Hemoglobin is a complex of which ion?

Which statement is true for sodium nitroprusside?

Identify the complexes formed in the following processes:
$I$. Mond's process for purification of nickel
$II$. Removal of unreacted $AgBr$ from photographic plates
$III$. Removal of lead poisoning from the body

If a didentate ligand ethane-$1, 2$-diamine is progressively added in the molar ratio $en : Ni :: 1: 1, 2: 1, 3: 1$ to $[Ni(H_2O)_6]^{2+}$ aqueous solution,the following coordination entities are formed:
$I$. $[Ni(H_2O)_4en]^{2+}_{(aq)} -$ pale blue
$II$. $[Ni(H_2O)_2(en)_2]^{2+}_{(aq)} -$ blue/purple
$III$. $[Ni(en)_3]^{2+}_{(aq)} -$ violet
The wavelengths in $nm$ of light absorbed in the case of $I$ and $III$ are respectively:

Prussian blue is due to the formation of

The formation of Prussian blue color by ferric ions is due to the formation of which compound?