Isomerism in coordination compounds

Isomers have the same formula but a different arrangement. Coordination compounds show two big families.

Stereoisomers: the same bonds, arranged differently in space.

• Geometrical isomers. In square planar [Pt(NH₃)₂Cl₂], the two Cl can sit next to each other (cis), like two sisters leaning in together for a photo, or straight across from each other (trans). Octahedral complexes such as [Co(NH₃)₄Cl₂]⁺ show cis and trans in the same way, and so does M(AA)₂B₂ with a didentate ligand AA.
• For MA₃B₃ octahedral complexes there are two arrangements: fac (facial: the three A on one face, a triangle) and mer (meridional: the three A round a meridian, in a line round the middle).
• Tetrahedral complexes cannot show geometrical isomerism: in a tetrahedron, every position is next to every other.

• Optical isomers are mirror images that cannot be placed on top of each other, like your left and right hands. Such a molecule is chiral: it has no mirror plane. They are common in octahedral complexes with didentate ligands, such as [Co(en)₃]³⁺ and cis-[PtCl₂(en)₂]²⁺. To check, draw the complex, draw its mirror image, and turn it: if it never fits exactly, it is chiral. (The trans form has a mirror plane, so it is not.)

Structural isomers: different bonds.

• Linkage: an ambidentate ligand bonds through a different atom (–NO₂ through N, –ONO through O).
• Coordination: ligands are swapped between a complex cation and a complex anion, as in [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].
• Ionisation: a ligand and the counter ion swap places, so the compounds give different ions in water: [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅SO₄]Cl.
• Solvate (hydrate): water moves between being a ligand inside the bracket and free water of crystallisation outside.