Question

How would you account for the following: 

1. Of the d⁴ species, Cr²⁺ is strongly reducing while manganese(III) is strongly oxidising. 
2. Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised. 
3. The d¹ configuration is very unstable in ions.

Answer 1

(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 \(t^3_2\)g 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.

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(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 3rd ionization 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.

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(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.

Answer 2

(i) The standard electrode potential for Cr³⁺/Cr²⁺ is negative (-0.41 V), while Mn³⁺/Mn²⁺ has a positive value (+1.57 V). Cr²⁺ can undergo oxidation and serves as a reducing agent, whereas Mn³⁺ can undergo reduction and acts as an oxidizing agent.

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(ii) Due to the greater stability of Co(III) compared to Co(II), the transition of cobalt from the +2 to +3 oxidation state is facilitated in the presence of a complexing agent.

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(iii) Once ns electrons are lost, the removal of a d¹ electron leads to a stable electronic configuration. Elements exhibiting this behavior either act as reducing agents or undergo disproportionation.