Question

For complexes of the same metal ion with no change in oxidation number, the stability increases as the number of electrons in the t₂g orbitals increases. Which complex in each of the following pairs of complexes is more stable?

a) [Fe(H₂O)₆]²⁺
b) [Fe(CN)₆]⁴⁻
c) [Mn(H₂O)₆]²⁺
d) [Mn(CN)₆]⁴⁻

Answer 1

The more stable complex in each pair is [Fe(CN)6]4- and [Mn(CN)6]4-.

Step-by-step explanation:

The stability of complexes with the same metal ion and no change in oxidation number increases as the number of electrons in the t2g orbitals increases. Let's analyze the given pairs of complexes:

[Fe(H2O)6]2+ vs. [Fe(CN)6]4-: [Fe(H2O)6]2+ has weak-field ligands (H2O), so the electrons occupy the d orbitals singly before pairing occurs. On the other hand, [Fe(CN)6]4- has strong-field ligands (CN-), resulting in pairing of the electrons in the lower energy t2g orbitals. Thus, [Fe(CN)6]4- is more stable.

[Mn(H2O)6]2+ vs. [Mn(CN)6]4-: The analysis is similar to the previous pair. The weak-field ligands in [Mn(H2O)6]2+ result in electrons occupying all d orbitals singly before any pairing occurs, while the strong-field ligands in [Mn(CN)6]4- cause pairing in the t2g orbitals. Therefore, [Mn(CN)6]4- is more stable.