The present invention relates to a new family of mono, bis or tris (2,2'-substituted bipyridine) complexes of a metal selected amongst iron, ruthenium, osmium or vanadium, the bipyridine being substituted by at least one electron donor group. According to the invention, said electron donor group is selected amongst a hydroxy group, an alkoxy group, an aryloxy group or a primary, secondary or tertiary amine group. The invention also relates to the method for the preparation of such complexes.