JEE Chemistry 2025 Question with Solution

Given: The complexes are $\left[\mathrm{CrCl}_{3}(\mathrm{py})_{3}\right]$ and $\left[\mathrm{CrCl}_{2}(\mathrm{ox})_{2}\right]^{3-}$.

Find: The number of stereoisomers possible for each complex.

For $\left[\mathrm{CrCl}_{3}(\mathrm{py})_{3}\right]$, chromium is coordinated by six ligands, so the complex has octahedral geometry. This is of the type $[\mathrm{MA}_{3}\mathrm{B}_{3}]$.

For an octahedral complex of the type $[\mathrm{MA}_{3}\mathrm{B}_{3}]$, two geometrical arrangements are possible:

• fac isomer
• mer isomer

Hence, $\left[\mathrm{CrCl}_{3}(\mathrm{py})_{3}\right]$ has 2 stereoisomers.

For $\left[\mathrm{CrCl}_{2}(\mathrm{ox})_{2}\right]^{3-}$, oxalate is a bidentate ligand, so the complex is again octahedral. The two chloride ligands can be arranged in cis and trans forms.

The trans form is not optically active. The cis form is chiral and gives a pair of enantiomers.

So the total number of stereoisomers is:

Therefore, the numbers of stereoisomers are $2$ and $3$ respectively. The correct option is C.

Given: Stereoisomers include geometrical and optical isomers.

Find: Count all stereoisomers for both complexes.

1. For $\left[\mathrm{CrCl}_{3}(\mathrm{py})_{3}\right]$:

• It is an octahedral complex.
• It belongs to the type $[\mathrm{MA}_{3}\mathrm{B}_{3}]$.
• Such complexes show facial (fac) and meridional (mer) isomerism.

Hence, total stereoisomers = $2$.

2. For $\left[\mathrm{CrCl}_{2}(\mathrm{ox})_{2}\right]^{3-}$:

• It is an octahedral complex with two chloride ligands and two bidentate oxalato ligands.
• Geometrical isomers: cis and trans.
• The cis arrangement is optically active and exists as two non-superimposable mirror images.
• The trans arrangement is not optically active.

Hence, total stereoisomers =

Therefore, the required pair is $2 \& 3$, so the correct option is C.