We have studied theoretically the complexes of model N-phenylguanidine/ium derivatives with PtCl3- and PtCl2 in different coordinating modes (mono- and bidentate) with different N atoms of the guanidine/ium moiety using the B3LYP/6-31+G** and LANL2DZ mixed basis set. This will aid the understanding of the complexation between platinum and the guanidine or guanidinium moiety in order to design dual anticancer agents that combine a guanidine-based DNA minor groove binder and a cisplatin-like moiety. Calculated interaction and relative energies, analysis of the electron density, and examination of the orbital interactions indicate that the most stable type of complex is that with a monodentate interaction between PtCl3- and guanidinium established through one of the NH2 groups. Next, we optimized the structure of three bis-guanidinium diaromatic systems developed in our group as DNA minor groove binders and their complexation with PtCl3-, finding that the formation of Pt complexes of these minor groove binders is favorable and would produce stable monodentate coordinated systems.