Problem 154 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following has largest number of isomers? \(\quad\) (a) \(\left[\mathrm{Ru}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right]^{2+}\) (c) \(\left[\mathrm{Ir}\left(\mathrm{PR}_{3}\right)_{2} \mathrm{H}(\overline{\mathrm{CO}})\right]^{2+}\) (d) \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\) \([\mathrm{R}=\) alkyl group, en \(=\) ethylenediamine \(]\)

Step-by-Step Solution

Answer

(d) \( \left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+} \) has the largest number of isomers.

Step 1: Analyze Complex (a)

The compound is \( \left[\mathrm{Ru}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]^{+} \). It is an octahedral complex that can exhibit geometric isomerism. The two chloride ions can be either adjacent to each other (cis) or opposite each other (trans). Thus, there are two possible isomers for this complex.

Step 2: Analyze Complex (b)

The compound \( \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right]^{2+} \) is an octahedral complex with one ligand different from the others. It can have only one possible arrangement because swapping the positions of the \(\text{Cl}^-\) with any \(\text{NH}_3\) molecule does not result in a new isomer. Therefore, it has only one isomer.

Step 3: Analyze Complex (c)

In the compound \( \left[\mathrm{Ir}\left(\mathrm{PR}_{3}\right)_{2} \mathrm{H}(\overline{\mathrm{CO}})\right]^{2+} \), the central atom Ir forms a square planar complex. Square planar complexes like this can show geometric isomerism. Here, the positions of \( \mathrm{PR}_{3} \) and hydrogen or \( \text{CO} \) can be changed, resulting in different isomers. This compound could have multiple isomers, more than one due to different seating arrangements of \( \mathrm{PR}_{3} \), \( \text{H}^+ \), and \( \overline{\text{CO}} \).

Step 4: Analyze Complex (d)

The compound \( \left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+} \) is an octahedral complex. Ethylenediamine (en) is a bidentate ligand that can cause multiple geometric isomers due to the two chloride ions present. This complex can exhibit both geometric (cis and trans) isomerism, and possibly optical isomerism as well. Generally, this type of complex can have multiple isomers, commonly more than two.

Step 5: Compare the Isomer Counts

In summary, complex (a) has 2 isomers, complex (b) has 1, complex (c) can have multiple (possibly more than two), and complex (d) can have 3 or more isomers due to both geometric and optical possibilities.

Key Concepts

Isomerism in Coordination Compounds

Coordination compounds are fascinating because they can form various isomers. An isomer is a compound that has the same chemical formula as another compound, but a different arrangement of atoms in space. In coordination chemistry, isomerism can significantly affect the physical and chemical properties of a compound. Coordination compounds can exhibit several types of isomerism, including geometric and optical isomerism. Understanding these might help in fields like pharmaceuticals, where the activity of a drug depends on its isomer form. Complexes can rearrange the ligands, the atoms, ions, or molecules bound to a central atom, leading to different isomers.

Geometric Isomerism

Geometric isomerism occurs when ligands can assume different positions around a central atom, leading to different spatial arrangements. This type of isomerism is common in coordination compounds with octahedral and square planar geometries. In an octahedral complex like \[\left[\mathrm{Ru}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\], geometric isomers can form when the two chloride ions are positionally distinct, either adjacent (cis) or opposite (trans) from each other. For square planar complexes such as \[\left[\mathrm{Ir}\left(\mathrm{PR}_{3}\right)_{2} \mathrm{H}(\overline{\mathrm{CO}})\right]^{2+}\], ligands can occupy adjacent or opposite positions forming isomers. Geometric isomers can exhibit different reactivities and properties, which can be crucial in catalysis and material science.

Optical Isomerism

Optical isomerism is a fascinating phenomenon where isomers (called enantiomers) are non-superimposable mirror images of each other. This often occurs in coordination complexes where the arrangements of the ligands cause asymmetry. When we observe a complex like \[\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\], optical isomerism can be seen due to the chiral nature of the structure when the ligands create a mirror image relationship. Enantiomers can have significant implications in areas like biochemistry and pharmacology, as the different forms may react differently with biological molecules, affecting processes such as drug efficacy.

Square Planar Complexes

Square planar complexes are one of the primary shapes seen in coordination compounds, particularly with metal ions such as palladium, platinum, and iridium. In a square planar complex, like \[\left[\mathrm{Ir}\left(\mathrm{PR}_{3}\right)_{2} \mathrm{H}(\overline{\mathrm{CO}})\right]^{2+}\], the central metal atom is at the center of a square made by the four bound ligands. This arrangement allows for a variety of isomers because the positions of the ligands can be switched with each other. They are vital in certain catalytic processes like those used in organic chemistry reactions, such as the fabrication of new organic compounds. Due to their planar nature, they lend themselves to very specific types of reactivity that may not be present in other coordination geometries.

Octahedral Complexes

Octahedral complexes are extremely common in coordination chemistry and usually form with a central metal ion surrounded by six ligands arranged symmetrically. An example is \[\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right]^{2+}\]where there is a specific arrangement of ligands around a co-central metal ion. Octahedral complexes often display both geometric and optical isomerism due to the vast number of ways ligands can be arranged, leading to multiple possible isomers. For example, \[\left[\mathrm{Ru}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\] can show cis or trans forms because of the flexible arrangement of ligands. These complexes play crucial roles in processes like electron transfer, light absorption, and catalysis in various chemical reactions and industries.