Which of the following metal hydroxide is not soluble in excess of $NH_3$ solution?

Write the chemical reactions of lanthanoids $(Ln)$ with the following:
$(1)$ With acid:
$2Ln + 6HCl \rightarrow 2LnCl_3 + 3H_2$
$(2)$ With halogens ($X_2$,where $X = F, Cl, Br, I$):
$2Ln + 3X_2 \rightarrow 2LnX_3$

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 __

Addition of high proportions of manganese makes steel useful in making rails of railroads,because manganese

The ${E^0}_{M^{3+}/M^{2+}}$ values for $Cr, Mn, Fe$ and $Co$ are $-0.41, +1.57, +0.77$ and $+1.97 \ V$ respectively. For which one of these metals is the change in oxidation state from $+2$ to $+3$ the easiest?

$Fe$,$Co$ and $Ni$ have valuable catalytic properties in processes involving