Q.60

 

The complex [Pt(NH3)2(py)2Cl2]2+[Pt(NH3)2(py)2Cl2]2+ has 3 optically inactive geometrical isomers (and 2 optically active ones, giving 5 geometrical isomers in total).

Geometry and ligand set

• Platinum(II) in such complexes is square planar with coordination number 4.

• The ligands are: two identical NH3NH3, two identical pyridines (py), and two identical chlorides, giving a planar arrangement of type Ma2b2c2Ma2b2c2.

• For Ma2b2c2Ma2b2c2 square‑planar complexes, 5 distinct geometrical arrangements (geometrical isomers) are possible.

Enumerating the 5 geometrical isomers

Label the four coordination positions as top (T), bottom (B), left (L), right (R) in the square plane around Pt. Each isomer is defined by how the two identical ligands are placed:

1. All three pairs trans

   • Example arrangement: T = NH33, B = NH33; L = py, R = py; Cl above/below plane (formal representation as trans‑NH3NH3, trans‑py, trans‑Cl).

   • This structure has multiple symmetry elements including a C22 axis and mirror planes, so it is optically inactive.

2. NH33 cis, py trans, Cl cis

3. NH33 trans, py cis, Cl cis

4. NH33 cis, py cis, Cl trans

   • These three belong to the set where exactly one ligand pair is trans and the other two are cis, or all three are cis in specific unsymmetrical patterns; two of them form a pair of non‑superposable mirror images (enantiomers), and one remains achiral.

5. All three pairs cis in a symmetric way

   • Arrangement with a mirror plane in the molecular plane (e.g., T = NH33, B = Cl, L = py, R = py with appropriate permutations) leads to another optically inactive isomer.

From systematic enumeration (standard result for Ma2b2c2Ma2b2c2 square‑planar complexes), two of the five are chiral and form an enantiomeric pair, while the remaining three possess a plane or center of symmetry and are therefore optically inactive.

Short SEO‑friendly introduction

In coordination chemistry exams, questions on the number of optically inactive geometrical isomers of [Pt(NH3)2(py)2Cl2]2+[Pt(NH3)2(py)2Cl2]2+ test understanding of square‑planar stereochemistry.
This complex belongs to the Ma2b2c2Ma2b2c2 type, which can form five geometrical arrangements, only some of which are optically active.
By examining cis–trans patterns and symmetry elements, one can quickly infer that three of these geometrical isomers are optically inactive.