Chemistry - Coordination Chemistry Question with Solution | TestHub

The compound ${\mathrm{CoCl}}_3\cdot 4{\mathrm{NH}}_3$ exists as two different colored forms (green and violet) with distinct properties. This indicates they are isomers. Since the molecular formula is the same and the ligands are arranged differently in space, they are geometrical isomers (specifically, cis and trans isomers).

Explanation:

Geometrical isomers arise when ligands occupy different spatial positions around the central metal ion. For a square planar or octahedral complex, different arrangements of the same ligands can lead to distinct compounds with different physical and chemical properties, including color. In this case, ${\mathrm{CoCl}}_3\cdot 4{\mathrm{NH}}_3$ can be formulated as $[\mathrm{Co}({\mathrm{NH}}_3{)}_4{\mathrm{Cl}}_2]\mathrm{Cl}$, which is an octahedral complex. This complex can exhibit cis and trans isomerism, leading to different colors.

Let's analyze the options:

1. Ans.(B): This refers to the correct option.

2. Geometrical isomers: These isomers have the same chemical formula but different spatial arrangements of ligands around the central metal atom. This matches the description of two forms with different colors and properties.

3. Ionisation isomers: These isomers differ in the ion that is outside the coordination sphere versus inside. For example, $[\mathrm{Co}({\mathrm{NH}}_3{)}_5\mathrm{Br}]{\mathrm{SO}}_4$ and $[\mathrm{Co}({\mathrm{NH}}_3{)}_5{\mathrm{SO}}_4]\mathrm{Br}$.

4. Coordination isomers: These isomers occur when both the cation and anion are complex ions and the ligands are exchanged between the two complex ions. For example, $[\mathrm{Co}({\mathrm{NH}}_3{)}_6][\mathrm{Cr}(\mathrm{CN}{)}_6]$ and $[\mathrm{Cr}({\mathrm{NH}}_3{)}_6][\mathrm{Co}(\mathrm{CN}{)}_6]$.

The observed phenomenon of two distinct colors and properties for the same chemical formula ${\mathrm{CoCl}}_3\cdot 4{\mathrm{NH}}_3$ is characteristic of geometrical isomerism.