c
\(\left[{NiCl}_{4}\right]^{-2-}>\left[{Ni}\left({H}_{2} {O}\right)_{6}\right]^{-2+}>\left[{Ni}({CN})_{4}\right]^{-2-}\)

Splitting energy order : \(\Delta_{{t}}<\Delta_{0} \quad<\Delta_{{sq}}\)

absorbed energy order : \(\left[{NiCl}_{4}\right]^{-2-}<\left[{Ni}\left({H}_{2} {O}\right)_{6}\right]^{-2+}<\left[{Ni}({CN})_{4}\right]^{-2-}\)

intensity of colour of compound : \(\quad\left[{NiCl}_{4}\right]^{-2-}>\left[{Ni}\left({H}_{2} {O}\right)_{6}\right]^{2+}>\left[{Ni}({CN})_{4}\right]^{2-}\)