Stereoisomerism in Complex Ions (OCR A-Level Chemistry A): Revision Notes

Stereoisomerism in Complex Ions

Introduction to stereoisomerism

Stereoisomers are compounds that contain the same atoms bonded together in the same sequence, but these atoms are arranged differently in three-dimensional space. Complex ions can display two main types of stereoisomerism:

• Cis-trans isomerism (also called geometric isomerism)
• Optical isomerism

The type of stereoisomerism that a complex ion can exhibit depends on:

• The coordination number of the central metal ion
• The geometry of the complex (square planar or octahedral)
• The types of ligands present (monodentate or bidentate)
• The number of different ligands attached

Cis-trans isomerism in complex ions

Key differences from organic chemistry

You may have encountered cis-trans isomerism in organic chemistry, where it occurs around carbon-carbon double bonds ($\text{C}=\text{C}$). The double bond prevents rotation, which locks groups into different positions.

In complex ions, cis-trans isomerism works differently:

• No double bonds are involved
• The rigid geometry of the complex prevents ligands from moving
• The shape of the complex holds ligands in fixed orientations around the central metal ion

Cis-trans isomerism occurs in both square planar and octahedral complex ions.

Cis-trans isomerism in square planar complexes

Requirements and basic principles

For cis-trans isomerism to occur in square planar complexes, the complex must have:

• A coordination number of 4
• At least two pairs of identical ligands
• Square planar geometry (ligands arranged in a flat square around the metal)

In square planar geometry, ligands are positioned at the corners of a square in the same plane. Each ligand is separated from its neighbours by bond angles of $90°$.

Example: Dichlorodiamminepalladium(II)

The complex $[\text{Pd}(\text{NH}_3)_2\text{Cl}_2]$ is formed from:

• A $\text{Pd}^{2+}$ ion (central metal)
• Two $\text{NH}_3$ ligands (ammonia)
• Two $\text{Cl}^-$ ligands (chloride ions)

This complex exists as two distinct isomers:

In the diagram above:

• Cis isomer (left): Both ammonia ligands are adjacent, both chloride ligands are adjacent, with 90° angles between identical ligands
• Trans isomer (right): Ammonia ligands are opposite each other, chloride ligands are opposite each other, with 180° angles between identical ligands

Drawing cis-trans isomers in three dimensions

When drawing these isomers, it's important to represent them in three dimensions to clearly show the spatial arrangement. The standard convention uses:

• Solid wedge bonds: Ligands coming out of the page towards you
• Dashed wedge bonds: Ligands going behind the page away from you
• Normal lines: Ligands in the plane of the page

Cis-trans isomerism in octahedral complexes

Octahedral complexes with monodentate ligands

Octahedral complexes have a coordination number of 6, with six ligands arranged around the central metal ion. The geometry resembles two square-based pyramids joined at their bases.

Example: Tetraamminedichlorocobalt(III) ion

The complex $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+$ contains:

• A $\text{Co}^{3+}$ ion (central metal)
• Four $\text{NH}_3$ ligands
• Two $\text{Cl}^-$ ligands

This complex exists as two isomers with different colours:

• Cis isomer: Violet coloured
• Trans isomer: Green coloured

In the octahedral arrangement:

• Cis isomer: The two chloride ligands are adjacent to each other with a bond angle of 90° between their coordinate bonds
• Trans isomer: The two chloride ligands are at opposite corners of the octahedron with a bond angle of 180° between their coordinate bonds

Three-dimensional representation of octahedral isomers

Drawing octahedral complexes in three dimensions requires careful use of wedge-and-dash notation:

The 3D diagrams clearly show how the chloride ligands are positioned:

• In the cis isomer: both chlorides are $90°$ apart
• In the trans isomer: the chlorides are $180°$ apart (directly opposite)

Octahedral complexes with bidentate ligands

Bidentate ligands are ligands that can form two coordinate bonds to the central metal ion. This means one bidentate ligand occupies two of the coordination sites in the complex.

