$\mathrm{N}\mathrm{i}\mathrm{C}{\mathrm{l}}_2\left[\mathrm{P}\right({\mathrm{C}}_2{\mathrm{H}}_5{)}_2\left({\mathrm{C}}_6{\mathrm{H}}_5\right){]}_2$ exhibits temperature dependent magnetic behaviour (paramagnetic, diamagnetic). The coordination geometries of $\mathrm{N}{\mathrm{i}}^{2+}$ in the paramagnetic and diamagnetic states are respectively

For a four coordinated complexes paramagnetic species are generally tetrahedral and diamagnetic species are square planar. As in case of tetrahedral complex weak ligands are used so pairing of inner d-orbitals electrons do not occur so unpaired electrons are present hence there is paramagnetic nature. In case of square planar strong ligands are used so pairing of inner d-orbitals electrons occur so unpaired electrons are not present hence there is diamagnetic nature.
Example: $[\mathrm{N}\mathrm{i}\mathrm{C}{\mathrm{l}}_4{]}^{-2},\mathrm{n}=2$ , paramagnetic, tetrahedral
$[\mathrm{N}\mathrm{i}\left(\mathrm{C}\mathrm{O}{)}_4\right],\mathrm{n}=0$ , diamagnetic, square planar
Same thing is possible in the given complex according to temperature.