In order to understand the steric influence on excimer formation in square planar metal complexes, three different Pt(II) complexes were prepared by modifying the substituents in the main ligand: Pt(dfppy) (acac) (Pt-1); the bulky triphenyl silyl group was substituted at the pyridine moiety (Pt-2) and at the phenyl moiety (Pt-3) of the main ligand of Pt-1. The Pt-1 and Pt-3 showed excimer emission in the concentrated solution and the solid sample. The crystal structures of Pt-1 and Pt-3 were analyzed by X-ray crystallographic measurements. The results revealed that the LUMO moiety closely overlapped with that of another Pt-complex. We concluded that the LUMO-LUMO interaction between the pyridine moieties of the main ligand is the main factor responsible for excimer formation.