Explain geometrical isomerism in four-coordinated compounds.

(N/A) Four-coordinated complex compounds are either tetrahedral or square planar in shape.
Tetrahedral complexes do not show geometrical isomerism because all four ligands are adjacent to each other at an angle of $109.5^{\circ}$.
Hence,only square planar complexes can show geometrical isomerism.
In square planar complexes,if similar ligands are at adjacent positions $(90^{\circ})$,it is called a $Cis$-isomer,and if the similar ligands are at opposite positions $(180^{\circ})$,it is called a $Trans$-isomer.
$(i)$ Complexes of type $[M X_{2} L_{2}]$ where $X$ and $L$ are monodentate ligands.
Example: $[Pt(NH_{3})_{2} Cl_{2}]$ where $X=NH_{3}$,$L=Cl$.
$(ii)$ Complexes of type $[M A B X L]$ where $A, B, X$ and $L$ are unidentate ligands also show geometrical isomerism.
Example: $[Pt(NH_{3})(Py)(Cl)(Br)]^{2+}$
$A=NH_{3}$,$B=Py$,$X=Cl$,$L=Br$.