Question

Identify the type of structural isomerism that exists in each of the following pairs of compounds: (a) \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]\left[\mathrm{Co}(\mathrm{ox})_{3}\right]\) and \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]\left[\mathrm{Cr}(\mathrm{ox})_{3}\right]\) (b) \(\left[\mathrm{CoCl}_{2}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}^{3} \cdot \mathrm{H}_{2} \mathrm{O}\) and \(\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{OH}_{2}\right)\right] \mathrm{Cl}_{2}\) (c) \(\left[\mathrm{Co}(\mathrm{CN})_{5}(\underline{\mathrm{NCS}})\right]^{3-}\) and \(\left[\mathrm{Co}(\mathrm{CN})_{5}(\mathrm{NCS})\right]^{3-}\) (d) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{PtCl}_{6}\right]\) and \(\left[\mathrm{PtCl}_{2}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{PtCl}_{4}\right]\)

Short answer

Pair (a) exhibits coordination isomerism, pair (b) shows ionization isomerism, pair (c) displays linkage isomerism, and pair (d) does not exhibit structural isomerism.

Step-by-step solution

Identify Isomerism for Pair (a)

Examine the coordination compounds \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]\left[\mathrm{Co}(\mathrm{ox})_{3}\right]\) and \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]\left[\mathrm{Cr}(\mathrm{ox})_{3}\right]\). Here, 'en' stands for ethylenediamine and 'ox' refers to the oxalate ion. The two compounds have their metal centers and ligands interchanged. This type of structural isomerism is known as coordination isomerism.

Identify Isomerism for Pair (b)

Compare the coordination spheres in \(\left[\mathrm{CoCl}_{2}(\mathrm{NH}_{3})_{4}\right] \mathrm{Cl}^{3} \cdot \mathrm{H}_{2} \mathrm{O}\) and \(\left[\mathrm{CoCl}(\mathrm{NH}_{3})_{4}(\mathrm{OH}_{2})\right] \mathrm{Cl}_{2}\). In the first compound, two chloride ions are inside the coordination sphere, while in the second, only one chloride ion is inside with a water molecule taking the place of the second chloride ion. This indicates a type of structural isomerism called ionization isomerism.

Identify Isomerism for Pair (c)

Analyze the two complexes \(\left[\mathrm{Co}(\mathrm{CN})_{5}(\underline{\mathrm{NCS}})\right]^{3-}\) and \(\left[\mathrm{Co}(\mathrm{CN})_{5}(\mathrm{NCS})\right]^{3-}\). Both complexes contain the same ligands but one of them has the thio ligand coordinated through the nitrogen atom (indicated by underline) and the other through the sulfur atom. This represents linkage isomerism, a type of structural isomerism where the linkage point of a ligand to the metal center is different.

Identify Isomerism for Pair (d)

Consider the compounds \(\left[\mathrm{Pt}(\mathrm{NH}_{3})_{4}\right]\left[\mathrm{PtCl}_{6}\right]\) and \(\left[\mathrm{PtCl}_{2}(\mathrm{NH}_{3})_{4}\right]\left[\mathrm{PtCl}_{4}\right]\). The first compound has a completely different composition than the second, fulfilling neither the requirement for coordination isomerism, ionization isomerism, nor linkage isomerism. Therefore, this is not a case of structural isomerism but instead two different compounds without isomerism.

Key concepts

Coordination Isomerism

Let's delve into the concept of coordination isomerism using a real example from the textbook. This type of isomerism occurs in coordination compounds that contain more than one metal center. Here, you can think of the metals as different 'neighborhoods' and the ligands, which are molecules or ions that bind to the metal, as 'houses' that can switch neighborhoods.

Consider the complexes \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]\left[\mathrm{Co}(\mathrm{ox})_{3}\right]\) and \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]\left[\mathrm{Cr}(\mathrm{ox})_{3}\right]\). In these complexes, the ligands (houses) have swapped places between the two coordinating metals (neighborhoods). In other words, ethylenediamine ('en') and oxalate ('ox') ligands have changed their metal partners. The chromium metal was linked with 'en' in the first compound, while in the second, it's linked with 'ox', showing coordination isomerism clearly.

In essence, coordination isomerism is all about the exchange of entire ligand groups between metal ions within a compound. It's like a lively dance of elements, switching partners while keeping the music (overall formula) the same.

Ionization Isomerism

Moving on to ionization isomerism, this one's a bit like a magic trick in chemistry. Imagine a magician showing you a molecule and then, with a little flick, one of the parts appears outside the molecule! This is what happens in ionization isomerism.

Take the two compounds \(\left[\mathrm{CoCl}_{2}(\mathrm{NH}_{3})_{4}\right] \mathrm{Cl}^{3} \cdot \mathrm{H}_{2} \mathrm{O}\) and \(\left[\mathrm{CoCl}(\mathrm{NH}_{3})_{4}(\mathrm{OH}_{2})\right] \mathrm{Cl}_{2}\). Here we have a fascinating switcheroo. In the first compound, two chloride ions are cozily tucked inside the coordination sphere. However, in the second compound, only one chloride ion gets to stay inside, and the other gets ousted, with water 'moving in' to take its place.

This molecular makeover changes the formula of the compounds when they are dissolved in water, because the ions that were inside and outside the coordination sphere swap places. That's how you can tell it's ionization isomerism—it's all about ions 'moving' in or out, thus affecting the properties of the compound when it's ionized.

Linkage Isomerism

Last but definitely not least, let's explore linkage isomerism. In the world of chemistry, some ligands are like talented actors—they can play more than one role depending on how they are connected to the metal center.

The classic example is the pair of complexes \(\left[\mathrm{Co}(\mathrm{CN})_{5}(\underline{\mathrm{NCS}})\right]^{3-}\) and \(\left[\mathrm{Co}(\mathrm{CN})_{5}(\mathrm{NCS})\right]^{3-}\). Both have the same ligands, but one catch: the ligand marked as NCS can attach through the nitrogen (N) or sulfur (S) atom. Think of it as a reversible jacket—wear it one way, and it shows a nitrogen 'side'; flip it, and it's all about sulfur.

Linkage isomerism comes into play because these molecules wear their ligand 'jackets' in different styles. As a result, even though the compound's overall formula doesn't change, the point of attachment, or linkage, of the ligand to the metal does, and this small change is like changing the cut of the jacket—it gives the molecule a whole new 'look' and properties.