Display omitted •A new complex platinum compound is investigated.•Up to two DESO molecules may exist as neutral ligands in the platinum (IV) complex ion.•Strong cation-anion coordination explains an experimentally derived high melting point. Diethyl sulfoxide (DESO) is far less known than its shorter-alkyl-chain homolog, dimethyl sulfoxide. New fundamental and applied research works routinely appear from a few research groups in the world. Recently, the tetraethylammonium diethylsulfoxidopentachloroplatinate complex compound was synthesized containing the DESO molecule as a neutral ligand. In the present paper, we use a systematic computational method to rationalize the mentioned synthetic achievement in coordination chemistry. We show that only up to two DESO molecules may exist in the platinum (IV) complex ion, whereas all higher contents of DESO are thermodynamically unstable and the sterical factor plays an important role in their instabilities. Structural analysis of the tetraethylammonium diethylsulfoxidopentachloroplatinate ion pair reveals its rather strong cation–anion coordination and for the first time explains an experimentally derived high melting point. The reported results are expected to inspire experimental efforts to extend the universe of senior sulfoxides as neutral organic ligands in d-metal complexes.