomers; [M(AA)₃] → d and l
Worked examples
1. Which type of isomerism is shown by [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄? How can you tell them apart?
Step 1: The same atoms are present; Br and SO₄ swap inside/outside. Step 2: That is ionisation isomerism. Step 3: Add AgNO₃: the second gives no AgBr, the first gives a pale yellow AgBr precipitate. Add BaCl₂: only the second gives white BaSO₄. Answer: ionisation isomers; told apart by AgNO₃ / BaCl₂ tests.
2. Does [Pt(NH₃)₂Cl₂] (square planar) show geometrical isomerism? Does a tetrahedral MA₂B₂ like [Zn(NH₃)₂Cl₂]?
Step 1: Square planar has neighbouring (90°) and opposite (180°) positions, so two Cl can be cis or trans. Step 2: Tetrahedral: all 4 positions are neighbours, so any 2 Cl are always 109.5° apart — only one arrangement. Answer: square planar yes (2 isomers), tetrahedral no.
3. How many geometrical isomers does [Co(NH₃)₃Cl₃] have? Name them.
Step 1: It is MA₃B₃ octahedral. Step 2: The three Cl can be on one face (all 90° apart) or on a meridian (two opposite each other). Answer: 2 — fac and mer.
4. Is trans-[Co(en)₂Cl₂]⁺ optically active?
Step 1: In trans, the two Cl are opposite each other, above and below the plane of the two en rings. Step 2: The plane holding Co and both en rings is a mirror plane. Step 3: A molecule with a mirror plane is not chiral. Answer: No, trans is optically inactive; only the cis form is active.
5. How many stereoisomers in total does [Co(en)₂Cl₂]⁺ have?
Step 1: Geometrical: cis and trans. Step 2: trans has a mirror plane → 1 form. Step 3: cis is chiral → d and l forms (2). Answer: 1 + 2 = 3 stereoisomers.
6. Which isomerism is possible for [Co(NH₃)₅(NO₂)](NO₃)₂? List all types.
Step 1: NO₂⁻ inside is ambidentate → linkage isomers (nitrito-N / nitrito-O). Step 2: NO₂⁻ inside and NO₃⁻ outside can swap → ionisation isomers, e.g. [Co(NH₃)₅(NO₃)](NO₃)(NO₂). Step 3: Octahedral MA₅B has only one geometrical form. Answer: linkage and ionisation isomerism.
7. How many geometrical isomers does square planar [Pt(NH₃)(Br)(Cl)(py)] have?
Step 1: All four ligands differ: MABCD. Step 2: Fix A. The ligand opposite A can be B, C or D. Step 3: Each choice gives a different arrangement. Answer: 3 geometrical isomers.
Common mistakes
- Saying tetrahedral MA₂B₂ has cis and trans. It has none: every corner of a tetrahedron is next to every other.
- Calling trans-[Co(en)₂Cl₂]⁺ optically active. It has a mirror plane; only the cis form is chiral.
- Mixing up linkage and coordination isomerism. Linkage = one ambidentate ligand changes its donor atom. Coordination = ligands swap between two metals.
- Counting a molecule turned in space as a new isomer. If turning makes them match, they are the same compound.