The relation between structure and absorption of three symmetrical 2:1 octahedral Co(III) azo complexes was studied. Quantum chemical calculations based on density functional theory (DFT) relate well with E-azo configuration of ligands with coordinated nitrogen atoms coming solely from phenolic residues. DFT calculations estimate the mer stereoisomer as remarkably preferred with respect to any fac arrangement. Time dependent DFT calculations using B3LYP and CAM-B3LYP functionals were successfully used to interpret the absorption spectra in a visible range. (C) 2014 Elsevier B.V. All rights reserved.