Chemistry - Coordination Chemistry Question with Solution | TestHub

The stability of complexes is influenced by factors like the nature of the metal ion, oxidation state, and ligand properties (e.g., chelate effect, ligand field strength).

A. The Co(III) complex, $[\mathrm{Co}({\mathrm{NO}}_2{)}_6{]}^{3-}$, is more stable than the Co(II) complex, $[\mathrm{Co}({\mathrm{NO}}_2{)}_6{]}^{4-}$, due to the higher oxidation state of cobalt.

B. $[\mathrm{Co}({\mathrm{NH}}_3{)}_6{]}^{3+}$ is more stable than $[\mathrm{Co}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{3+}$ because ${\mathrm{NH}}_3$ is a stronger field ligand than ${\mathrm{H}}_2\mathrm{O}$.

C. $[\mathrm{Cu}(trien){]}^{2+}$ is more stable than $[\mathrm{Cu}(\mathrm{en}{)}_2{]}^{2+}$ due to the chelate effect. Trien (triethylenetetramine) is a tetradentate ligand, forming more stable complexes than two bidentate en (ethylenediamine) ligands.

D. $[\mathrm{Ni}({\mathrm{NH}}_3{)}_6{]}^{2+}$ is generally more stable than $[\mathrm{Mn}({\mathrm{NH}}_3{)}_6{]}^{2+}$ due to higher crystal field stabilization energy for ${\mathrm{Ni}}^{2+}$ (d⁸) compared to ${\mathrm{Mn}}^{2+}$ (d⁵ high spin).

Therefore, option C correctly states that the first complex is more stable than the second.