Isomerism in Coordination Compounds — Six Types, Verified Diagrams

5.4

The map before the detail

orientation

The one distinction everything hangs on

Stereoisomers have the same chemical formula and the same bonds — only the arrangement in space differs. Structural isomers have different bonds altogether: something has swapped places between inside and outside the coordination sphere, or a ligand has turned round and bonded through a different atom.

If you can tell which of these two families a question belongs to, you have already eliminated half the options.

Geometrical Isomerism

stereoisomerism

What it is

Arises in heteroleptic complexes when identical ligands can sit either next to each other (cis, 90°) or opposite each other (trans, 180°). Found at coordination numbers 4 (square planar only) and 6.

For [Ma₃b₃] octahedral complexes the two arrangements get different names: fac when the three like ligands cap one triangular face, mer when they lie around a meridian.

Rotate the polyhedron — the angle readout is computed live

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Structure — Highlighted L–M–L — Chiral —

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Complex type	Geometry	Geometrical isomers	Names
MA₂B₂	Square planar	2	cis, trans
MA₂B₂	Tetrahedral	0	— all positions equivalent
MABCD	Square planar	3	one per ligand placed trans to A
MA₄B₂	Octahedral	2	cis, trans
Ma₃b₃	Octahedral	2	fac, mer
[MX₂(AA)₂]	Octahedral	2	cis, trans
[M(AA)₃]	Octahedral	0	— but 2 optical isomers

Where marks are lost Tetrahedral complexes never show geometrical isomerism — in a tetrahedron every pair of corners is adjacent, so there is no cis/trans distinction to make. This was verified computationally: the tetrahedral rotation group maps all six possible placements of two like ligands onto one another, leaving exactly one arrangement.

Quick check How many geometrical isomers does [Co(NH₃)₃Cl₃] have?

Reveal

Two — it is Ma₃b₃, so fac and mer. Neither is optically active: both keep a mirror plane. Set the viewer above to fac and mer and watch the chirality readout stay “no” for both.

Optical Isomerism

stereoisomerism

What it is

Two forms that are non-superimposable mirror images — enantiomers. A complex is chiral when it has no plane and no centre of symmetry. The two forms are labelled d (dextro, rotates polarised light right) and l (laevo, left), or Δ and Λ for tris-chelates.

Common in octahedral complexes containing didentate ligands. A chelate ring can only span a cis pair of sites, which is why chelation and chirality travel together.

Try to superimpose them — rotate each side independently

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mirror plane

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Relationship — Superimposable by rotation —

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Where marks are lost trans is almost never optically active. Putting two identical ligands on one axis creates a mirror plane. Switch the viewer to trans-[CoCl₂(en)₂]⁺ above: the “mirror image” on the right can be rotated onto the left one, so it is not a second isomer at all. Only the cis form of [MX₂(AA)₂] is chiral — giving 1 trans + 2 cis = 3 stereoisomers in total.

Quick check Which is optically active: cis-[CrCl₂(ox)₂]³⁻ or trans-[CrCl₂(ox)₂]³⁻?

Reveal

The cis form. It is the same [MX₂(AA)₂] skeleton as the en complex, with oxalate replacing en. The trans form has a symmetry plane through the two chlorides.

Linkage Isomerism

structural

What it is

Occurs when the complex contains an ambidentate ligand — one with two different donor atoms, of which only one binds at a time. The same ligand attached through a different atom gives a genuinely different compound, often with a different colour.

The classic case, discovered by Jørgensen: [Co(NH₃)₅(NO₂)]Cl₂ exists as a red O-bonded form and a yellow N-bonded form.

Switch the donor atom

Bonded through — Written as — IUPAC ligand name —

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Where marks are lost Ambidentate is not the same as didentate. An ambidentate ligand could bind through either atom but uses only one; a didentate ligand uses both at once and forms a ring. NO₂⁻ and SCN⁻ are ambidentate. en and oxalate are didentate. Only the first kind causes linkage isomerism.

Quick check Why can Cl⁻ never cause linkage isomerism?

Reveal

It has only one kind of donor atom. Linkage isomerism needs two chemically different donor atoms in the same ligand — N and O in NO₂⁻, S and N in SCN⁻.

Coordination Isomerism

structural

What it is

Requires both the cation and the anion to be complex ions, built on two different metals. The isomers differ by swapping the ligand sets between the two metals.

Swap the ligand sets between the metals

Complex cation

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⇆

Complex anion

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Where marks are lost Check for two sets of square brackets. If both ions are inside brackets, the isomerism is coordination. If only one ion is bracketed and the other is a bare ion, it is ionisation isomerism instead. That single visual cue separates types 4 and 5.

Quick check Name the coordination isomer of [Cr(NH₃)₆][Co(CN)₆].

Reveal

[Co(NH₃)₆][Cr(CN)₆] — the ammine set moves to cobalt and the cyanide set to chromium. Both metals keep the +3 state, so the charges still balance.

Ionisation Isomerism

structural

What it is

Arises when the counter ion is itself a potential ligand. It can trade places with a ligand from inside the coordination sphere, so the two isomers release different ions in solution and react differently to the same reagent.

Move the sulphate in and the bromide out — then run the tests

Coordination sphere

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⇆

Counter ion (free in solution)

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Add BaCl₂ solution

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Add AgNO₃ solution

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Where marks are lost The reagent test is the whole point of this type. Only the ion that is outside the bracket is free to react. A sulphate locked inside the sphere gives no precipitate with Ba²⁺, however much sulphate the formula appears to contain.

