Octahedral complexes of copper (II) undergo structural distortion...

According to John teller any nonlinear molecular system in a degenerate electronic state will be unstable and will undergo some kind of distortion which will lower its symmetry and energy and split the degenerate state. In case of octahedral d9 configuration, the last electron may occupy either dz² or ${dx}^{2} - {y}^{2}$orbitals of eg set. If it occupies dz² orbital most of the electron density will be concentrated between the metal and the two ligands on the z axis. Thus there will be greater electrostatic repulsion associated with these ligands than with the other four on xy plane. The Jahn Teller effect is mostly observed in octahedral environments. The considerable distortions are usually observed in high spin d⁴, low spin d⁷ and d⁹ configuration

There is unsymmetrical filling of ${e}_{g}$ subset of ${Cu}^{+ 2}$ ion, while there is symmetrical distribution in ${t}_{2g}$ set, if the complex has same ligand there will be equal repulsion which leads to symmetrical bond length along ${t}_{2g}$, but due to uneven filling of electron in ${e}_{g}$ subset, either octahedral will be elongated or compressed.

Hence,\left[Cu{\left({H}_{2}O\right)}_{6}\right]{SO}_{4}maximum structural distortion