Aqueous CuSO₄ exists as [Cu(H₂O)₄]SO₄. It is blue in colour due to the presence of [Cu[H₂O)₄]²⁺ ions.
1) When KF is added:
\(\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}+4 F \rightarrow\left[\mathrm{Cu}(F)_{4}\right]^{2+}+4 \mathrm{H}_{2} \mathrm{O}\) 
2) When KCl is added:
\(\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}+4 \mathrm{Cl} \rightarrow\left[\mathrm{CuCl}_{4}\right]^{2-}+4 \mathrm{H}_{2} \mathrm{O}\) 
In both these cases, the weak field ligand water is replaced by the F⁻ and Cl⁻ ions.

(ii) Oxidation state of Ni in the complex = + 2
Hybridisation = dsp² (because it is a planar molecule)
(iii) It is diamagnetic (because there is no unpaired electron present)

This is because Cu²⁺ oxidizes I^- to I₂(2Cu²⁺ + 4I^{- \(\rightarrow \)} Cu₂I₂(s)+I₂) or I^- ions reduce Cu²⁺ to Cu⁺.

There are three t_2g orbitals with lower energy and two e_g orbitals with higher energy. Three electrons with parallel spins can first enter into the lower energy three t_2g orbitals, i.e., for complexes with d¹. d² and d³ ion. For d ⁴, d ⁵, d⁶ etc. ions, the electrons can either enter into t_2g orbitals and pair up or they may enter into e_g orbitals. The former gives rise to low spin complex while the latter gives rise to high spin complex. If crystal field splitting energy (difference of energy between t_2g and eg orbitals) is large, low spin state is more stable and if it is small, high spin state is more stable.