Quantum-chemical investigation of exchange interaction mechanisms in transition-metal complexes with stable organic radicals

Coordination compounds of transition metal ions with paramagnetic organic ligands are promising in designing of novel class of ferro- and ferrimagnetic materials. Very interesting in this respect are the recently synthesized complexes of bis-chelates Cu(II) and Ni(II) with stable nitroxides. The present work is devoted to the detailed quantum-chemical (ab initio) investigation of possible exchange interaction mechanisms in the molecular polyspin systems (M)...(L) and (L)...(M)...(L) of such complexes and critical analysis of available methods for calculating the spin-exchange parameters J (H= -JSaSb). Here (M) is the chelating moiety of the complexes containing metal ion M = Cu(II) or Ni(II), and (L) is the fragment of nitroxide involving the O-N group. The main results of our study are the following: The observed magnetic properties of such complexes cannot be explained in terms of the direct exchange interaction mechanism determined by the overlap only between magnetic orbitals of paramagnetic fragmets (M) and (L). The traditional molecular-orbital approach also inadequately describes the magnetic properties of these complexes. The reason is that this approach considers the configuration interactions (CI) of only two (ground and doubly excited) singlet configurations (2x2 CI) in the basis of frontier MO's. Of utmost importance for correct estimation of the values and the sign of the exchange parameters J for complexes containing asymmetric exchange systems like (M)...(L), with essentially localized electrons, is the incorporation of the third (singly excited) singlet configuration (3x3 CI) into CI. The dominating role in the realization of ferro-magnetic exchange interaction in all complexes of this type belongs to the delocalization mechanism of interaction caused by slight spin density redistribution between paramagnetic fragments of complexes in the directions (M)=> (L) and (L) = > (M)