MAGNETIC-PROPERTIES OF CU(II) BISCHELATE COMPLEXES WITH 3-IMIDAZOLINENITROXIDES - 2 - AN AB-INITIO ANALYSIS OF EXCHANGE INTERACTION MECHANISMS

The mechanism responsible for the emergence of ferromagnetic exchange interactions in bischelate complexes of Cu2+ with enaminoketone deriva tives of 3-imidazoline nitroxide CuL2 is studied by ab initio quantum chemical methods. The parameters J(Cu-L) and J(L-L') of exchange inter actions between the unpaired electrons of the paramagnetic centers (Cu 2+ ion and N-O groups of nitroxyl ligands L and L') of these complexes were calculated in terms of the full 3x3 configuration interaction be tween the singlet states constructed in a basis set of molecular orbit als of unpaired electrons. It is shown that for variations of the stru cture of the coordination polyhedron around the Cu2+ ion from square p lanar to tetrahedral the exchange interactions between the unpaired el ectrons of the paramagnetic centers is ferromagnetic J(Cu-L) >> J(L-L' ) > 0, which agrees with the data of magnetic measurements. The princi pal mechanism of exchange interactions in CuL2 complexes is the deloca lization mechanism that is due to a minor transfer of spin density fro m the 3d-orbitals of CU2+ to the sigma-orbitals of the N-O groups of L and L' ligands.