Isomerism which occurs due to different relative arrangements of ligands around central metal atom is known as geometrical isomerism. Complexes of formula MABCD may exist in three isomeric forms. e.g., [Pt(NH3)(Br)(Cl)(Py)]
square planar complexes do not show optical isomerison since they are not optically active as they have all the ligands and metal atoms in one plane, that's why there is a plane of symmetry.