Question

One of the most important properties of `d`-block metal cations is the formation of coordinate compounds. Widely used theories that explain bonding in these compounds are valence bond theory and crystal field theory.

Which of the following complex ions/compounds is obtained as a racemic mixture on mixing an
isomer with its mirror image in $1:1$ molar ratio?

• A. $\left[Ni{\left(dmg\right)}_2\right]$
• B. ${\left[Cr{\left(ox\right)}_3\right]}^{3-}$
• C. $\text{trans}\left[CrBrCl{\left(en\right)}_2\right]$
• D. $\left[IrB{r}_3{\left(H_{2}O\right)}_2\left(N{H}_3\right)\right]$

Answer 1

The correct option is B ${\left[Cr{\left(ox\right)}_3\right]}^{3-}$

We will be able to obatin a racemic mixture only when we mix equal parts of both forms of an optically active compound. Hence, we can eliminate the complexes which show no optical activity.
1. $\left[Ni{\left(dmg\right)}_2\right]$ - optically inactive due to a plane of symmetry.
2. ${\left[Cr{\left(ox\right)}_3\right]}^{3-}$ has two forms, cis and trans. It's cis form is optically active.
3. $\text{trans}\left[CrBrCl{\left(en\right)}_2\right]$ is optically inactive because it is a symmetrical molecule i.e. it has a plane of symmetry.
4. $\left[IrB{r}_3{\left(H_{2}O\right)}_2\left(N{H}_3\right)\right]$ is optically inactive because of the presence of plane of symmetry.

Hence, we can say that ${\left[Cr{\left(ox\right)}_3\right]}^{3-}$ is the required complex.