Which transition metal reduces steam to evolve hydrogen?

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.

Vanadium $(III)$ oxide $(V_2O_3)$ is a strong:

Give examples and suggest reasons for the following features of the transition metal chemistry:
$(i)$ The lowest oxide of transition metal is basic,the highest is amphoteric/acidic.
$(ii)$ $A$ transition metal exhibits highest oxidation state in oxides and fluorides.
$(iii)$ The highest oxidation state is exhibited in oxoanions of a metal.

Transition elements with lower oxidation states act as........