Question

Background:
Creating the copper complex [Cu(H2O)6] 2+ .
Ammonia added to solution which causes water molcueles to be progressivly replaced by ammonia. This shifts downwards the complex's wavelength of absorption.

Question:
Why is ammonium sulphate required to be added during preparation of copper complex? It prevents the formation of hydroxides - why is this important?

Answer 1

The ammonia acts as both base and ligand . With small amount of Ammonia, hydroxide ions are pulled off from the complex to form neutral complex.

Cu(H₂O)₆²⁺ + 2NH₃   Cu(H₂O)₄(OH)₂ + 2NH₄⁺

^{The precipitate dissolves if added excess of ammonia.}

^{The ammonia replaces water as a ligand to give tetraammineaquacopper(II) ions. Only 4 out of 6 water molecule are replaced.}

Cu(H₂O)₆²⁺ + 4NH₃Cu(NH₃)₄(H₂O)₂²⁺ + 4H₂O

The color changes from pale blue to dark blue upon addition of excess of ammonia.