Question

Which of the following is heteroleptic complex ?

• A. \( [\text{Co}(\text{NH}_3)_6]^{3+} \)
• B. \( [\text{Cr}(\text{NH}_3)_6]^{3+} \)
• C. \( [\text{Ni}(\text{H}_2\text{O})_6]^{2+} \)
• D. \( [\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+ \)

Hint:

Heteroleptic complexes are important because they are the ones that typically show Geometrical Isomerism (cis/trans forms). Homoleptic complexes like \( [\text{Co}(\text{NH}_3)_6]^{3+} \) cannot show cis-trans isomerism.

Answer 1

The Correct Option is D

Concept: Coordination compounds are formed when ligands (donor atoms/molecules) donate electron pairs to a central metal ion. Depending on the variety of ligands present in the coordination sphere, complexes are categorized into two types:

• Homoleptic Complexes: These are complexes in which the central metal atom or ion is bound to only one kind of donor group/ligand. For example, a metal surrounded only by ammonia molecules.

• Heteroleptic Complexes: These are complexes in which the central metal atom or ion is bound to more than one kind of donor group/ligand. These complexes often exhibit various types of isomerism, such as geometrical or optical isomerism.
Step 1: Analysis of Homoleptic options (A, B, and C).
(A) \( [\text{Co}(\text{NH}_3)_6]^{3+} \): Here, the Cobalt(III) ion is surrounded by six ammine (\(\text{NH}_3\)) ligands. Since all six ligands are identical, it is a homoleptic complex. (B) \( [\text{Cr}(\text{NH}_3)_6]^{3+} \): Similarly, the Chromium(III) ion is coordinated with six identical ammine ligands. This is also a homoleptic complex. (C) \( [\text{Ni}(\text{H}_2\text{O})_6]^{2+} \): In this case, the Nickel(II) ion is coordinated with six aqua (\(\text{H}_2\text{O}\)) molecules. Since only one type of ligand (water) is present, it is homoleptic.

Step 2: Detailed analysis of option (D).
In the complex \( [\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+ \), the Cobalt ion is the central metal. Looking at the coordination sphere (inside the square brackets), we find:

• Four ammine (\(\text{NH}_3\)) ligands.

• Two chlorido (\(\text{Cl}^-\)) ligands.
Because there are two different species (ammine and chlorido) acting as ligands to the same metal center, this complex perfectly fits the definition of a heteroleptic complex.

Step 3: Conclusion.
The presence of multiple types of ligands in the coordination sphere of \( [\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+ \) distinguishes it from the other options which contain only a single type of ligand.