Which of the following compounds would exhibit co-ordination isomerism?

[Pt(NH₃)₄] [CuCl₄]

The correct answer is option 2. \([Pt(NH_3)_4][CuCl_4]\).

Coordination isomerism is a type of structural isomerism observed in coordination compounds where there is an exchange of ligands between the coordination sphere (the central metal ion and its directly attached ligands) and the counter ions (the ions that balance the charge of the complex in the overall compound). This phenomenon occurs when the same ligands can coordinate either to the central metal ion in the complex or to the counter ions outside the complex.

Let us break down the concept with an example: \([Pt(NH_3)_4][CuCl_4]\)

In the compound \([Pt(NH_3)_4][CuCl_4]\), we have two parts:

\([Pt(NH_3)_4]^{2+}\): This is the complex ion where platinum \((Pt)\) is the central metal ion coordinated with four ammonia \((NH_3)\) ligands. The overall charge of the complex ion is \(+2\).

\([CuCl_4]^{2-}\): This is the counter ion where copper \((Cu)\) is coordinated with four chloride \((Cl^-)\) ligands. The overall charge of the counter ion is -2.

Analysis for Coordination Isomerism:

Ligands Involved:

\(NH_3\) (ammonia): Can act as a ligand coordinating to the central metal ion \((Pt)\) in \([Pt(NH_3)_4]^{2+}\).

\(Cl^-\) (chloride): Can act as a ligand coordinating to the central metal ion \((Cu)\) in [CuCl4]^{2-}\).

In the compound \([Pt(NH_3)_4] [CuCl_4]\), there is a possibility for ammonia \((NH_3)\) ligands to exchange between the \(Pt\) complex ion and the \(Cu\)) counter ion. Similarly, chloride \((Cl^-)\) ligands can also exchange between the \(Cu\) complex ion and the Pt counter ion.

This exchange leads to two possible isomers:

\([Pt(NH_3)_4] [CuCl_4]\): Ammonia\((NH_3)\) ligands are coordinated to \(Pt\) and chloride \((Cl^-)\) ions are coordinated to \(Cu\).

\([Cu(NH_3)_4] [PtCl_4]\): Ammonia \((NH_3)\) ligands are coordinated to \(Cu\) and chloride \((Cl^-)\) ions are coordinated to \(Pt\).

Coordination isomerism highlights the flexibility of ligands to switch between the central metal ion and the counter ions, leading to different structural forms of the compound with the same chemical formula but different arrangements of ligands. This phenomenon is particularly important in understanding the behavior of coordination compounds in solution and their chemical properties.

In summary, \([Pt(NH_3)_4][CuCl_4]\) is an example of a compound that exhibits coordination isomerism because it allows for the exchange of ligands (\(NH_3\) and \(Cl^-\)) between the complex ion and the counter ion. This exchange results in structural isomers with distinct coordination arrangements, illustrating the concept of coordination isomerism in coordination chemistry.