Question

Why is $C{r}^{2+}$ reducing and $M{n}^{3+}$ oxidising when both have $d^4$ configuration?

Answer 1

$\text{Oxidising / Reducing nature of}C{r}^{2+}\text{and}M{n}^{3+}\text{ions}$

$C{r}^{2+}$ is reducing as its configuration changes from $d^4$ to $d^3$ , the latter having a half-filled $t_g^2$ level.


On the other hand, the change from $M{n}^{3+}$ to $M{n}^{2+}$ results in the half-filled $\left(d^5\right)$ configuration which has extra stability. That is why $C{r}^{2+}$ is reducing and $M{n}^{3+}$ is oxidising when both have $d^4$ configuration.