In the scope of the Goodenough model, a brief review of the electronic properties of some 3d transition elements (M = Fe, Co, Ni and Cu ) is given; the main relevant electronic and structural parameters are emphasized. Then a special attention is focussed on N/(N−1) and N/(N−2) electron mixed valence compounds.
Finally some particular features of copper are discussed and their relevance to high Tc superconductors is pointed out:
-the srong covalency of Cu-O bondings involving a possible delocalization of the hole between trivalent copper and oxygen,
-the existence of low and high spin electronic configurations for CuIII,
-the disproportionation of Cu2+ and its role in the formation of hole pairs,
Cu-O distances very close for the three successive oxidation states of copper favouring the disproportionation process.