Importance of t2g-eg hybridization in transition metal oxides

We studied the influence of the trigonal distortion of the regular octahedron along the (111) direction, as found in the CoO2 layers. Under this distortion the t(2g) orbitals split into one a(1g) and two degenerate e'(g) 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 crystalline field theory; that is, metal-ligand hybridization, the long-range crystalline field, screening effects, and orbital relaxation. We found that none of them is responsible for the relative order of the t2g orbitals. In fact, the trigonal distortion allows a mixing of the t(2g) and e(g) orbitals of the metallic atom. This hybridization is at the origin of the a(1g)-e(g)(') relative order and of the incorrect prediction of crystalline field theory.