What is the cause of the color change observed when #HCl# is added to the complex ion #[Cu(NH_3)_4]^(2+)#?
I have explained a similar ligand displacement reaction to this in the answer below:
In this case the complex we have is tetraammine copper(II):
If excess chloride ions are added the ammonia ligands are displaced and the green, tetrachloro copper(II) complex forms:
The solution looks like this: