Which of the following is not optically active?

• A. [Co(en)₃]³⁺
• B. [Cr(ox)₃]³⁻
• C. cis - [CoCl₂(en)₂]⁺
• D. trans - [CoCl₂(en)₂]⁺

To determine which of the given complexes is not optically active, we need to consider their symmetry and structure.

Understanding Optical Activity

Optical activity refers to the ability of a compound to rotate plane-polarized light. This property is typically found in chiral molecules, which lack an internal plane of symmetry.

Analyzing the Options

• A. [Co(en)₃]³⁺: This complex is chiral due to the arrangement of the ethylenediamine (en) ligands, making it optically active.
• B. [Cr(ox)₃]³⁻: The oxalate (ox) ligands create a symmetrical structure, leading to no optical activity.
• C. cis - [CoCl₂(en)₂]⁺: The cis arrangement of ligands results in chirality, thus it is optically active.
• D. trans - [CoCl₂(en)₂]⁺: The trans configuration has a plane of symmetry, making it not optically active.

Final Thoughts

Among the options, both B and D are not optically active. However, if you need to choose one, B. [Cr(ox)₃]³⁻ is the best answer as it is inherently symmetrical and does not exhibit optical activity.