Final answer:
The red [Co(NH₃)₅NO₂]₂ complex changes to yellow due to a change in its structural configuration, affecting the d-orbital splitting and the wavelengths of light that the complex absorbs.
Step-by-step explanation:
When the red complex [Co(NH₃)₅NO₂]₂ forms and then changes color to yellow, a structural change is occurring within the complex ion. This change involves the ligands attached to the metal ion, cobalt, in this particular complex. Ligands such as ammonia (NH₃) and nitrite (NO₂) influence the complex's electronic structure and thus its color, as the color of coordination complexes is affected by the types of ligands bound to the metal ion, due to their impact on the d-orbital splitting and the resulting absorption of light. The change in color is indicative of this ligand exchange or rearrangement, where either the nitrite ligand changes its bonding mode or its position relative to the cobalt center, thus affecting the overall energy of d-orbital splitting and the wavelengths of light absorbed. The yellow color suggests that the energy gap between the d-orbitals has decreased, typically associated with a change to weaker-field ligands or a change in their arrangement and an increase in the wavelength of light absorbed.
In short, the transformation from red to yellow over time is a reflection of an internal equilibrium within the complex, which can be shifted by various factors including temperature, light, and the presence of other chemicals, altering the energy of absorbed light and therefore the color observed.