Isomerism in Complex Ions Revision Notes | A Level AQA Chemistry

1. Why Isomerism Matters in Complex Ions

Two molecules can share an identical molecular formula yet behave completely differently — one curing cancer, the other doing nothing. That is not a hypothetical scenario; it is the story of cisplatin and its trans counterpart. The difference comes down to the spatial arrangement of ligands around a central metal ion, which is exactly what isomerism in complex ions is about.

At A Level, you need to recognise two distinct types of stereoisomerism in complex ions: cis-trans isomerism (a special case of E-Z isomerism) and optical isomerism. The type that arises depends on the geometry of the complex and the nature of the ligands present. This lesson builds from geometry outward — once you understand the shape, the isomerism follows logically.

2. Octahedral Complexes and Cis-Trans Isomerism

Octahedral complexes have six ligand positions arranged around the central metal ion — four in a square plane and one above and one below. When a complex has the general formula [ML4X2], where L and X are two different monodentate ligands, the two X ligands can sit either next to each other or directly opposite each other.

When the two identical ligands are adjacent (at 90° to each other), the complex is the cis isomer. When they are opposite (at 180° to each other), it is the trans isomer. These two arrangements cannot be interconverted without breaking and reforming coordinate bonds, so they are genuinely different compounds with different properties.

A classic AQA example is [CoCl2(NH3)4]+. In the cis form, the two chloride ligands occupy adjacent positions in the octahedron. In the trans form, they sit directly opposite each other across the central cobalt ion. The two isomers have different colours and different chemical reactivities — a reminder that spatial arrangement has real chemical consequences.

3. Optical Isomerism in Octahedral Complexes

Cis-trans isomerism requires only monodentate ligands and the right formula type. Optical isomerism in octahedral complexes is a different matter — it arises when the complex is chiral, meaning it lacks a plane of symmetry and is non-superimposable on its mirror image. In practice, this condition is most commonly met when bidentate ligands are present, because they wrap around the metal ion and create a three-dimensional arrangement that can lack the symmetry required for superimposability.

Consider the complex [Co(en)3]3+, where 'en' is 1,2-diaminoethane (a bidentate ligand). Three bidentate ligands coordinate to the cobalt centre, each spanning two adjacent positions. The resulting structure has no plane of symmetry — it exists as two mirror-image forms called enantiomers, labelled the Δ (delta) and Λ (lambda) forms. These rotate plane-polarised light in opposite directions.

A key point for the exam: optical isomerism in complex ions requires the complex to be chiral — it must lack an internal plane of symmetry. With three bidentate ligands in an octahedral arrangement, this condition is met. With monodentate ligands alone, octahedral complexes retain a plane of symmetry and do not display optical isomerism.

It is worth noting that the cis isomer of [CoCl2(en)2]+ does display optical isomerism, because the cis arrangement removes the plane of symmetry. The trans isomer does not, because a mirror plane passes through the two chloride ligands. This distinction — cis vs trans affecting whether optical isomerism is possible — is a favourite examiner test point.

4. Square Planar Complexes and Cisplatin

Not all transition metal complexes are octahedral. Some adopt a square planar geometry, in which four ligands surround the central metal ion in a flat, square arrangement. This geometry is particularly common for Pt(II) and Ni(II) complexes. Square planar complexes can also display cis-trans isomerism when two different types of monodentate ligand are present.

The most important example for AQA is cisplatin, the complex [PtCl2(NH3)2]. In the cis isomer, the two chloride ligands are on the same side of the platinum centre and the two ammonia ligands are on the same side. In the trans isomer, each chloride is directly opposite an ammonia. These are genuinely different compounds — cisplatin is the cis isomer and is used as an anti-cancer drug, while the trans isomer is biologically inactive.

Cisplatin works because the two chloride ligands in the cis arrangement are close enough together to simultaneously bind to adjacent guanine bases on a DNA strand, forming a cross-link that prevents DNA replication in cancer cells. The trans isomer cannot achieve this because its chloride ligands point in opposite directions. This is a powerful illustration of why spatial arrangement — not just composition — determines chemical behaviour.

5. The Linear Complex [Ag(NH3)2]+ and Tollens' Reagent

Not every complex ion is octahedral or square planar. Silver(I) ions form a linear complex with ammonia: [Ag(NH3)2]+. In this complex, two ammonia ligands coordinate to the Ag+ ion in a straight line, giving a bond angle of 180°. Linear geometry is characteristic of d10 metal ions such as Ag+ and Cu+.

This complex is the active species in Tollens' reagent (also called ammoniacal silver nitrate solution). When Tollens' reagent is warmed with an aldehyde, the silver(I) in [Ag(NH3)2]+ is reduced to silver metal, which deposits as a silver mirror on the inside of the test tube. Ketones do not reduce Tollens' reagent under these conditions, making this a useful test to distinguish aldehydes from ketones.

The formation of [Ag(NH3)2]+ is itself a ligand substitution reaction: water molecules in the aqua complex [Ag(H2O)2]+ are replaced by ammonia ligands. The linear geometry arises because Ag+ has a full d10 configuration, which favours a two-coordinate linear arrangement rather than the higher coordination numbers seen with transition metal ions in the middle of the d block.

6. Comparing All Three Geometries

Now that all three geometries have been introduced, it helps to see them side by side. The type of isomerism a complex can display depends directly on its geometry and the nature of its ligands — so recognising the geometry is the first step in any isomerism question.

7. Exam Technique and Common Mistakes

Isomerism questions in AQA papers tend to follow predictable patterns. The most common demand is to draw or identify isomers, explain why a complex displays a particular type of isomerism, or compare the properties of cis and trans isomers. Each of these requires a slightly different approach.

Worked example: Answering an 'Explain' Question on Isomerism

Explain why [CoCl2(en)2]+ can display both cis-trans isomerism and optical isomerism. [4 marks]

1. Co3+ with 4 ligand groups (2 Cl- and 2 en) — each en is bidentate, so total coordination number = 6. Geometry is octahedral. — Always establish geometry first. Six coordinate bonds = octahedral. [M1]
2. The two Cl- ligands can be adjacent (cis, 90° apart) or opposite (trans, 180° apart) in the octahedral arrangement. — State the spatial difference explicitly — 'adjacent' and 'opposite' are the key words. [M1]
3. The cis isomer has no plane of symmetry. The two bidentate en ligands and two Cl- ligands create a chiral centre, giving two non-superimposable mirror images (enantiomers). — Link chirality to the absence of a plane of symmetry. Specify it is the cis isomer — the trans isomer has a plane of symmetry and does NOT show optical isomerism. [M1]
4. The trans isomer has a plane of symmetry passing through the two Cl- ligands and the Co centre, so it is superimposable on its mirror image — no optical isomerism. — This distinguishing point is often worth a mark and shows higher-level understanding. [M1]

Answer: The complex is octahedral. The two Cl- ligands can be cis (adjacent, 90°) or trans (opposite, 180°), giving cis-trans isomerism. The cis isomer lacks a plane of symmetry due to the bidentate en ligands, producing two non-superimposable mirror images (optical isomers). The trans isomer has a plane of symmetry and does not display optical isomerism.