Ligands confusion Watch
The O has a lone pair, with which it can form coordinate bonds.
If OH- deprotonates a water, it leaves behind an OH- attached to the central ion. It might not be actually substitution, but the end result is much the same.
Co(H2O)6]2 + 2(NH3) = Co(H2O)4(OH)2 + 2(NH4+)
or when it acts like a Lewis base, when it donates an electron pair-This is when NH3 acts like a ligand, as it donates its lone pair to form a coordinate bond with the metal ion.
Co(H2O)6]2 + 6(NH3) = Co(NH3)6 + 6(H2O)
you just have to learn that NH3 only acts as a lewis base and does a ligand exchange with the 3 metal ions (Cu, Co, Cr) and that Cu doesnt loose all of its water ligands.