Two or more compounds having the same molecular formula but different arrangement of atoms are called isomers and the phenomenon is called isomerism.

Because of different arrangement of atoms, isomers differ in one or more physical or chemical properties. Isomers can be broadly classified into two major categories:

(A) Structural isomers (B) Stereoisomers.

These are further sub-divided as shown below:

Consider the following isomerisms:

(I) Ionization

(II) Hydrate

(III) Coordinate

(IV) Ligand

Which of the above isomerism are exhibited by \([Cr(NH_3)_2(H_2O)_4]Cl_2\)?

Only II

The answer is 2. The complex \([Cr(NH_3)_2(H_2O)_4]Cl_2\) can exhibit only hydrate isomerism because it does not have any ionizable groups and it has only one metal ion in the coordination sphere.

Here are the two hydrate isomers of \([Cr(NH_3)_2(H_2O)_4]Cl_2\):

\([Cr(NH_3)_2(H_2O)_4]Cl_2\)

\([Cr(NH_3)_2(H_2O)_3]Cl .H_2O\)

The first isomer has four water molecules coordinated to the metal ion, while the second isomer has three water molecules coordinated to the metal ion and one water molecule as a ligand. The two isomers have the same chemical formula, but they have different physical properties.