Question

why cu++ is more stable in aqueous solution them cu+???

Answer 1

And the reason why Cu(II) is more stable is because its complexes exhibit a high stability. Especially Cu(II) is famous for the Jahn-Teller-effect meaning an additional splitting of those degenerated d-orbitals making them even more stable!

The result is very often a quadratic planar coordination as found in this complex [Cu(NH3)4]2+