Question

The complex which has facial and meridional isomers is
(Given : \(\text{py} = \text{pyridine}\) and \(\text{en} = \text{H}_2\text{N}-\text{CH}_2-\text{CH}_2-\text{NH}_2\))

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

Hint:

Keep this structural template locked in memory: - fac-mer isomerism requires an octahedral system strictly following the configuration \(\mathbf{[MA_3B_3]}\). - Scan options immediately for a combination of \(3+3\) monodentate ligands. Here, 3 pyridine + 3 chloride make option (2) an instant match.

Answer 1

The Correct Option is B

Concept: Facial (fac) and meridional (mer) isomerism is a specific category of geometrical isomerism observed exclusively in octahedral complexes with an empirical formula of the type \([\text{MA}_3\text{B}_3]\), where M denotes the central metal ion, and A and B represent monodentate ligands.

• Facial (fac) Isomer: Occurs when the three identical ligands occupy three adjacent positions on a single octahedral face (clumped together corner-to-corner).

• Meridional (mer) Isomer: Occurs when the three identical ligands occupy positions around an arc or meridian that encircles the metal ion (forming a T-shape pattern).

Step 1: Classifying Option (1)
The complex \([\text{Ni(en)}_2(\text{H}_2\text{O})_2]^{2+}\) matches the structural profile \([\text{M(XX)}_2\text{B}_2]\), where en behaves as a didentate chelating ligand. This class of coordination compound displays standard cis- and trans- geometrical isomerism, but lacks the structural symmetry required to define fac/mer spatial configurations.

Step 2: Classifying Option (2)
The complex \([\text{Cr(py)}_3(\text{Cl})_3]\) features a central chromium ion surrounded by three monodentate pyridine (py) ligands and three monodentate chloride (\(\text{Cl}^-\)) ligands. This aligns perfectly with the generic structural format: \[ [\text{MA}_3\text{B}_3] \quad \text{where } \text{M} = \text{Cr}^{3+}, \, \text{A} = \text{py}, \, \text{B} = \text{Cl}^- \] Because it contains exactly two distinct sets of three identical monodentate ligands, it can be arranged into distinct facial and meridional stereoisomers.

Step 3: Classifying Option (3)
The complex \([\text{Cr(H}_2\text{O})_6]^{3+}\) takes the basic molecular formula \([\text{MA}_6]\). Since all six coordinating groups are completely identical ligand molecules, no structural or geometric isomerism can exist.

Step 4: Classifying Option (4)
The complex \([\text{Co(NH}_3)_4(\text{H}_2\text{O})_2]^{3+}\) conforms to the standard coordination blueprint \([\text{MA}_4\text{B}_2]\). This type of complex possesses two geometric orientations, designated as cis- (ligands separated by \(90^\circ\)) and trans- (ligands separated by \(180^\circ\)), rather than exhibiting fac/mer isomerism. Thus, only complex (2) supports facial and meridional isomerism.