Revisiting the Spectrum of Co(CN) 6 3- : The Role of Correlation, Solvation, and Vibronic and Spin-Orbit Couplings.

In the present work, we revisit the spectrum of the hexacyanocobaltate(III) ion, [Co(CN) 6 ] 3- , which has been considered a prototype complex in the coordination chemistry, with modern quantum chemistry methods. The main features have been describing by revealing the role of different effects, such as vibronic coupling, solvation and spin-orbit coupling. The UV-vis spectrum is composed by two bands ( 1 A 1 g → 1 T 1 g and 1 A 1 g → 1 T 2 g ), characterized by singlet-singlet metal-centered transitions, and a more intense third one, characterized by charge transfer transition. There is also a small band shoulder. The first two are symmetry-forbidden transitions in the O h group. Their intensity can only be explained by a vibronic coupling mechanism. For the band shoulder, additional to vibronic coupling, spin-orbit coupling is also necessary, since the transition is characterized as singlet to triplet, 1 A 1 g → 3 T 1 g .