Problem 72 - General Chemistry: Principles and Modern Applications

Question

Of the following complex ions, the one that exhibits isomerism is (a) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+} ;\) (b) \(\left[\mathrm{CoNO}_{2}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+};\) \((\mathrm{c}) \quad\left[\mathrm{Pt}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+};\) (d) \(\quad\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+};\) (e) \(\left[\mathrm{PtCl}_{6}\right]^{2-}\).

Step-by-Step Solution

Answer

The complex ion that exhibits isomerism is \(\quad\left[\mathrm{Pt}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\).

Step 1: Understanding Structural Isomerism

Structural isomerism occurs when the compounds have the same formula but different bonding structures. Each compound must have a central atom or cation, and ligands around it. A complex ion would be able to exhibit geometric isomerism if it has a coordination number of 6 and the ligands surrounding the central atom are not all identical.

Step 2: Analyzing Complex Ions

Now let's analyze the complex ions provided in each option:\n(a) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\) - This complex has two identical ligands and cannot show isomerism.\n(b) \(\left[\mathrm{CoNO}_{2}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}\) - This complex Ion also cannot exhibit isomerism as all five of its ligands are identical. The nitro group does not act as a ligand.\n(c) \(\quad\left[\mathrm{Pt}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\) - This complex ion shows geometric isomerism as it has a coordination number of 6 and the ligands are not identical.\n(d) \(\quad\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}\) - Similar to option (b), this ion also cannot have isomerism because of identical ligands.\n(e) \(\left[\mathrm{PtCl}_{6}\right]^{2-}\) - All ligands are identical in this case, so no isomerism can be observed.

Step 3: Determining the Correct Option

From the analysis in step 2, it is clear that \(\quad\left[\mathrm{Pt}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\) is the only complex ion among the given options that can exhibit geometric isomerism.

Key Concepts

Structural Isomerism

Structural isomerism is a phenomenon observed in chemistry where molecules with the same molecular formula have different structures. Imagine you have a set of building blocks with the same shape and number of pieces, yet you can put them together in different ways to create various structures. In coordination chemistry, structural isomerism occurs among complex ions which can form diverse structures around a central metal ion due to different possible arrangements of their ligands — the molecules or ions that bind to the metal. Structural isomerism includes various types such as linkage, coordination, and ionization isomerism. This is critical in understanding how identical formulas can lead to complex ions with very different properties, reactivities, and functionalities.

For instance, in coordination chemistry, a ligand that can attach to the central metal in more than one way can result in linkage isomers. In the exercise provided, the evaluation of structural isomerism is important to determine whether the given complex ions can exhibit different forms based purely on the arrangement of atoms within them.

Coordination Chemistry

Coordination chemistry is a branch of chemistry that deals with complex compounds formed when central metal atoms or ions bond with surrounding groups called ligands. This area of chemistry is essential because it explains a lot of biological processes, helps in designing new materials, and plays a role in environmental chemistry. In the educational exercise, complex ions are the focus — these are species formed when a central metal ion is surrounded by ligands. The number of ligand atoms directly bonded to the central atom is known as the coordination number, which plays a key role in the structure of the complex ion.

Ligands can be simple ions like chloride, Cl-, or molecules like ammonia, NH3. They can be monodentate, having one point of attachment to the metal, or multidentate with multiple binding sites. An example from the exercise is [Pt(en)(NH3)2]2+, where en (ethylenediamine) is a bidentate ligand, coordinating through two nitrogen atoms to the platinum ion. Understanding coordination chemistry is a stepping stone to grasping other essential concepts like the types of isomerism.

Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, is a specific type of stereoisomerism. This type of isomerism occurs due to different possible spatial arrangements of ligands around the central metal ion in a complex ion. In the context of coordination compounds, you can think of geometric isomerism as the different ways of arranging furniture in a room, where the room represents the coordination sphere around the central metal atom, and the pieces of furniture are the ligands.

In a complex ion with a coordination number of six, such as [CoCl(NH3)5]2+, geometric isomerism can arise when there are at least two different types of ligands present. The ligands can be arranged in distinct patterns around the metal center, resulting in isomers with different spatial arrangements and physical properties. For instance, when a complex has two types of ligands in varying patterns, such as in the case of [Pt(en)(NH3)2]2+ from the exercise, two isomers can be formed: one where the same type of ligands are on the same side (cis-isomer), or where they are on opposite sides (trans-isomer). Each isomer can have vastly different chemical and biological activities, making the concept a crucial aspect of coordination chemistry.