Question

Which of the following complex is optically active?

• A. $trans-[CrCl_{2}(ox)_{2}]^{3-}$
• B. $trans-[PtCl_{2}(en)_{2}]^{2+}$
• C. $cis-[Pt(NH_{3})_{2}]Cl_{2}$
• D. $trans-[Pt(NH_{3})_{2}]Cl_{2}$
• E. $cis-[PtCl_{2}(en)_{2}]^{2+}$

Hint:

Cis-chelates are almost always chiral in octahedral geometry.

Answer 1

Answer: (E)

Step 1: Understanding the Concept:
A complex is optically active if it is chiral, which means its mirror image is non-superimposable. In coordination chemistry, this is often tested by looking for elements of symmetry like a plane of symmetry or a center of inversion. If a molecule has a plane of symmetry, it is achiral and therefore optically inactive.
Step 2: Detailed Explanation:
Let's analyze the geometry and symmetry of each complex. (ox = oxalate, a bidentate ligand; en = ethylenediamine, a bidentate ligand). - (A) trans-[CrCl\(_2\)(ox)\(_2\)]\(^{3-}\): This is an octahedral complex of the type M(AA)\(_2\)b\(_2\), where AA is a bidentate ligand. In the trans isomer, the two Cl ligands are opposite each other (180° apart). The two bidentate oxalate ligands lie in the plane perpendicular to the Cl-Cr-Cl axis. This structure has a plane of symmetry containing the Cr and the two oxalate ligands, and also a center of inversion. It is achiral and optically inactive. - (B) trans-[PtCl\(_2\)(en)\(_2\)]\(^{2+}\): Similar to (A), this is an octahedral complex of type M(AA)\(_2\)b\(_2\). The trans isomer, with the two Cl ligands opposite each other, has multiple planes of symmetry and a center of inversion. It is achiral and optically inactive. - (C) cis-[Pt(NH\(_3\))\(_2\)]Cl\(_2\): The complex ion is [Pt(NH\(_3\))\(_2\)]\(^{2+}\) which is square planar as Pt\(^{2+}\) is a d\(^8\) ion. This is cis-[Pt(NH\(_3\))\(_2\)] which seems wrong, probably meant [Pt(NH\(_3\))\(_2\)Cl\(_2\)]. Assuming cis-[Pt(NH\(_3\))\(_2\)Cl\(_2\)], it is a square planar complex. Square planar complexes have a plane of symmetry (the molecular plane itself) and are therefore always achiral and optically inactive. - (D) trans-[Pt(NH\(_3\))\(_2\)]Cl\(_2\): Similar to (C), this is a square planar complex and is achiral. - (E) cis-[PtCl\(_2\)(en)\(_2\)]\(^{2+}\): This is the cis isomer of the M(AA)\(_2\)b\(_2\) octahedral complex. The two Cl ligands are adjacent to each other (90° apart). This arrangement lacks a plane of symmetry and a center of inversion. The molecule is asymmetric (chiral). Its mirror image is non-superimposable. Therefore, this complex is optically active. Step 4: Final Answer:
The complex cis-[PtCl\(_2\)(en)\(_2\)]\(^{2+}\) is optically active.