Stabilization of Highest‐Oxidation States of Transition Metals and the Induced Electronic Phenomena in Oxidic Perovskite Structures

In oxides the stabilization of the highest oxidation states n+ of transition metals leads to the stabilization of strong covalent M n+ ‐O bonds. Such very covalent bonds can induce electronic phenomena as spin transitions through the increase of the local crystal‐field energy (Dq) or in the specific case of an anisotropic eg 1 electronic configuration some strong interactions with the perovskite structure (3D character, structural distortion…) can lead to a insulator→metal transition or a disproportionation phenomenon. [2M n+ ⇔(M (n—ϵ)+ )+(M (n+ϵ)+ )] In order to illustrate such a topic, three unusual oxidation states have been selected: Co 3+ (d 6 ), Fe 4+ (d 4 ) and Ni 3+ (d 7 ) in perovskite‐type structures.