$Cr^{2+}$ and $Mn^{3+}$ both possess $d^4$ electronic configuration. Therefore,

Fill in the blanks:
$(a)$ The only $Mn$ halide compound that shows an oxidation state of $+7$ is ........
$(b)$ The $E^{\ominus}_{M^{II}/M}$ value is maximum negative for ........ in the $3d$ series.
$(c)$ Between $Cr^{2+}$ and $Fe^{2+}$,the stronger reducing agent is ........

$\underset{\text{coloured}}{P(\text{soln.})}$ $\xrightarrow{\text{Air}} \underset{\text{coloured}}{Q(\text{soln.})}$ $\xrightarrow{KOH} R \downarrow$
$(R = \text{ppt. Insoluble in both excess } NaOH \text{ and excess } NH_3 \text{ solution})$
Then $P$ contains

Which of the following is the correct order of negative values of $E^o\,M^{2+} | M$ for the elements $Cr, Mn, Fe,$ and $Co$?

The correct order of oxidizing power of the given ions is

Which one of these is not known?