Animated Solution for Chemistry - Coordination Compounds: A reaction of cobalt (III) chloride and ethylene diamine in a 1:2 mole ratio generates two isomeric products A (violet coloured) and B (green coloured). A can show optical activity, but B is optically inactive. What type of isomers does A and B represent ?

The Tale of Two Isomers: Cis, Trans, and the Magic of Symmetry

Have you ever wondered how two molecules with the exact same atoms can have completely different colors and optical properties? Welcome to the fascinating world of coordination chemistry! In this problem, we explore the reaction between cobalt(III) chloride and ethylene diamine, a classic experiment that beautifully demonstrates the concept of geometrical isomerism.

Our journey begins with a chemical reaction. We are mixing cobalt(III) chloride ($CoC{l}_3$) with ethylene diamine (en) in a strict 1:2 molar ratio. Ethylene diamine is a bidentate ligand, meaning it has two nitrogen atoms that can simultaneously bite onto the central metal atom.

Because we only have two molecules of 'en', they occupy four coordination sites around the cobalt ion. To satisfy cobalt's preferred coordination number of six, two chloride ions must remain inside the coordination sphere. This gives us the complex cation $[Co(en{)}_{2}C{l}_2{]}^{+}$.

The complex $[Co(en{)}_{2}C{l}_2{]}^{+}$ is an octahedral complex of the general type $[M(AA{)}_2{a}_2]$, where 'AA' represents a symmetrical bidentate ligand and 'a' represents a monodentate ligand. This specific geometry is famous for existing in two distinct spatial arrangements: the cis and trans forms. These are known as geometrical isomers.

Cis-Isomer

Imagine placing the two chloride ligands right next to each other, at a $90^{\circ }$ angle. This arrangement is the cis-isomer. If you try to find a plane of symmetry in this molecule—a mirror plane that cuts it into two identical halves—you will fail. The molecule is asymmetric, or chiral.

Because it lacks a plane of symmetry, the cis-isomer is optically active, meaning it can rotate plane-polarized light. The problem states that Product A is optically active and violet in color. Therefore, Product A is the cis-isomer!

Trans-Isomer

Now, let's rearrange the furniture. What if we place the two chloride ligands exactly opposite to each other, at a $180^{\circ }$ angle? This gives us the trans-isomer.

Take a close look at the equatorial plane containing the cobalt atom and the two ethylene diamine ligands. This plane acts as a perfect mirror, slicing the top chloride and the bottom chloride symmetrically. Because it possesses this plane of symmetry, the trans-isomer is achiral and optically inactive. This perfectly matches the description of the green Product B.

So, what is the relationship between Product A and Product B? They share the same molecular formula and the same atom-to-atom connectivity. However, they differ entirely in how their ligands are arranged in 3D space. This makes them Geometrical Isomers.

We can confidently rule out the other options: ionisation isomerism requires an exchange of counter ions, linkage isomerism requires ambidentate ligands, and coordination isomerism requires both a complex cation and a complex anion. Geometrical isomerism is the only piece that completes this chemical puzzle!