Nature of the interaction between Ln(III) and Cu-II ions in the ladder-type compounds {Ln(2)[Cu(opba)](3)}center dot S (Ln = lanthanide element; opba= ortho-phenylenebis(oxamato), S = solvent molecules)

To date, most of the studies dealing with the magnetic properties of 4f-3d compounds have been limited to the case in which the 4f ion was Gd-III, wit h a pure spin ground state. For the lanthanide(III) ions with a first-order orbital momentum, the determination of the nature of the 4f-3d interaction is still a challenge. This paper addresses this problem. The magnetic prop erties of the compounds of formula {Ln(2)[M(opba)](3)}. S (abbreviated here after as Ln(2)M(3)) have been investigated; Ln stands for a lanthanide elem ent, M for Cu or Zn, opba for ortho-phenylenebis-(oxamato), and S for solve nt molecules. All of these compounds have similar one-dimensional structure s consisting of ladder-type motifs. Our approach consisted of comparing the magnetic properties of Ln(2)Cu(3) and Ln(2)Zn(3) for each Ln(III) ion. The former are governed by both the thermal population of the Stark components of Ln(III) and the Ln(III)-Cu-II interaction; the latter are only governed by the thermal population of the Ln(III) Stark components. It has been con firmed that the Gd-III-Cu-II interaction was ferromagnetic, and it was foun d that the Tb-III-Cu-II and Dy-III-Cu-II interactions were ferromagnetic as well. The Tm-III-Cu-II interaction might also be ferromagnetic; the situat ion, however, is uncertain. On the other hand, in all other cases the Ln(II I)-Cu-II interaction is not ferromagnetic; it is either not detectable by t he magnetic technique or antiferromagnetic. The difference between Dy-III-C u-II (ferromagnetic) and Ho-III-Cu-II (not ferromagnetic) is particularly s triking. These findings have been discussed.