How would you account for the following:
A) Of the `d^(4)` species `Cr^(2+)` is strongly reducing while manganese(III) is strongly oxidizing.
B) Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidized.
C) The `D^(1)` configuration is very unstable in ions.

Explanation

(a) E values for `Cr^(3+) | Cr^(2+)` is negative (-0.41 V) and for `Mn^(3+)| Mn^(2+)` is positive (+1.57 V). Thus, `Cr^(2+)` can undergo oxidation and, therefore, is reducing agent. On the other hand, Mn (III) can undergo reduction, and therefore, acts as an oxidizing agent.
(b) In the presence of complexing agents, cobalt gets oxidised from +2 to +3 state because Co (III) is more stable than Co (II).
(c) After loss of ns electrons, `d^1` electron can easily be lost to give a stable configuration. Therefore, the elements having `d^1` configuration are either reducing or undego disproportionation.