Geometrical shapes of the complexes formed by the reaction of \(\mathrm{Ni}^{2+}\) with \(\mathrm{Cl}^{-}\), \(\mathrm{CN}^{-}\)and \(\mathrm{H}_2 \mathrm{O}\), respectively, are

\(\mathrm{Ni}^{2+}+4 \mathrm{CN}^{-} \longrightarrow\left[\text{Ni} (\text{CN})_4\right]^{2-}\). Here \(\text{Ni} ^{2+}\) has \(\text d^8\) configuration with \(\text{CN} ^{-}\)as strong ligand.


\(d^8\)-configuration in strong ligand field gives \(d s p^2\)-hybridisation, hence square planar geometry.
\(\mathrm{Ni}^{2+}+4 \mathrm{Cl}^{-} \longrightarrow\left[\mathrm{NiCl}_4\right]^{2-}\)
Here \(\mathrm{Ni}^{2+}\) had \(d^8\)-configuration with \(\mathrm{Cl}^{-}\)as weak ligand.


\(d^8\)-configuration in weak ligand field gives \(s p^3\) hydridisation, hence tetrahedral geometry. \(\mathrm{Ni}^{2+}\) with \(\mathrm{H}_2 \mathrm{O}\) forms \(\left[\mathrm{Ni}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}\) complex and \(\mathrm{H}_2 \mathrm{O}\) is a weak ligand.

\(\text { Therefore, } \mathrm{Ni}\left(\mathrm{H}_2 \mathrm{O}\right)^{2+} \text { has octahedral geometry. }\)