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?


