inorganic chemistry - Why does copper(II) sulfate react with ammonia?

We've done preparation of Tetraamminecopper(II) sulphate $\ce{[Cu(NH_3)_4(H_2O)_n]SO_4}$ using copper sulphate and 50% ammonia solution. Initially, a pale blue color of $\ce{Cu(OH)_2}$ appeared with turbidity then after a dark blue color solution is obtained. It is a substitution reaction Initially, why formation of $\ce{Cu(OH)_2}$ is taking place ??

Is initially NH3 acting as a base and ripping off the proton from the complex $\ce{[Cu(H_2O)_6]^2+}$? Then on the further increment of the concentration of $\ce{NH_3}$, it starts to act as Nucleophile?? My high school teacher taught me that acid-base reactions are always fast, so in this example is everything is about kinetics? What is the role of the thermodynamics? From where can I get a quantitative explanation with proper data?