Structure of ethylenediamine

A common bidentate ligand is ethylenediamine (also called 1,2-diaminoethane), with the formula $\text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2$:

Ethylenediamine has:

• A two-carbon chain
• An $\text{NH}_2$ group at each end
• Two nitrogen atoms, each with a lone pair of electrons
• The ability to form two coordinate bonds to a metal ion

Example: Dichlorobis(ethylenediamine)cobalt(III) ion

The complex $[\text{Co}(\text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2)_2\text{Cl}_2]^+$ contains:

• A $\text{Co}^{3+}$ ion
• Two ethylenediamine ligands (occupying four coordination sites)
• Two $\text{Cl}^-$ ligands (occupying two coordination sites)

This complex can exist as both cis and trans isomers:

The difference between the isomers:

• Cis isomer: Both chloride ligands are adjacent ($90°$ apart)
• Trans isomer: Both chloride ligands are opposite each other ($180°$ apart)

The ethylenediamine ligands "wrap around" the metal ion, occupying two coordination sites each.

Optical isomerism in octahedral complexes

What is optical isomerism?

Optical isomerism is a special type of stereoisomerism that produces enantiomers - molecules that are non-superimposable mirror images of each other. Think of your left and right hands: they are mirror images, but you cannot place one exactly on top of the other to make them match perfectly.

Requirements for optical isomerism

Why trans isomers don't show optical isomerism

An important point to remember: trans isomers cannot form optical isomers. The mirror image of a trans isomer is exactly the same as the original - it can be superimposed. Only cis isomers with bidentate ligands can exist as optical isomers.

Example: Complexes with two bidentate ligands

The cis form of $[\text{Co}(\text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2)_2\text{Cl}_2]^+$ (which we looked at earlier for cis-trans isomerism) also exists as two optical isomers:

Example: Complexes with three bidentate ligands

Optical isomerism can also occur in octahedral complexes containing three bidentate ligands. For example, $[\text{Ru}(\text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2)_3]^{2+}$ exists as two optical isomers:

In this complex:

• The ruthenium(II) ion is at the centre
• Three ethylenediamine ligands wrap around the metal
• The overall charge is 2+
• The two forms are non-superimposable mirror images

The role of cis-trans isomerism in medicine

Discovery of cisplatin

The medical importance of cis-trans isomerism in complex ions was discovered accidentally. Scientists were researching the effects of electric fields on bacteria using platinum electrodes. When they applied an electrical current to a colony of E. coli bacteria, they noticed the bacteria failed to divide but continued to grow.

Initially, they thought the electric current was preventing cell division. However, further investigation revealed that a platinum compound was being formed at the electrode. This compound was cis-platin (cisplatin).

Structure of cisplatin

Cisplatin has the formula $[\text{Pt}(\text{NH}_3)_2\text{Cl}_2]$ and has a square planar geometry:

The structure shows:

• A platinum(II) metal centre
• Two ammonia ligands
• Two chloride ligands
• The cis configuration (both ammonia ligands adjacent, both chloride ligands adjacent)

How does cisplatin work?

Cisplatin is effective as an anticancer drug because:

1. It enters cancer cells
2. It binds to DNA strands, forming cross-links between DNA bases
3. This binding prevents the DNA from replicating
4. The cell's own repair mechanisms detect the damage
5. Unable to repair the damage, the cell undergoes apoptosis (programmed cell death)

Clinical use and limitations

After extensive clinical trials, cisplatin was proven to be effective at attacking tumours. In many cases, tumours treated with cisplatin shrank significantly in size.

Unfortunately, cisplatin has significant side effects:

• Kidney damage
• Nausea
• Other unpleasant effects

Despite these side effects, cisplatin is still used extensively in cancer treatment today. Researchers continue to search for other platinum-based drugs that can inhibit tumour growth with fewer side effects.

Oxaliplatin

Another platinum-based anticancer drug is oxaliplatin, which is used for the treatment of colorectal cancer:

Oxaliplatin contains:

• A platinum(II) centre
• A cyclohexane ring with two amino groups attached (a bidentate ligand)
• An oxalato ligand (also bidentate)

The coordination number of platinum in oxaliplatin is 4, and the geometry is square planar. The presence of different ligands compared to cisplatin gives oxaliplatin different properties and effectiveness against different types of cancer.

Exam tips and common mistakes

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