Importance of spin–orbit coupling effect and solvent effect in electronic transition assignments of PtII complexes: In the case of cis/trans-[PtIICl2(NH3)2]

Electronic excitation spectra of cis-platin (cis-[PtIICl2(NH3)2]), which is a well-known antitumor drug, and trans-platin (trans-[PtIICl2(NH3)2]) were experimentally observed and theoretically analyzed using ab initio spin–orbit CASPT2 (SO-CASPT2) calculations. Cis-platin showed two series of absorption bands in the wavelength regions of 250–350 nm and 350–450 nm. On the other hand, trans-platin showed three series of absorption bands in the wavelength regions of 260–280 nm, 300–340 nm and 360–400 nm. Comparing SO-CASPT2 calculations at the optimum structures in gas-phase conditions with those in DMF solvent, which were obtained by PCM implicit solvation model, it was clarified that both spin–orbit effect and solvent effect cannot be negligible even for quantitative assignments of electronic excitation spectra of cis/trans-platin.