Question

Once the red complex (Co(NH₃)₅NO₂)₂* is formed, it slowly changes its color to yellow. What is happening and why?

(a) The gaseous NH3 ligands dissociate.
(b) The complex undergo redox reaction.
(c) It undergoes isomerization reaction.
(d) The reaction is both kinetic- and thermodynamic-controlled.
(e) The reaction is only thermodynamically controlled.

Answer 1

The red complex (Co(NH₃)₅NO₂)₂* turns yellow due to an isomerization reaction, where the nitro group repositions itself, altering the electronic structure and changing the light absorption characteristics without a redox reaction or dissociation of NH₃ ligands. So the correct answer is option C.

Step-by-step explanation:

When the red complex (Co(NH₃)₅NO₂)₂* changes its color to yellow, it is undergoing an isomerization reaction. This type of reaction involves a rearrangement of ligands within the molecule without disrupting the coordination center.

Specifically, the nitro group (NO₂) may change its coordination from an O-bonded nitrito form to an N-bonded nitro form. Such changes affect the electronic structure and therefore, the light absorption characteristics of the complex, leading to a color change from red to yellow.

This process does not involve the dissociation of NH₃ ligands or a redox reaction. It is a common phenomenon in coordination chemistry where the optical properties of a complex are altered by the spatial rearrangement of its ligands.