Question

Which one of the following compounds does not show geometrical isomerism?

• A. \(\mathrm{MnO_4^-}\)
• B. \(\mathrm{[Fe(NH_3)_2(CN)_4]^-}\)
• C. \(\mathrm{[CrCl_2(ox)_2]^{3-}}\)
• D. \(\mathrm{[CoCl_2(en)_2]}\)

Hint:

Look for the tetrahedral species. A tetrahedron with four identical ligands has no cis-trans forms.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
Geometrical isomerism (cis-trans) appears when the same ligands can sit in two different relative positions around the metal. It needs a geometry where such different arrangements exist, such as octahedral or square planar with at least two different kinds of ligands.

Step 2: Key Formula or Approach
Check the geometry of each species. Then check the ligand pattern: \(MA_2B_4\), \(M(AA)_2B_2\) and \(M(AA)_2X_2\) types are octahedral and show cis-trans isomers.

Step 3: Check option (1)
Permanganate \(MnO_4^-\) is a simple tetrahedral ion. All four oxygen atoms are the same and every position is equivalent to every other. A tetrahedron has no cis or trans pair, so it cannot show geometrical isomerism. So (1) does NOT show it.

Step 4: Check option (2)
\([Fe(NH_3)_2(CN)_4]^-\) is an octahedral \(MA_2B_4\) complex. The two \(NH_3\) ligands can be next to each other (cis) or opposite each other (trans). So (2) shows geometrical isomerism.

Step 5: Check option (3)
\([CrCl_2(ox)_2]^{3-}\) is octahedral \(M(AA)_2B_2\) with two bidentate oxalate ligands. The two chloride ligands can be cis or trans. So (3) shows geometrical isomerism.

Step 6: Check option (4)
\([CoCl_2(en)_2]\) is also \(M(AA)_2B_2\) with two bidentate en ligands. The two chloride ligands can be cis or trans. So (4) shows geometrical isomerism.

Final Answer:
Only permanganate, a tetrahedral ion with identical ligands, has no geometrical isomers. This is option (1). \[ \boxed{\text{Option (1)}} \]