Which of the following pairs of compounds do not exist?

How would you account for the following:
$(i)$ Of the $d^{4}$ species,$Cr^{2+}$ is strongly reducing while $Mn^{3+}$ is strongly oxidising.
$(ii)$ $Co^{2+}$ 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.

Explain the trends in $E_{M^{3+}/M^{2+}}^{o}$ electrode potential.

The incorrect statement$(s)$ among $(a)-(c)$ is (are) :-
$(a)$ $W(VI)$ is more stable than $Cr(VI)$
$(b)$ In the presence of $HCl$,permanganate titrations provide satisfactory results.
$(c)$ Some lanthanoid oxides can be used as phosphors.

What is the general molecular formula of the products obtained on heating lanthanoids $(Ln)$ with sulphur?

Which transition metal reacts with steam to produce hydrogen gas?