We report a theoretical study of the structures, energetics, and electronic spectra of the Pt-II/Ru-II mixed-metal complex RuPt2(tppz)(2)Cl-2(PF6)(4) (tppz = 2,3,5,6-tetra(2-pyridyl)-pyrazine) in acetonitrile. The hybrid B3LYP density functional theory and its TDDFT methods were used with a complete basis set (CBS) extrapolation scheme and a conductor polarizable continuum model (C-PCM) for solvation effects. Results showed that the trinuclear complex has four types of stable conformers and/or enantiomers. They are separated by high barriers owing to the repulsive H/H geometrical constraints in tppz. A strong entropy effect was found for the dissociation of RuPt2(tppz)(2)Cl-2(PF6). in acetonitrile. The UV-visible and emission spectra of the complex were also simulated. They are in good agreement with experiments. In this work we have largely focused on exploring the origin of anomalous electronic circular dichroism (ECD) spectra of the RuPt2(tppz)(2)Cl-2(PF6)(4) complex in acetonitrile. As a result, a new mechanism has been proposed together with a clear illustration by using a physical model.