Which of the following species show geometrical isomerism?

Solution:

Geometrical isomerism in coordination compounds arises due to different possible spatial arrangements of ligands around the central metal ion. Let's analyze each option:

1. [Cr(H₂O)₅Cl]Cl₂: This complex is of the type [MA₅B]. Octahedral complexes of this type do not exhibit geometrical isomerism because there is only one possible arrangement for the ligands.

2. [Pt(NH₃)₃Cl]Cl: This complex is of the type [MA₃B]. For a square planar complex (common for Pt(II)), MA₃B type does not show geometrical isomerism. If it were an octahedral complex, it would also not show geometrical isomerism.

3. [Co(en)₃]³⁺: This complex is of the type [M(AA)₃], where 'en' (ethylenediamine) is a bidentate ligand. Complexes of this type are octahedral and do not show geometrical isomerism (cis/trans) because all positions are equivalent relative to the bidentate ligands. However, they do exhibit optical isomerism.

4. [Co(NH₃)₃(NO₂)₃]: This complex is of the type [MA₃B₃]. Octahedral complexes of the MA₃B₃ type exhibit geometrical isomerism, specifically facial (fac) and meridional (mer) isomers. In the fac isomer, the three identical ligands (e.g., NH₃) occupy positions on one face of the octahedron. In the mer isomer, the three identical ligands lie in a plane passing through the central metal ion.

Therefore, [Co(NH₃)₃(NO₂)₃] shows geometrical isomerism.