A general approach is developed in order to calculate the energy levels and magnetic properties of high nuclearity mixed valence clusters. The approach considers all the relevant electronic processes, namely (i) interelectronic Coulomb repulsion between the moving electrons, which depend on the different distributions of the electronic pairs over the available metal sites; (ii) single and double electron transfer processes, which promotes the electron delocalization; (iii) magnetic exchange interactions. This approach is applied here to tetranuclear iron (II)-iron (III) clusters for symmetries comprised between tetrahedral and square planar.