The electronic structure of [PtL2](2)Cl-2 6H(2)O crystal is studied focusing on the bonding origin of the dimer formation, using theoretical calculations, where ligand L is a redox-active ligand and refers either o-phenylenediamido (L2-), o-semibenzoquinonediimine(L1-), or o-benzoquinonediimine(L-0). It is a non-bridging dimer with 3.01 angstrom of the inter-molecular Pt Pt distance. Considering the stoichiometry, the monomer should have +1 charge and 1/2 spin. For the dimer, there are two possibilities of the electronic structure; singlet or triplet state. On the other hand, the observed bond length is shorter than usual Pt-II Pt-II (d(8)-d(8)) interaction, while longer than usual Pt-III Pt-III (d(7)-d(7)) interaction. According to the structures and potential energy profiles on these possible spin states, we concluded that the dimer prefers the singlet state, formed by face-to-face bonding between the ligands of the two cationic monomers in the non-bridging Pt dimer. The analysis of molecular orbitals revealed that the Pt Pt interaction is explained by the d(8)-d(8) interaction and partial oxidation by the redox-active ligands. This study highlights the nature of the electronic structure of stacked radical monocations of Pt complex dimer.