Question

Although the cis configuration is known for \(\left[\mathrm{Pt}(\mathrm{en}) \mathrm{Cl}_{2}\right], \mathrm{no}\) trans form is known. (a) Explain why the trans compound is not possible. (b) Would \(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) be more likely than en \(\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\right)\) to form the trans compound? Explain.

Short answer

In summary, the trans compound is not possible for the [Pt(en)Cl₂] complex due to the chelate effect caused by the bidentate ethylenediamine (en) ligand, which forces both nitrogen atoms to bind to the Pt atom on the same side. On the other hand, the ligand NH₂CH₂CH₂CH₂CH₂NH₂, with a longer linear chain between the amine groups, is more flexible and likely to form the trans compound, as it can coordinate with the Pt atom in a trans configuration.

Step-by-step solution

Part (a) Understanding the cis configuration for [Pt(en)Cl₂]

The given compound has the formula [Pt(en)Cl₂], where en is ethylenediamine (NH₂CH₂CH₂NH₂). It is a coordination complex containing the central metal atom Pt, which is connected to two ethylenediamine (en) and two chlorine atoms (Cl). The "cis" isomer involves arranging the ligands in a way such that both Cl atoms and both N atoms from en are on the same plane. Now, let's answer why the trans compound is not possible for this configuration.

Part (a) Explaining the impossibility of the trans form of [Pt(en)Cl₂]

In the trans isomer, for the given compound [Pt(en)Cl₂], we'd expect to have one Cl atom and one N atom from en on the same plane, while the other Cl and N atoms from en are on the opposite side. However, it is important to note that the ethylenediamine (en) is a bidentate ligand, which means it attaches to the Pt atom simultaneously through both its N atoms. This creates a chelate ring (consisting of the Pt and en), and the chelate effect is responsible for stabilizing the complex. As a result, keeping both N atoms of en on the same side becomes necessary, thus making it impossible to form the trans isomer in this scenario. Now, let's move to part (b) of the exercise and compare the likelihood of forming trans compounds for NH₂CH₂CH₂CH₂CH₂NH₂ and en (NH₂CH₂CH₂NH₂).

Part (b) Understanding the ligands NH₂CH₂CH₂CH₂CH₂NH₂ and en (NH₂CH₂CH₂NH₂)

The two ligands given in this part are: 1. NH₂CH₂CH₂CH₂CH₂NH₂ - This is a linear chain with amine groups at both ends and 5 carbon atoms between them. 2. en (NH₂CH₂CH₂NH₂) - Ethylenediamine, also with amine groups at both ends and 2 carbon atoms between them. Now, let's determine the likelihood of trans compound formation for each ligand.

Part (b) Comparing the likelihood of trans compound formation

For trans compound formation, we need each ligand to be able to bind to the Pt atom with one N atom on one side and the other N atom on the opposite (trans) side. 1. For the ligand NH₂CH₂CH₂CH₂CH₂NH₂, it’s more flexible due to the longer linear chain between the amine groups. This allows it to coordinate with the Pt atom in a trans configuration, where one N atom can be on one side and the other can be on the opposite side. 2. In contrast, the en ligand (NH₂CH₂CH₂NH₂) is less flexible due to a shorter chain between amine groups, and as a result, the chelate effect forces both N atoms to bind to the Pt atom on the same side, restricting the formation of a trans isomer. Therefore, the ligand NH₂CH₂CH₂CH₂CH₂NH₂ is more likely than en (NH₂CH₂CH₂NH₂) to form the trans compound.

Key concepts

Chelate Effect

When studying coordination compounds, an important concept to understand is the chelate effect. This phenomenon occurs when a ligand forms a ring that includes the central metal atom within the coordination complex. Ligands that can do this are known as chelating ligands.

These chelating ligands often create more stable complexes compared to compounds with monodentate ligands, which only bind through a single atom. The stability arises due to the formation of two or more bonds with the central metal ion, effectively increasing the overall entropy of the system - less ordered species (monodentate ligands and their associated ions) morph into a more ordered structure (the chelate complex).

Let's consider ethylenediamine (en), a bidentate ligand, in the [Pt(en)Cl₂] complex. Here, the chelate effect contributes to the complex's stability by ensuring that en forms a ring with the platinum ion. This ring structure locks the two nitrogen atoms in the 'cis' position, making the formation of a 'trans' isomer impossible due to the inherent rigidity of the ring.

Bidentate Ligand

Diving into the intricacies of coordination chemistry reveals various types of ligands, including the bidentate ligand. A bidentate ligand has two donor atoms which can simultaneously bind to a single metal ion, creating a five-membered or six-membered chelate ring in the process.

Characteristics of Bidentate Ligands

Bidentate ligands, like ethylenediamine found in [Pt(en)Cl₂], contain multiple binding sites – typically nitrogen, oxygen, or sulfur atoms. These sites are often part of a flexible molecular structure that allows the ligand to wrap around the central metal atom. The ligand's ability to form such chelates has a large impact on the compound's chemical properties and stability.

In the given example, ethylenediamine (en) coordinates with the platinum atom in such a way that it forms a stable, ring-like structure. As we attempt to envision a 'trans' isomer, the rigid structure imposed by the chelating en ligand prevents any such isomer from forming, which is why the 'trans' configuration of [Pt(en)Cl₂] does not exist.

Coordination Compounds

Coordination compounds are a class of compounds where a central metal atom is bonded to a surrounding array of molecules or ions called ligands. These ligands provide electrons to the metal, which creates coordinate covalent bonds. The entire entity, comprised of the central metal and its ligands, is referred to as a coordination complex.

One common case is platinum complexes. They often have square planar geometries and are of great interest in fields like medicinal chemistry. The reason for the interest lies in the cis and trans isomers these compounds can form. In pharmaceutical applications, the different isomers can have dramatically diverse effects on biological systems.

The metal-ligand bonding greatly affects the properties of these compounds. For instance, the presence of a bidentate ligand, like ethylenediamine, restricts the isomerism due to the chelate effect. As a result, such considerations are crucial in the synthesis and application of these compounds in various fields, including medicinal chemistry, where specific isomers can be leveraged for targeted therapeutic effects.