Isomerism in Coordination Compounds: Quick Revision

A brief note on Isomerism in Coordination Compounds. Read it before a test. Try each question at the end before you open its answer.

Structural isomers

Same overall formula, different arrangement of the parts.

• Ionisation: a ligand and a counter ion swap. Example: $[\text{Pt}(\text{NH}_{3})_{4}\text{Cl}_{2}]\text{Br}_{2}$ and $[\text{Pt}(\text{NH}_{3})_{4}\text{Br}_{2}]\text{Cl}_{2}$. Silver nitrate gives AgBr with the first and AgCl with the second.
• Hydrate: water inside or outside the bracket. Example: $[\text{Cr}(\text{H}_{2}\text{O})_{6}]\text{Cl}_{3}$, $[\text{CrCl}(\text{H}_{2}\text{O})_{5}]\text{Cl}_{2}\cdot \text{H}_{2}\text{O}$ and $[\text{CrCl}_{2}(\text{H}_{2}\text{O})_{4}]\text{Cl}\cdot 2\text{H}_{2}\text{O}$.
• Linkage: an ambidentate ligand bonds through its other atom, such as $\text{NO}_{2}^{-}$ (N or O) and $\text{SCN}^{-}$ (S or N).
• Coordination: ligands swap between the cation and the anion of one salt, such as $[\text{Cu}(\text{NH}_{3})_{4}][\text{PtCl}_{4}]$ and $[\text{Pt}(\text{NH}_{3})_{4}][\text{CuCl}_{4}]$.
• Only free ions outside the bracket precipitate with silver nitrate or barium chloride.

Geometrical isomers

• Cis: $90^\circ$ apart. Trans: $180^\circ$ apart.
• Tetrahedral: none. Square planar: $\text{MA}_{2}\text{B}_{2}$ 2, $\text{MA}_{2}\text{BC}$ 2, $\text{MABCD}$ 3.
• Octahedral: $\text{MA}_{4}\text{B}_{2}$ 2, $\text{MA}_{3}\text{B}_{3}$ 2 (fac, mer), $\text{MA}_{2}\text{B}_{2}\text{C}_{2}$ 5, $\text{MABCDEF}$ 15.
• To count, write the ligands as three trans pairs and count the different sets.
• The cis isomer of a square planar $\text{MA}_{2}\text{B}_{2}$ is polar. The trans isomer has no dipole moment.

Optical isomers

• A complex with no plane of symmetry is chiral and has a mirror-image form.
• Chiral: $[\text{M}(\text{AA})_{3}]$, cis-$[\text{M}(\text{AA})_{2}\text{B}_{2}]$, $[\text{M}(\text{AB})_{3}]$. Not chiral: square planar complexes and trans-$[\text{M}(\text{AA})_{2}\text{B}_{2}]$.
• Stereoisomers = geometrical isomers + mirror-image pairs.
• Enantiomers rotate plane-polarised light by equal angles in opposite directions. A racemic mixture does not rotate it.

A worked example

Find the geometrical isomers of $[\text{Co}(\text{NH}_{3})_{4}\text{BrCl}]^{+}$ and say whether any is optically active.

1. Type. Four A ligands, one B, one C: $\text{MA}_4\text{BC}$.
2. Trans pairs. The sets are AA, AA, BC (B trans to C) and AA, AB, AC (B cis to C). There are 2 geometrical isomers.
3. Mirror images. Each has a plane of symmetry, so neither is optically active.

Practice with new numbers

A program to try

The program lists the sets of trans pairs for several types and counts them.

Mistakes to avoid

• Counting a turned drawing as a new isomer.
• Looking for cis and trans in a tetrahedral complex.
• Counting water outside the bracket as an ion.
• Saying every chelate complex is chiral. Test for a plane of symmetry.
• Counting a mirror-image pair as one stereoisomer.

Check yourself

1. Which two ligands in this lesson can give linkage isomers?

2. How many geometrical isomers has a square planar complex of type $\text{MABCD}$?

3. Is cis-$[\text{Co}(\text{en})_{2}\text{Cl}_{2}]^{+}$ optically active?

4. What rotation does a racemic mixture give, and why?