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^(ɵ)` value for `((Cr^(3+))/(Cr^(2+)))` is negative `(-0.41V)` whereas `E^(ɵ)` value for `((Mn^(3+))/(Mn^(2+)))` is positive `(+1.57V)`. Hence `Cr^(2+)` ions can easily undergo oxidation to give `Cr^(3+)` ions and therefore, act as strong reducing agent whereas `Mn^(2+)` can easily undergo reduction to give `Mn^(2+)` and hence act as oxidising agent.
(ii). `Co^(3+)` has greater tendecny to form coordination complexes than `Co^(2+)`. Hence in the presence of ligands `Co^(2+)` changes to `Co^(3+)` 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. Hence, they are unstable and undergo oxidation or disproportionation.