The reaction between hexaaquacopper(II) and ammonium reacts as follows
$$\ce{[Cu(H2O)6]^2+ + 4NH3 -> [Cu(NH3)4(H2O)2]^2+ + 4H2O}$$
Apparently it's known from physical measurements that the hexaamine complex $\ce{[Cu(NH3)6]^2+}$ is not formed? Why is this?
Edit: This isn't a homework question by the way, I'm genuinely curious seeing as the hexaamine complex is formed when ammonium is reacted with complex cobalt ions.