Crystal field and exchange effects in rare earth semiquinone complexes

We describe here the synthesis, the magnetic properties, and a comparison o f the X-band EPR spectra of complexes belonging to the homologous families Ln(III)(HBPz(3))(2)(3,5-DTBSQ) and Ln(III)(HBPz(3))(2) (tropolonate) (where HBPz(3) = hydrotrispyrazolborare, 3,5-DTUSQ = 3,4-di-tert-butylsemiquinone and Ln = Yb and Ho). This study shows new examples of violations to the ru le that ferromagnetic coupling should hold for ions of the second half of t he lanthanide series and an unpaired spin. (C) 2001 Academic des sciences / Editions scientifiques el medicales Elsevier SAS.