28.4 Stereoisomerism in Complexes
Learning outcomes
Candidates should be able to:
- describe the types of stereoisomerism shown by complexes, including those associated with bidentate ligands:
- (a) geometrical (cis/trans) isomerism, e.g. square planar such as [Pt(NH₃)₂Cl₂] and octahedral such as [Co(NH₃)₄(H₂O)₂]²⁺ and [Ni(en)₂(H₂O)₂]²⁺
- (b) optical isomerism, e.g. [Ni(en)₃]²⁺ and [Ni(en)₂(H₂O)₂]²⁺
- deduce the overall polarity of complexes such as those described in 28.4.1(a) and (b)
Structural isomerism, and an introduction to stereochemistry
Some of the types of isomerism found in organic chemistry are also found in complex ions. An example of structural isomerism occurs in compounds of formula CrCl₃·6H₂O. Three such compounds are known, with different numbers of free Cl⁻ ions (shown by reaction with Ag⁺ ions) and free H₂O molecules (shown by dehydration).
| Colour | Free Cl⁻ ions | Free H₂O molecules | Structure |
|---|---|---|---|
| purple | 3 | 0 | [Cr(H₂O)₆]³⁺ 3Cl⁻ |
| blue-green | 2 | 1 | [Cr(H₂O)₅Cl]²⁺ 2Cl⁻·H₂O |
| green | 1 | 2 | [Cr(H₂O)₄Cl₂]⁺ Cl⁻·2H₂O |
Complexes can also show stereochemistry — differences in the spatial arrangement of ligands around the same central metal ion, with the same bonds. Stereoisomerism in complexes falls into two types, both covered below: geometric (cis/trans) isomerism and optical isomerism.
A good example is the complex ion [Co(en)₂(NH₃)₂]²⁺, where en is 1,2-diaminoethane, H₂NCH₂CH₂NH₂. This complex exists as cis and trans forms. The cis form (but not the trans) has a chiral centre at the cobalt atom, and can be resolved into optical isomers. The trans form has zero overall polarity, since all the bond dipoles cancel; the cis form does have a dipole moment.
Geometric isomerism: square planar complexes
Any square planar complex with the general formula [M(A)₂(B)₂] (M = transition element; A, B = ligands) can show cis/trans isomerism.
Geometric isomerism: octahedral complexes
Octahedral complexes with the general formula [M(A)₄(B)₂] can also show geometric isomerism. An example is the cobalt(II) complex ion [Co(NH₃)₄(H₂O)₂]²⁺.
trans-[Co(NH₃)₄(H₂O)₂]²⁺: the two H₂O ligands are opposite each other → symmetric → bond dipoles cancel → non-polar.
Octahedral complexes with bidentate ligands, general formula [M(AA)₂(B)₂], show the same cis/trans pattern. An example is nickel(II) with two bidentate en ligands and two monodentate H₂O ligands, [Ni(en)₂(H₂O)₂]²⁺.
Optical isomerism
Ni²⁺ forms an octahedral complex with three en ligands, [Ni(en)₃]²⁺. This has only one geometric form — the en “bite” is too short to stretch across opposite (trans) positions — but that one form is chiral, existing as a non-superimposable mirror-image pair.
Now back to [Ni(en)₂(H₂O)₂]²⁺: only the cis isomer is optically active, with two non-superimposable mirror images. The trans isomer is symmetrical, so its mirror image can be rotated and superimposed onto the original — it does not show optical isomerism.
Deducing polarity: summary
Identical ligands on the same side (cis) → bond dipoles don’t cancel → polar.
The size of any dipole depends on the electronegativity difference between the donor atoms of the two ligand types.
| Complex | cis polarity | trans polarity | Optically active? |
|---|---|---|---|
| [Pt(NH₃)₂Cl₂] | polar | non-polar | no (square planar) |
| [Co(NH₃)₄(H₂O)₂]²⁺ | polar | non-polar | no |
| [Ni(en)₂(H₂O)₂]²⁺ | polar | non-polar | cis only |
| [Ni(en)₃]²⁺ | only one geometric form exists | yes | |
Exam tips
- Square planar [M(A)₂(B)₂] and octahedral [M(A)₄(B)₂] / [M(AA)₂(B)₂] complexes can all show cis/trans isomerism — learn one worked example of each.
- To deduce polarity: identical ligands opposite each other → dipoles cancel → non-polar (trans); identical ligands on the same side → dipoles don’t cancel → polar (cis).
- Only complexes with no plane of symmetry are chiral. A bidentate ligand’s “bite” is often too short to bridge opposite (trans) positions — this is why [Ni(en)₃]²⁺ and [Ni(en)₂(H₂O)₂]²⁺ cis only exist in one geometric arrangement each for those positions.
- State clearly which isomer (cis or trans) is optically active, and why the other one is not (it has a plane of symmetry / its mirror image is superimposable).
Quick summary
- Structural isomerism (e.g. CrCl₃·6H₂O) differs from stereoisomerism, which is split into geometric (cis/trans) and optical (enantiomer) types.
- Geometric isomerism occurs in square planar [M(A)₂(B)₂] and octahedral [M(A)₄(B)₂] / [M(AA)₂(B)₂] complexes.
- Cisplatin (cis-[Pt(NH₃)₂Cl₂]) is an anticancer drug that cross-links DNA; trans-platin is inactive.
- Optical isomers (enantiomers) are non-superimposable mirror images, e.g. [Ni(en)₃]²⁺ and cis-[Ni(en)₂(H₂O)₂]²⁺.
- Polarity: identical ligands trans (opposite) → dipoles cancel → non-polar; identical ligands cis (same side) → dipoles don’t cancel → polar.