Problem 7 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

Which of the following pairs of compounds are isomers? Name the type of any isomerism present. (Section 28.3 ) (a) \(\quad \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{8}\right]\) and \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (b) \(\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{3}\right] \mathrm{Se} \mathrm{O}_{4}\) and \(\left[\mathrm{Co}\left(\mathrm{SeO}_{4}\right)\left(\mathrm{NH}_{3}\right)_{3}\right] \mathrm{C}\) (c) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{Pt} \mathrm{Br}_{4}\right]\) and \(\left[\mathrm{PtBr}\left(\mathrm{NH}_{3}\right)\right]\left[\mathrm{PtBr}_{3}\left(\mathrm{NH}_{3}\right)\right]\) (d) \(\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{3}\right] \mathrm{Cl}_{2}\) and \(\left[\mathrm{Co}(\mathrm{ONO})\left(\mathrm{NH}_{3}\right)\right] \mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

Pairs (b) and (d) are isomers with ionization and linkage isomerism, respectively.

Step 1: Determine Isomerism in Pair (a)

For the pair \( \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{8}\right] \) and \( \mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \), evaluate the coordination number and oxidation state of iron. The first complex has coordination of 8, while the second has coordination of 6. These are not isomers since they do not have the same chemical formula. Therefore, no isomerism type is present.

Step 2: Determine Isomerism in Pair (b)

For \( \left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{3}\right] \mathrm{SeO}_{4} \) and \( \left[\mathrm{Co}\left(\mathrm{SeO}_{4}\right)\left(\mathrm{NH}_{3}\right)_{3}\right] \mathrm{Cl} \), notice the positions of \( \mathrm{Cl} \) and \( \mathrm{SeO}_{4} \) in the complexes. They represent a coordination sphere change, suggesting ionization isomerism, where an anion inside the coordination sphere exchanges with one outside.

Step 3: Determine Isomerism in Pair (c)

Consider \( \left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{Pt}\mathrm{Br}_{4}\right] \) and \( \left[\mathrm{PtBr}\left(\mathrm{NH}_{3}\right)\right]\left[\mathrm{PtBr}_{3}\left(\mathrm{NH}_{3}\right)\right] \). The number of ligands around each Pt and their oxidation states differ, indicating these compounds do not share the same molecular formula and thus are not isomers.

Step 4: Determine Isomerism in Pair (d)

Examine \( \left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{3}\right] \mathrm{Cl}_{2} \) and \( \left[\mathrm{Co}(\mathrm{ONO})\left(\mathrm{NH}_{3}\right)_{3}\right] \mathrm{Cl}_{2} \). Both have the same chemical formula. The difference is in the ligand connectivity: \( \mathrm{NO}_{2}^- \) as a nitro group vs. \( \mathrm{ONO}^- \) as a nitrito group. This represents linkage isomerism.

Key Concepts

Coordination Compounds

Coordination compounds are fascinating structures composed of a central metal atom or ion that is bonded to a surrounding array of molecules or ions known as ligands. These compounds are significant due to their complex 3D structures and wide range of properties. A key feature of coordination compounds is the coordination sphere, which includes the metal and all its directly bonded ligands.
The properties of coordination compounds are determined largely by the arrangement of ligands around the metal. These can affect color, reactivity, and biological activity. For instance, the coordination sphere dictates an important property of these compounds: their ability to exhibit isomerism.
In everyday life, coordination compounds are impactful, ranging from the vibrant colors in paints to vital roles in biological systems, such as hemoglobin transporting oxygen in the blood.

Ionization Isomerism

Ionization isomerism is a type of isomerism seen in coordination compounds where two or more compounds have the same composition but yield different ions in solution. This occurs when an anion inside the coordination sphere is exchanged with an anion outside of it.
A classic example of ionization isomerism is observed in the pair \( [\mathrm{CoCl}(\mathrm{NH}_{3})_{3}] \mathrm{SeO}_{4} \) and \( [\mathrm{Co}(\mathrm{SeO}_{4})(\mathrm{NH}_{3})_{3}] \mathrm{Cl} \). Despite having similar chemical formulas, the primary distinction is the interchange of \( \mathrm{Cl}^- \) and \( \mathrm{SeO}_{4}^{2-} \) between the coordination sphere and the ionizable sphere.
This switch results in different ions being produced when these compounds dissolve in water, influencing properties like solubility and conductivity. This type of isomerism highlights the subtle yet significant influence of structure on the behavior and properties of coordination compounds.

Linkage Isomerism

Linkage isomerism occurs when a ligand can coordinate to a metal atom in more than one way. It involves a change in the way a ligand bonds to the central atom, though the overall chemical formula remains unchanged.
A well-known example is the pair \( [\mathrm{Co}(\mathrm{NO}_{2})(\mathrm{NH}_{3})_{3}] \mathrm{Cl}_{2} \) and \( [\mathrm{Co}(\mathrm{ONO})(\mathrm{NH}_{3})_{3}] \mathrm{Cl}_{2} \). The ligand \( \mathrm{NO}_{2}^- \), commonly referred to as the nitro group when the nitrogen atom forms the bond, can also bond through oxygen, resulting in the nitrito linkage.
Linkage isomers are intriguing because despite having identical empirical formulas, they exhibit different properties such as color, stability, and reactivity due to the different connections of the ligand to the metal center. This underlines the profound effects that seemingly minor structural changes can have on compound characteristics.

Coordination Number

The coordination number of a complex is the total number of ligand attachments to the central metal atom. It essentially reflects the number of coordinate covalent bonds between the ligand and metal.
Various factors influence the coordination number, including the size and charge of the central metal ion, as well as the size and charge of the ligands.

• Common coordination numbers are 6 and 4, which often produce octahedral and square planar geometries, respectively.
• A coordination number of 8 can lead to a square antiprismatic structure.

Understanding the coordination number is crucial in predicting the geometry and subsequent properties of the compound. It's a foundational element in the design and application of coordination compounds in fields like medicinal chemistry and catalysis.

Oxidation State

The oxidation state of a metal in a coordination compound is a conceptual charge that helps to determine the electron distribution in the complex. It is important for identifying the nature of coordination compounds and hints at potential reactivity.
To calculate the oxidation state, consider the total charge of the compound and subtract the charges contributed by the ligands. For example, in the complex \( [\mathrm{Pt}(\mathrm{NH}_{3})_{4}] \mathrm{Br}_{4} \), \( \mathrm{Br}_{4}^- \) contributes a charge of -4, indicating that platinum is in the +4 oxidation state.
Understanding oxidation states is vital as it affects not only the stability and color of the coordination compounds but also their magnetic and catalytic properties. It is a key concept in both inorganic and analytical chemistry, helping scientists to predict how these compounds will interact and react under various conditions.