How would you account for the following :
(i) Out 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 oxidized.
(iii) The `d^(1)` configuration is very unstable in ions.

Answer

(i) `E^(0)` value for `Cr^(3+)//Cr^(2+)` is negative `(-0.41V)` whereas `E^(0)` value for `Mn^(3+)//Mn^(2+)` is positve `(+1.57V)` Hence, `Cr^(2+)` ions can easily undergo oxidation to give `Cr^(3+)` ions and , therefore , act as strong reducing agent whereas `Mn^(3+)` can easily undergo reduction to give `Mn^(2+)` and hence acts as oxidizing agent
(ii) `3d^(6)` has higher `CFSE`
`(C ) 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 agent.