5.3.1 (f,g) Stereoisomerisim in Complex ions

(f) types of stereoisomerism shown by complexes, including those associated with bidentate and multidentate ligands:

(i) cis–trans isomerism e.g. Pt(NH₃)₂Cl ₂

(ii) optical isomerism e.g. [Ni(NH₂CH₂CH₂NH₂)₃]²⁺

{Learners should be able to draw 3-D diagrams to illustrate stereoisomerism.}

(g) use of cis-platin as an anti-cancer drug and its action by binding to DNA preventing cell division

{Benefits of chemotherapy; risks from unpleasant side effects.}

What does this mean?

"Cis" means the same side, "Trans" means the other side

The Ciskei and Transkei regions of South Africa are either side of the Kei river, for instance.

It's possible to arrange square planar complexes in both Cis and Trans form if they have two different ligands

Cisplatin is a cancer drug that is thought to bind to DNA and stop it dividing. Its Chloride ligands are both on the same side of the central metal ion.

Transplatin is an isomer but is the wrong shape to do the job and so doesn't have the same beneficial effect. Its Chloride ligands are on opposite sides of the metal ion.

This can stop a tumour in its tracks but such chemotherapy drugs also affect healthy tissue and so are not a "magic bullet" as the side-effects are very serious.

The Chromium complex ions above both have two CN^- ligands and two bidentate ligands such as ethane diamine H₂NCH₂CH₂NH₂

The lefthand example is Trans because the CN^- ligands are on opposite sides of the metal ion

The righthand example is Cis because the CN^- ligands are next to each other.

Optical isomers are non-superimpossible mirror images of each other.

For reasons that we don have to consider, they rotate polarised light in opposite directions.

The simplest examples generally have a Carbon atom bonded tetrahedrally to 4 different atoms/groups.

Note, it's impossible to rotate the lefthand molecule and end up as the righthand one.

Metal ion complexes can show the same effect if they are octahedral and have bidentate ligands, although you'll never be asked to explain how.

Unfortunately, it's quite a bit harder to draw the isomers.

This would be an acceptable representation. It's hard to picture the rotation without making the models but we have shown them to be mirror images.

This representation is also usually acceptable, though the examiner sometimes insists on seeing which atoms are donating the electron pairs in the dative bonds (usually N or O atoms)

This arrangement also shows optical isomerism because the two isomers are non-superimposible mirror images.

So it's possible to have Cis/Trans and Optical isomers of the same complex.

Note, you can only have optical isomers of the Cis form.

So, three isomers is the maximum. Any question asking for 3 isomers of a metal complex with a couple of bidentate ligands is looking for this.