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

In both states (paramagnetic and diamagnetic) of the given complex, $\mathrm{Ni}\text{ exists as }{\mathrm{Ni}}^{2+}$whose electronic configuration is $\left[\mathrm{Ar}\right]3{\mathrm{d}}^{8}4{\mathrm{s}}^0$ 

In the above paramagnetic state the geometry of the complex is sp³ giving tetrahedral geometry. The diamagnetic state is achieved by pairing of electrons in 3d orbital. 

Thus the geometry of the complex will be dsp² giving square planar geometry.