A reaction of cobalt (III) chloride and ethylenediamine in a 1:2 mole ratio generates two isomeric products A (violet coloured) and B (green coloured). A can show optical activity, while B is optically inactive. What type of isomers do A and B represent?

We know Ethylenediamine is a bidentate ligand and Co³⁺ forms an octahedral complex having a coordination number 6. Here, 2 moles of ethylenediamine can satisfy four coordination numbers. Then the remaining two would be satisfied by existing Chloride ions. The reaction is,
CoCl₃ + 2C₂H₈N₂ → [CoCl₂(en)₂]Cl
According to the given ratio, the above product can only be formed. As it says there are two products, so another product should be an isomer. Now the possibility is that the two Cl⁻ ions can be either in cis form or in trans form. And on seeing this in cis form, there is no plane of symmetry and hence it is chiral and optically active, and the trans form will be optically inactive. Hence they are geometrical isomers of each other.