The complex which has facial and meridional isomers is: (py = pyridine, en = H₂N-CH₂-CH₂-NH₂)

Solution & Explanation

Concept: facial (fac) and meridional (mer) isomerism is shown only by octahedral complexes of the type [Ma₃b₃] — three of one ligand and three of another. In fac, the three identical ligands occupy one triangular face (mutually cis, 90° apart); in mer, they lie on a meridian (two trans, one cis). Check each option: - A. [Cr(py)₃Cl₃] is [Ma₃b₃] (three pyridine + three chloride) → shows fac and mer isomers. ✓ - B. [Cr(H₂O)₆]³⁺ is [Ma₆] → only one arrangement, no fac/mer. - C. [Co(NH₃)₄(H₂O)₂]³⁺ is [Ma₄b₂] → shows cis/trans, not fac/mer. - D. [Ni(en)₂(H₂O)₂]²⁺ is [M(en)₂b₂] → shows cis/trans (and the cis is chiral), not fac/mer. Trap: fac/mer requires the Ma₃b₃ pattern specifically; Ma₄b₂ and Ma₂b₂c₂ give cis/trans isomerism instead. Hence the answer is A — [Cr(py)₃Cl₃].