Edexcel A-Level Chemistry A2 17.5 Dative Coordinate Covalent Bonding Is Involved in the Formation of Complex Ions Questions

Transition metal compounds can show a number of different types of isomerism. Hydration isomerism is where different numbers of water molecules act as ligands. The name chromium(III) chloride is given to several chemical compounds with the formula $\mathrm{CrCl}_{3} \cdot \times \mathrm{H}_{2} \mathrm{O}$, including a number of hydration isomers.

Complexes can exhibit both cis-trans and optical isomerism. The cis- and trans-isomers of diamminedichloroplatinum(II) are commonly known as cis-platin and trans-platin.

In chemotherapy medication, cis-platin is used to treat a number of cancers including testicular cancer and breast cancer, while trans-platin has no beneficial effect against cancer. The cis-isomer is effective because it binds with the deoxyribonucleic acid (DNA) molecules in a cancerous cell through adenine and guanine groups. This interferes with the replication of the cell and results in its destruction.

This works in three steps.
Step 1 Slow substitution of one chloride ligand by a water.
Step 2 This water ligand is easily displaced by guanine or adenine in the DNA strand.

Step 3 Finally the second chloride ligand is displaced by a different guanine or adenine from a different part of the strand.

Describe how guanine and adenine can bind to the platinum ion.