Investigation of the Ligand-Field States of the Hexaammine Cobalt(III) Ion with Quantum Chemical Methods

The ligand-field (LF) transition energies of the Co(NH3)(6)(3+) ion have been computed with multiconfiguration quasidegenerate second-order perturbation theory (MCQDPT2). The water solvent was treated with the polarizable continuum model (PCM), and the environment in crystals was modeled by the Co(NH3)(6).Cl-4(-) complex. The Co-N bond lengths, calculated for the hydrated cation and the Co(NH3)(6).Cl-4(-) model compound, agree with those in the crystal structures. The vertical transition energies agree with experiment, whereby those based on Co(NH3)(6).Cl-4(-) are more accurate than those for the hydrated ion. The 0-0 transitions were based on the OPBE geometries of ground and excited T-1(1g), T-3(1g), T-5(2g) states of the hydrated ion. The T-3(1g) state is the lowest excited state; the T-5(2g) state lies higher by >0.6 eV.