We studied the influence of the trigonal distortion of the regular octahedron along the (111) direction, found in the CoO2​ layers. Under such a distortion the t2g​ orbitals split into one a1g​ and two degenerated eg′​ orbitals. We focused on the relative order of these orbitals. Using quantum chemical calculations of embedded clusters at different levels of theory, we analyzed the influence of the different effects not taken into account in the crystalline field theory~; that is metal-ligand hybridization, long-range crystalline field, screening effects and orbital relaxation. We found that none of them are responsible for the relative order of the t2g​ orbitals. In fact, the trigonal distortion allows a mixing of the t2g​ and eg​ orbitals of the metallic atom. This hybridization is at the origin of the a1g​--eg′​ relative order and of the incorrect prediction of the crystalline field theory