How would you account for the following:
Of the species, is strongly reducing while manganese(III) is strongly oxidising.
Cr²⁺ has d⁴ configuration and acts as a strong reducing agent because it can easily oxidize to Cr³⁺ (d³), which has a stable configuration. Mn³⁺ also has d⁴ configuration but acts as a strong oxidizing agent because it can easily reduce to Mn²⁺ (d⁵), which has a stable half-filled d-orbital configuration. As mentioned in the context, the 'Much larger third ionisation energy of Mn (where the required change is d⁵ to d⁴) is mainly responsible for this' and explains 'why the +3 state of Mn is of little importance.'
Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.
In aqueous solution, Co²⁺ is stable. However, in the presence of complexing reagents, Co(II) is easily oxidized to Co(III) because the complex stabilizes the higher oxidation state. The stability of oxidation states depends on the nature of the species formed.
The configuration is very unstable in ions.
The d¹ configuration is very unstable in ions because such ions can easily lose the single d-electron to achieve a stable configuration or gain electrons to achieve more stable configurations like d³ or d⁵, which provide extra stability.
Explanation
This question tests understanding of oxidation states and stability of transition metal ions. Part (i) relates to the stability of d³ and d⁵ configurations and uses the context about Mn's third ionization energy. Part (ii) involves how complexing agents stabilize higher oxidation states. Part (iii) relates to the instability of d¹ configuration due to the tendency to achieve more stable electronic configurations.