Iron is rendered passive by the action of

The highest oxidation state of manganese in fluoride is $+4$ $(MnF_4)$,but the highest oxidation state in oxides is $+7$ $(Mn_2O_7)$ because __

$I$. $Na_2Cr_2O_7 + X \xrightarrow{\Delta} Cr_2O_3$
$II$. $Cr_2O_3 + Y \xrightarrow{\Delta} Cr$
$X$ and $Y$ are :

The compound $AgF_2$ is an unstable compound. However,if formed,the compound acts as a very strong oxidizing agent. Why?

The copper $(II)$ halide which does not exist is:

Highest oxidation state of manganese in fluoride is $+4$ $(MnF_4)$ but highest oxidation state in oxides is $+7$ $(Mn_2O_7)$ because $..........$