Question:
How would you account for the following:

(i) Of the d⁴ species, Cr²⁺ is strongly reducing while manganese (III) is strongly oxidising.

(ii) Cobalt (II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.

(iii) The d¹ configuration is very unstable in ions.

Answer:

(i) Cr²⁺ is strongly reducing in nature. It has a d⁴ configuration. While acting as a reducing agent, it gets oxidized to Cr³⁺ (electronic configuration, d³). This d³ configuration can be written as t³_2g configuration, which is a more stable configuration. In the case of Mn³⁺(d⁴), it acts as an oxidizing agent and gets reduced to Mn²⁺(d⁵). This has an exactly half-filled d-orbital and is highly stable.

(ii) Co (II) is stable in aqueous solutions. However, in the presence of strong field complexing reagents, it is oxidized to Co (III). Although the 3^rdionization energy for Co is high, but the higher amount of crystal field stabilization energy (CFSE) released in the presence of strong field ligands overcomes this ionization energy.

(iii) The ions in d¹ configuration tend to lose one more electron to get into stable d⁰ configuration. Also, the hydration or lattice energy is more than sufficient to remove the only electron present in the d-orbital of these ions. Therefore, they act as reducing agents

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