POLYMORPHOUS MODIFICATIONS OF A NI2-O-N BONDS - QUANTUM-CHEMICAL INVESTIGATION OF EXCHANGE INTERACTIONS IN HETEROSPIN SYSTEMS( COMPLEX WITHSTABLE NITROXIDE INVOLVING NI2+)

It has been established that the Ni2+ bischelate with the deprotonante d enamine ketone derivative of stable 3-imidazoline luoro-1'-propenyl) -2-oxy-O',N3)]-nickel(II)-exists in the form of two polymorphous modif ications (alpha-NiL2 and beta-NiL2). We succeeded in obtaining each of these modifications in its pure state. X-ray data show both alpha-NiL 2 and beta-NiL2 to be layered polymeric structures. The polymeric stru cture is accounted for by the fact that each nickel ion coordinates N. -O group oxygens of two adjacent molecules along with the enamineketon e oxygens and nitrogens of its ''own'' ligands. However, the coordinat ed N.-O groups in alpha-NiL2 are in trans-position (Ni-O.-2.206(7)angs trom) while those in beta-NiL2 are in cis-position (Ni-O.-2.302(7) and 2.486(7) angstrom). The -.O-Ni-O.- angles are 180.0(2) and 85.1(2)-de grees in alpha-NiL2 and beta-NiL2, respectively. The structural differ ence of the polymorphous modifications determines the fundamental diff erence of their magnetic properties: in a-NiL2, exchange interactions are antiferromagnetic (J = -69.4 cm-1), whereas beta-NiL2 contains bot h antiferromagnetic (J congruent-to -115 cm-1) and ferromagnetic (J co ngruent-to 9.8 cm-1) exchange clusters. A detailed quantum-chemical in vestigation of exchange interaction mechanisms in the (N.-O-Ni2+-O.-N) heterospin exchange clusters and a critical analysis of available met hods for calculating the exchange parameters J for heterospin systems have been performed in order to account for the fundamental difference in magnetic properties of alpha-NiL2 and beta-NiL2 as well as the une xpected fact of ferromagnetic exchange in beta-NiL2 in the presence of direct contact of nonorthogonal magnetic orbitals. Both the conceptio n of direct exchange interaction and the traditional molecular-orbital approach were found to be inadequate to describe the magnetic propert ies of alpha-NiL2 and beta-NiL2 observed experimentally. The reason is that the molecular-orbital approach considers the configuration inter actions (CI) of only two (ground and doubly excited) singlet configura tions (2 x 2 CI) in the basis of frontier molecular orbitals (MO) and is meant primarily for the analysis of exchange interactions in metal complexes containing symmetrical exchange systems like {M}...{M}, wher e {M} is a fragment involving the paramagnetic metal ion. Of utmost im portance for the correct estimation of exchange interaction values for heterospin complexes of transition metals with nonsymmetrical exchang e systems like {M}...{L}, where {M} is the fragment containing the par amagnetic metal ion and {L} is the fragment containing the O.-N group, is the incorporation of the third (singly excited) singlet configurat ion (3 X 3 CI) into the configuration interaction as well as to accoun t for the contributions from the small delocalization of spin density in the directions {M} <-- {L} and {M} --> {L}. Crystal data: for alpha -NiL2, orthorhombic Pbca, a = 20.090(10) angstrom, b = 10.685(4) angst rom, c = 11.051(5) angstrom, Z = 4, R = 0.0692, R(W) = 0.0705; for bet a-NiL2, orthorhombic Pbca, a = 19.921(1) angstrom, b = 11.1 12(2) angs trom, c = 21.549(3) angstrom, Z = 8, R = 0.0476, R(W) = 0.0476.