Question

(I) [ Co(EDTA)] - has two optical isomers. (II) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{NO}_{2}\right)_{2}\right]^{+}\)show linkage isomers. (III) For [ \(\left.\left.\mathrm{Pt}(\mathrm{py})\left(\mathrm{NH}_{3}\right)\left(\mathrm{NO}_{2}\right) \mathrm{Cl} \mathrm{Br}\right]\right]\), theoretically fifteen different geometrical isomers are possible. (IV) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) can show hydrate as well as ionisation isomerism. Amongs the following correct statements are: (1) II, III (2) III (3) I, III (4) I, II \& III

Short answer

Correct statements II.

Step-by-step solution

- Analyze Statement I

Examine the given compound \[\mathrm{Co(EDTA)}^{-1}\]. EDTA (Ethylenediaminetetraacetic acid) is a hexadentate ligand forming very stable complexes with transition metals like Co(III). Since EDTA forms a large, rigid, and stable structure around the metal ion, it restricts the possibility of optical isomerism. Therefore, optical isomers are not expected for this complex.

- Analyze Statement II

Examine the given compound \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{NO}_{2}\right)_{2}\right]^{+}\). The ligand \(\mathrm{NO}_{2}^{-}\) can coordinate to the metal center through nitrogen (nitro) or oxygen (nitrito). This type of isomerism, where the ligand can bind to the central atom through different atoms, is called linkage isomerism. Therefore, this statement is correct.

- Analyze Statement III

Examine the given compound \[\left[\mathrm{Pt}(\text{py})(\mathrm{NH}_{3})(\mathrm{NO}_{2})\mathrm{Cl} \mathrm{Br}\right]\]. For Pt(IV) complexes with 6 different ligands, a number of geometric isomers are possible. However, to state that precisely 15 different geometrical isomers are possible is an incorrect assertion without detailed stereochemical analysis. Therefore, theoretically 15 geometrical isomers are not necessarily correct, and this statement can be deemed dubious or incorrect.

- Analyze Statement IV

Examine the given compound \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\). In this complex, water molecules can either be inside the coordination sphere (as coordinated ligands) or outside the sphere (as hydration water). The complex can show hydrate isomerism. Also, ionization isomerism can occur when a ligand within the inner coordination sphere can exchange with an anion outside the sphere, although this compound can't exchange anions like \(\mathrm{Cl}^{-}\) from the sphere, the assertions need detailed condition. This does not necessarily confirm ionization isomerism for the given complex.

Key concepts

Optical Isomerism

Optical isomerism occurs in coordination compounds when the arrangement of ligands around the central metal ion leads to non-superimposable mirror images. These compounds do not have a plane of symmetry and can rotate the plane of polarized light. This is similar to how your left and right hands are mirror images but cannot be superimposed.
In the original exercise, the compound \(\text{Co(EDTA)}^-\) was expected to exhibit optical isomerism. However, EDTA, being a hexadentate ligand, forms highly stable and rigid complexes which typically restrict optical isomerism. Thus, \(\text{Co(EDTA)}^-\) does not show this property.

Linkage Isomerism

Linkage isomerism occurs when a ligand can coordinate to the metal ion through two or more different atoms. For example, the \(\text{NO}_2^-\) can bind through the nitrogen atom (nitro) or through an oxygen atom (nitrito).
In the provided problem, \(\text{[Co(NH}_3\text{)}_4\text{(NO}_2\text{)}_2]^+\) exhibits linkage isomerism because the \(\text{NO}_2^-\) ligand can attach to the cobalt ion via either the nitrogen or the oxygen atom. This leads to the creation of different forms of the compound, demonstrating linkage isomerism.

Geometrical Isomers

Geometrical isomerism arises in coordination compounds when ligands can occupy different positions around the central metal ion, leading to distinct arrangements. This is commonly seen in octahedral and square planar complexes.
For instance, the compound \(\text{[Pt(py)(NH}_3\text{)(NO}_2\text{)ClBr]}\) can theoretically have multiple orientations of its six ligands around the Pt(IV) center. However, the solution correctly argues that without a detailed stereochemical analysis, it's incorrect to definitively state that precisely 15 geometric isomers are possible. Thus, this statement is dubious.

Hydrate Isomerism

Hydrate isomerism occurs when water molecules from the solvent get included inside or outside the coordination sphere of the complex. This leads to variations like hydrate isomers, where the number of water molecules inside the coordination sphere changes.
The compound \(\text{[Cr(H}_2\text{O)}_4\text{Cl}_2]\text{Cl}_2 \text{2H}_2\text{O}\) shows hydrate isomerism since water can either be coordinated to the chromium ion or exist outside the coordination sphere. The presence of hydrate isomers often shows differing properties like color or solubility for the different isomers.