Q5(b). Isomers of [CoCl₂(en)₂]⁺; is there a meso form?

1. Formula: [CoCl₂(en)₂]⁺, Co(III) d⁶, octahedral; each en (H₂NCH₂CH₂NH₂) spans two cis positions (it is too short to span trans).
2. trans isomer: the two Cl on opposite apices, the two en rings in the equatorial plane. It has a mirror plane (and a centre of symmetry), so it is superimposable on its mirror image: optically inactive. Historically the green praseo salt.
3. cis isomer: the two Cl at 90°. No mirror plane and no centre of symmetry: the molecule and its mirror image are non-superimposable, so cis exists as two enantiomers (d and l, also called Δ and Λ from the sense of the chelate rings). Historically the violet violeo salt. Werner's resolution of such cis bis(ethylenediamine)cobalt(III) ions into optical isomers proved the octahedral model.
4. Count: 2 geometrical isomers (cis, trans); 3 stereoisomers in all (trans, d-cis, l-cis).

Does a meso form exist? No.

1. A meso compound has two or more stereogenic centres of opposite configuration, so that its two halves are mirror images of each other and their rotations cancel inside the molecule (meso-tartaric acid).
2. [CoCl₂(en)₂]⁺ has one stereocentre, the cobalt atom (the en rings are not chiral centres). A molecule with one stereocentre can be chiral or achiral, but not meso.
3. The trans isomer is optically inactive because it is achiral (it has a plane of symmetry); it is a diastereomer of the cis pair, not a meso form of it. The racemic cis mixture is inactive by external compensation, again not meso.

Application. the cis and trans isomers are told apart by the number of products on substitution and by their colours; the optically active cis ion was among the first chiral complexes resolved, showing that optical activity does not need carbon.