Quick check [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅SO₄]Cl — which gives a white precipitate with BaCl₂?

Reveal

The first. Its sulphate sits outside the bracket and is free to precipitate as BaSO₄. The second gives AgCl with AgNO₃ instead, since its chloride is the free ion.

Solvate (Hydrate) Isomerism

structural

What it is

The same idea as ionisation isomerism, but the species trading places is a solvent molecule. When that solvent is water it is called hydrate isomerism. The isomers differ in whether a water molecule is bonded to the metal or merely sitting in the crystal lattice.

Move chlorides into the sphere and watch the AgCl count fall

Coordination sphere

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Outside the sphere

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Chlorides inside the sphere 0

Full formula — Colour — AgCl per mole — Electrolyte —

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Where marks are lost Coordination number stays at 6 throughout. Each chloride that moves in must push a water molecule out into the lattice — the total inside the bracket never changes. Counting the AgCl is how these isomers are told apart experimentally, and it is the most common numerical form of this question.

Quick check A sample of CrCl₃·6H₂O gives 2 mol AgCl per mole. Write its formula.

Reveal

[Cr(H₂O)₅Cl]Cl₂·H₂O — two free chlorides outside, one chloride and five waters inside, and the sixth water in the lattice. Set the slider to 1 above to see it.

✓

How these diagrams were verified

audit trail

Why this section exists

A diagram that looks like an octahedron can still be wrong — a chelate drawn across a trans pair, a “pair of enantiomers” that is really the same molecule rotated. Every structure above was defined as a set of ligand-to-vertex assignments and then tested against the mathematics before being rendered.

The tests used the 24 proper rotations of the octahedron. Two structures are the same isomer if some rotation maps one onto the other; a structure is chiral if no rotation maps it onto its mirror image. Bond angles come from dot products of the actual position vectors, which is also what the live angle readout above reports.

PASS sq-cis occupies every vertex exactly once (4 sites)

PASS sq-trans occupies every vertex exactly once (4 sites)

PASS oct-cis occupies every vertex exactly once (6 sites)

PASS oct-trans occupies every vertex exactly once (6 sites)

PASS fac occupies every vertex exactly once (6 sites)

PASS mer occupies every vertex exactly once (6 sites)

PASS en-trans occupies every vertex exactly once (6 sites)

PASS en-cis occupies every vertex exactly once (6 sites)

PASS tris-delta occupies every vertex exactly once (6 sites)

PASS tris-lambda occupies every vertex exactly once (6 sites)

PASS en-trans chelate en spans x+-y+ = 90 deg (must be 90)

PASS en-trans chelate en spans x--y- = 90 deg (must be 90)

PASS en-cis chelate en spans y+-z- = 90 deg (must be 90)

PASS en-cis chelate en spans y--x- = 90 deg (must be 90)

PASS tris-delta chelate en spans x+-y+ = 90 deg (must be 90)

PASS tris-delta chelate en spans y--z+ = 90 deg (must be 90)

PASS tris-delta chelate en spans x--z- = 90 deg (must be 90)

PASS tris-lambda chelate en spans x--y+ = 90 deg (must be 90)

PASS tris-lambda chelate en spans y--z+ = 90 deg (must be 90)

PASS tris-lambda chelate en spans x+-z- = 90 deg (must be 90)

PASS sq-cis labelled angle x+-M-y+ = 90 deg (claimed 90)

PASS sq-trans labelled angle x+-M-x- = 180 deg (claimed 180)

PASS oct-cis labelled angle z+-M-x+ = 90 deg (claimed 90)

PASS oct-trans labelled angle z+-M-z- = 180 deg (claimed 180)

PASS en-cis labelled angle z+-M-x+ = 90 deg (claimed 90)

PASS en-trans labelled angle z+-M-z- = 180 deg (claimed 180)

PASS fac has 0 trans pair(s) among the three like ligands (expected 0)

PASS fac all three pairs are 90 deg -> one face

PASS mer has 1 trans pair(s) among the three like ligands (expected 1)

PASS sq-cis and sq-trans are genuinely distinct (no rotation maps one to the other)

PASS oct-cis and oct-trans are genuinely distinct (no rotation maps one to the other)

PASS fac and mer are genuinely distinct (no rotation maps one to the other)

PASS en-cis and en-trans are genuinely distinct (no rotation maps one to the other)

PASS sq-cis chirality = False (expected False)

PASS sq-trans chirality = False (expected False)

PASS oct-cis chirality = False (expected False)

PASS oct-trans chirality = False (expected False)

PASS fac chirality = False (expected False)

PASS mer chirality = False (expected False)

PASS en-trans chirality = False (expected False)

PASS en-cis chirality = True (expected True)

PASS tris-delta chirality = True (expected True)

PASS tris-lambda chirality = True (expected True)

PASS tris Delta and Lambda are NOT superimposable by rotation

PASS tris Lambda IS the mirror image of Delta

PASS tetrahedron rotation group has 12 elements (expected 12)

PASS tetrahedral MA2B2 gives 1 distinct arrangement(s) -> no geometrical isomerism

PASS [MX2(AA)2] total stereoisomers = 1 trans + 2 cis enantiomers = 3

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