What is the fundamental difference between a Double Salt (like Mohr's Salt) and a Coordination Complex?

According to Werner's Theory, which valency is ionisable and corresponds to the oxidation state?

In the complex $CoCl_3 \cdot 4NH_3$ , precipitation with $AgNO_3$ yields only 1 mole of $AgCl$ . What is the secondary valency of Co?

Which valency in Werner's theory gives the complex its definitive geometry?

Define a Chelating Ligand.