Concept 03 / 06Coordination Compounds
Isomerism in Coordination Compounds
Structural isomerism (ionisation, solvate or hydrate, linkage and coordination isomerism) and stereoisomerism: geometrical (cis-trans in square planar and octahedral complexes, fac-mer) and optical isomerism in octahedral chelate complexes, with counting of isomers.
Isomers have the same formula but different arrangements of atoms.
Structural isomerism.
Ionisation isomerism: a counter ion itself could be a ligand and exchanges with a ligand, giving different ions in solution: (gives AgBr) and (gives ).
Solvate (hydrate) isomerism: water is inside or outside the coordination sphere: (violet), (grey-green), (dark green).
Linkage isomerism: an ambidentate ligand binds through different atoms: (nitrito-N, yellow) and (nitrito-O, red).
Coordination isomerism: exchange of ligands between cationic and anionic complexes: and .
Geometrical isomerism (see figure). Arises in heteroleptic complexes because ligands can occupy different relative positions.
Square planar (such as ): cis (like ligands adjacent) and trans (opposite). Also gives three isomers. Tetrahedral complexes do not show geometrical isomerism (all positions are adjacent).
Octahedral (such as ): cis and trans. (such as ): cis and trans.
Octahedral (such as ): facial (fac: the three A on one triangular face) and meridional (mer: the three A in a plane containing the metal, around the meridian).
Optical isomerism. Complexes that are non-superimposable on their mirror images (chiral) exist as d and l enantiomers. Common in octahedral complexes with didentate ligands:
(such as ): a pair of enantiomers.
cis- (such as cis-): optically active; the trans isomer has a plane of symmetry and is inactive. So has three stereoisomers in total.
Square planar complexes are rarely optically active (the plane is a symmetry plane).
Key Takeaways:
- Structural: ionisation, hydrate, linkage (ambidentate), coordination.
- square planar and octahedral: cis/trans.
- : fac/mer.
- and cis- are optically active.
- Pitfall: tetrahedral shows no geometrical isomerism.
Brain-Teaser 1: How many isomers (including stereoisomers) does have?
Solution: Geometrical: cis and trans. The trans isomer is achiral; the cis isomer is chiral and exists as d and l forms. Total .
Brain-Teaser 2: Identify the type of isomerism: (a) and ; (b) and ; (c) and .
Solution: (a) linkage; (b) coordination; (c) ionisation.
Brain-Teaser 3: Why does show geometrical isomerism but does not?
Solution: () forms a square planar complex, in which two positions can be adjacent (cis) or opposite (trans). () forms a tetrahedral complex, where every pair of positions is adjacent, so only one arrangement exists.
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