Modelling from quantum chemical data of the chemical bonding in NiO and La₂NiO₄ crystals comparatively to CuO and La₂CuO₄, is proposed. The increase of the ionicity degree of the Ni(II)–O bonding in comparison with Cu(II)‐O bonding is observed. The covalency is low and equally distributed in the equatorial plane and apical direction in the NiO₆ octahedra in La₂NiO₄. As a supplementary data, which is common to La₂CuO₄ and La₂NiO₄, the covalency of the apical La(III)–O bonding plays an important role in increasing the overall covalent character of the apical oxygen and triggering a further elongation of MO₆ octahedra.