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.

  • $\begingroup$ "Simply speaking, the missing $\ce{[Cu(NH3)6]2+}$ in aqueous solution can be attributed to the high concentration of water which is competing with ammonia for the coordination sites." $\endgroup$
    – thylmanoid
    Oct 5, 2017 at 13:50
  • $\begingroup$ The six ligand positions are not equivalent. I think the best answer is this. chemistry.stackexchange.com/a/29172/22102 $\endgroup$
    – MaxW
    Oct 5, 2017 at 15:59