Part 2: Complex isomerism, ligand exchange and biological roles

Keep three-dimensional arrangements, ligand changes and oxidation states separate when interpreting complex reactions.

Geometric isomers have different ligand arrangements

In square-planar Pt(NH₃)₂Cl₂, cis has the two chloride ligands adjacent and trans has them opposite. Both have the same connections and formula; the different arrangement cannot be removed by rotating the entire molecule in space. A tetrahedral MA₂B₂ complex does not have this cis/trans pair because its positions are equivalent.

Octahedral complexes with two identical ligands and four of another type can also have adjacent cis and opposite trans arrangements. Identify which ligands you are comparing and draw all six coordination positions. A flat cross with four ligands is not an octahedral drawing.

Optical isomerism does not require a carbon chiral centre

Some complexes with bidentate ligands form non-superimposable mirror images. For [Ni(en)₃]²⁺, all six donor atoms can be nitrogen and there need not be a carbon bonded to four different groups. Chirality comes from the overall three-dimensional arrangement of the chelating ligands.

To decide whether two diagrams are isomers, preserve the connections within each en ligand while imagining rotations. Treating all six nitrogen atoms as independent monodentate ligands destroys the feature that produces the handed arrangement. Cis/trans and optical are distinct classifications; do not label every pair of complex drawings “optical”.

Ammonia and chloride can replace water ligands

Adding a small amount of aqueous ammonia to Cu²⁺ first gives a pale-blue hydroxide precipitate because ammonia acts as a base. In excess ammonia it dissolves to a deep-blue solution containing [Cu(NH₃)₄(H₂O)₂]²⁺. Four waters have been replaced; two remain in the OCR complex formula.

Adding a high chloride concentration to the aqua complex establishes [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O. The tetrachloro complex is yellow; mixtures can look green because blue aqua and yellow chloro species coexist. Dilution with water favours the aqua complex again.

The coordination number changes from six to four and the course model changes from octahedral to tetrahedral. Copper remains +2 throughout. A colour change therefore does not prove that redox has occurred: ligand substitution alone can change the observed colour.

Use the OCR chromium complex and recognise slow substitution

The specified ammonia complex of chromium(III) is [Cr(NH₃)₆]³⁺, described as purple/violet. The six water ligands of [Cr(H₂O)₆]³⁺ can be replaced by six ammonia ligands. Chromium(III) substitution can be slow, so observed development depends on time and conditions rather than always being instantaneous.

Do not import the copper formula into chromium chemistry: copper retains two water ligands in the specified tetraammine complex, whereas chromium forms the hexaammine model. Complex formulae are needed in ligand-substitution equations because they show what has actually been replaced.

A ligand change can alter a biological function

Iron in haemoglobin provides a site for reversible oxygen binding, enabling transport. Carbon monoxide binds strongly at the metal site and competes with oxygen, reducing effective oxygen transport. Explain this as a ligand-binding effect rather than claiming that carbon monoxide simply removes all iron from blood.

Cis-platin acts as an anticancer drug by binding to DNA and disrupting replication and cell division. Its cis arrangement permits particular binding interactions; the trans isomer is not biologically interchangeable merely because its formula is the same. Chemotherapy can also damage healthy cells, which explains the need to weigh benefit and side effects. This is the specification’s chemical model, not treatment advice.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources