Question

How would you account for the following:
(i) Of the $d^4$ species, $C{r}^{2+}$ 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

Answer 1

(i) $C{r}^{3+}/C{r}^{2+}$ has negative value of standard electrode potential ($-0.41V$) and $M{n}^{3+}/M{n}^{2+}$ have positive value (+1.57 V). $C{r}^{2+}$ can undergo oxidation and acts as reducing agent. $M{n}^{3+}$ can undergo reduction and is oxidizing agent.
(ii) Co (III) being more stable than Co(II), the change in the oxidation state of cobalt from +2 to +3 is easy in presence of complexing agent.
(iii) Once ns electrons are lost, removal of $d^1$ electron will result in a stable electronic configuration. Such elements either acts as reducing agents or undergo disproportionation.