X-Ray crystal structures and electronic aspects of two mononuclear iron(III) complexes of the class FeIIILL′ with the quadridentate ligand L = salen or nta and the bidendate ligand L′= oxalate [salen = bis(salicylidene)ethylenediamine anion, H3nta =N,N-bis(carboxymethylglycine)]