How would you Account for the following:
(i) Of the `d^4` species, `Cr^(2+)` 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 `d^1` configuration is very unstable in ions.

Explanation

(i) `E^0` vlaue for `Cr^(3+)//Cr^(2+)` is negative (-0.41 V) whereas `E^0` value for `Mn^(3+)//Mn^(2+)` is positive(+1.57V). Thus, `Cr^(2+)` ions can easily undergo oxidation to give `Cr^(3+)` ions and, therefore, acts as strong reducing agent. On the other hand, `Mn^(3+)` can easily undergo reduction to give `Mn^(2+)` and hence act as oxidising agent
(ii) Co (III) has greater tendency to form co-ordination complexes than Co (II). Thus, in the presence of ligands, Co (II) changes to Co (III). i.e., is easily oxidised
(iii) The ions with `d^1` configuration have the tendency to lose the only electron present in d-subshell to acquire stable `d^0` configuration. Therefore, they are unstable and undergo oxidation or disproportion.