Isomerism in Coordination Compounds: Every Type With Its Test
Seven kinds of isomerism, each distinguished by a specific experiment. Knowing which experiment goes with which type is what the questions test.
Structural isomerism
| Type | What differs | How it is detected |
|---|---|---|
| Ionisation | Which anion is inside the coordination sphere and which is outside | Precipitation test on the free ion in solution |
| Hydrate | Whether water is coordinated or merely in the lattice | Loss of water on drying; conductivity |
| Linkage | Which atom of an ambidentate ligand is bonded | Infrared spectroscopy; colour difference |
| Coordination | Distribution of ligands between two complex ions | Requires both cation and anion to be complexes |
Linkage isomerism
An ambidentate ligand can bind through either of two donor atoms — nitrite through nitrogen or oxygen, thiocyanate through sulphur or nitrogen. The two isomers differ in colour and in their infrared spectra, since the bonding changes the vibrational frequencies of the ligand.
Which atom binds follows HSAB reasoning: a hard metal ion prefers the harder donor, a soft one prefers the softer donor. That connection is worth making explicitly.
Stereoisomerism
Geometrical isomerism
Which geometries permit it is the first thing to establish, and it is frequently asked in that form:
| Complex type | Geometrical isomerism? |
|---|---|
| Square planar MA2B2 | Yes — cis and trans |
| Tetrahedral MA2B2 | No — all positions are adjacent |
| Octahedral MA4B2 | Yes — cis and trans |
| Octahedral MA3B3 | Yes — facial and meridional |
The tetrahedral entry is the one most often got wrong. In a tetrahedron every position is adjacent to every other, so there is no cis or trans distinction to make. Stating that reason is what earns the mark.
Optical isomerism
A complex is optically active if it has no improper axis of symmetry — no mirror plane and no inversion centre. Octahedral complexes with two or three bidentate chelate rings are the classic optically active examples.
Among geometrical isomers, the cis form is often chiral while the trans form is not, because the trans arrangement usually possesses a mirror plane. Being asked which of a cis/trans pair is optically active is a standard question, and drawing the mirror plane is the way to answer it.
Working out how many isomers exist
- Establish the geometry from the coordination number.
- Enumerate distinct arrangements of the ligands, carefully avoiding duplicates related by rotation.
- For each, check for an improper axis to decide optical activity.
- Count enantiomeric pairs as two isomers.
Step two is where errors occur, because rotating a structure does not produce a new isomer. Building or sketching the arrangements systematically avoids double counting.
Frequently asked questions
Why does a tetrahedral complex show no geometrical isomerism?
Because all four positions are mutually adjacent, so there is no cis or trans relationship to distinguish. Any two arrangements are related by rotation.
How is ionisation isomerism demonstrated experimentally?
By adding a reagent that precipitates the free anion. Only the anion outside the coordination sphere is available, so the precipitate identifies which isomer is present.
What is the difference between facial and meridional?
In the facial isomer three identical ligands occupy one triangular face; in the meridional isomer they lie around a meridian of the octahedron. Both arise for MA3B3.
Are all cis isomers optically active?
No. Optical activity requires the absence of any improper axis. Some cis isomers still possess a mirror plane and are therefore achiral, so each case must be checked rather than assumed.
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