  The properties of AMO3 perovskite oxides, where M is a 3d transition metal,
depend strongly on the level of covalency between the metal d and oxygen p
orbitals. With their complex spin orders and metal-insulator transition,
rare-earth nickelates verge between dominantly ionic and covalent characters.
Accordingly, the nature of their ground state is highly debated. Here, we
reconcile the ionic and covalent visions of the insulating state of nickelates.
Through first-principles calculations, we show that it is reminiscent of the
ionic charge disproportionation picture (with strictly low-spin 4+ and
high-spin 2+ Ni sites) while exhibiting strong covalence effects with oxygen
electrons shifted toward the depleted Ni cations, mimicking a configuration
with identical Ni sites. Our results further hint at strategies to control
electronic and magnetic phases of transition metal oxide perovskites.
