Question

The square-planar complex \(\left[\mathrm{Pt}(\mathrm{en}) \mathrm{Cl}_{2}\right]\) only forms in one of two possible geometric isomers. Which isomer is not observed: cis or trans?

Short answer

The cis isomer is not observed for the square-planar complex \(\left[\mathrm{Pt}(\mathrm{en})\mathrm{Cl}_{2}\right]\) as it would not allow the ethylenediamine ligand to occupy adjacent sites on the metal center due to the placement of chloride ligands. The trans isomer is observed as it allows for a stable square-planar configuration with the platinum center, ethylenediamine, and the two chloride ligands.

Step-by-step solution

Understand the structure of the complex

The complex \(\left[\mathrm{Pt}(\mathrm{en})\mathrm{Cl}_{2}\right]\) contains a platinum(II) metal center (Pt), with ethylenediamine (en) and two chloride ligands (Cl) attached to it. Ethylenediamine is a bidentate ligand, meaning it can attach to the metal center at two points. The square planar configuration implies that all ligands lie in the same plane.

Identify cis and trans isomers

In a square-planar complex, the cis isomer has similar ligands adjacent to each other, while the trans isomer has similar ligands opposite each other. For \(\left[\mathrm{Pt}(\mathrm{en})\mathrm{Cl}_{2}\right]\), the cis isomer would have the two chloride ligands next to each other, and the trans isomer would have them diagonally opposite.

Determine the stability and existence of the isomers

The ethylenediamine ligand is a chelating compound that can form stable, ring-shaped structures with metal ions. In the complex, the en ligand occupies two adjacent coordination sites on the platinum center due to its bidentate nature. In a cis configuration, the chloride ligands would be on adjacent sites, leaving no space for the en ligand to occupy adjacent sites. Therefore, a cis isomer cannot form. On the other hand, in a trans configuration, the chloride ligands would be opposite each other, leaving two adjacent sites for the en ligand to attach to and form a stable chelate.

Identify the unobserved isomer

Based on the above reasoning, the cis isomer cannot form as it would not allow the ethylenediamine ligand to occupy adjacent sites on the metal center. Therefore, the cis isomer is not observed for the given complex. The trans isomer is observed as it allows both the ethylenediamine and the two chloride ligands to form a stable square-planar configuration with the platinum center.

Key concepts

Geometric Isomerism

Geometric isomerism occurs when compounds have the same formula but differ in spatial arrangement. This isomerism is common in coordination chemistry, particularly in square-planar complex ions. In these complexes, ligands are attached to the central metal in a plane, creating different possible arrangements.
This can lead to two main types of isomers:

• **Cis Isomers**: Ligands of the same type are adjacent.
• **Trans Isomers**: Ligands of the same type are opposite.

For example, in the complex \([\mathrm{Pt}(\mathrm{en})\mathrm{Cl}_{2}]\), the chloride ions can either sit next to each other (cis) or opposite each other (trans). However, structural requirements can sometimes prevent the formation of certain isomers. Understanding these spatial arrangements helps explain why some isomers are observed while others are not.

Bidentate Ligands

Bidentate ligands are molecules that can attach to a central metal atom at two points. This feature allows them to form chelate rings, which are generally more stable than complexes with monodentate ligands.
Ethylenediamine (en) is a classic example of a bidentate ligand. When en binds to a metal like platinum, it forms a ring structure by utilizing two adjacent coordination sites. This setup stabilizes the metal complex, often dictating the arrangement of other ligands.
In the square-planar complex \([\mathrm{Pt}(\mathrm{en})\mathrm{Cl}_{2}]\), ethylenediamine occupies two positions, leaving room only for the trans arrangement of chloride ligands. Understanding bidentate ligand behavior is critical in predicting the geometry of coordination complexes.

Square-Planar Complexes

Square-planar complexes involve a metal center with ligands arranged at the corners of a square plane. This geometry is typical for some complex ions, especially with metals like platinum.
In the complex \([\mathrm{Pt}(\mathrm{en})\mathrm{Cl}_{2}]\), the geometry requires careful placement of ligands to maintain stability. Bidentate ligands like ethylenediamine demand two adjacent sites, which affects the placement possibilities for other ligands, such as chloride ions. This constraint leads to only certain geometric isomers being feasible.
The square-planar framework allows for considerable predictability in terms of isomer preference, often leading to one stable form, as observed with the trans arrangement in this example. These properties illustrate why square-planar structures are significant in coordination chemistry, providing insights into molecular stability and reactivity.