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^(@)` value for `Cr^(3+)//Cr^(2+)` is negative `(-0.41" V")`, whereas `E^(@)` value for `Mn^(3+)//Mn^(2+)` is positive `(+1.57" V")`. Hence, `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 acts as oxidising agent.
(ii) Co (III) has greater tendency to form coordination complexes than Co (II). Hence, 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 too acquire stable `d^(0)` configuration. Hence, they are unstable and undergo oxidation or disproportionation.