It has been established that the Ni2+ bischelate with the deprotonanted enamine ketone derivative of stable 3-imidazoline nitroxide-bis[(1-(2,2,5,5-tetramethyl-1-oxyl-3-imidazolin-4-yl)-3’,3’,3’-trifluoro-1’-propenyl)-2-oxy-O’,N3)]-nickel(II)-exists in the form of two polymorphous modifications (alpha-NiL2 and beta-NiL2). We succeeded in obtaining each of these modifications in its pure state. X-ray data show both alpha-NiL2 and beta-NiL2 to be layered polymeric structures. The polymeric structure is accounted for by the fact that each nickel ion coordinates N.-O group oxygens of two adjacent molecules along with the enamineketone oxygens and nitrogens of its "own" ligands. However, the coordinated N.-O groups in alpha-NiL2 are in trans-position (Ni-O.-2.206(7)angstrom) 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)-degrees in alpha-NiL2 and beta-NiL2, respectively. The structural difference of the polymorphous modifications determines the fundamental difference of their magnetic properties : in a-NiL2, exchange interactions are antiferromagnetic (J = -69.4 cm-1), whereas beta-NiL2 contains both antiferromagnetic (J congruent-to -115 cm-1) and ferromagnetic (J congruent-to 9.8 cm-1) exchange clusters. A detailed quantum-chemical investigation of exchange interaction mechanisms in the (N.-O-Ni2+-O.-N) heterospin exchange clusters and a critical analysis of available methods 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 unexpected fact of ferromagnetic exchange in beta-NiL2 in the presence of direct contact of nonorthogonal magnetic orbitals. Both the conception of direct exchange interaction and the traditional molecular-orbital approach were found to be inadequate to describe the magnetic properties of alpha-NiL2 and beta-NiL2 observed experimentally. The reason is that the molecular-orbital approach considers the configuration interactions (CI) of only two (ground and doubly excited) singlet configurations (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}, where {M} is a fragment involving the paramagnetic metal ion.Of utmost importance for the correct estimation of exchange interaction values for heterospin complexes of transition metals with nonsymmetrical exchange systems like {M}...{L}, where {M} is the fragment containing the paramagnetic metal ion and {L} is the fragment containing the O.-N group, is the incorporation of the third (singly excited) singlet configuration (3 X 3 CI) into the configuration interaction as well as to account 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) angstrom, c = 11.051(5) angstrom, Z = 4, R = 0.0692, R(W) = 0.0705; for beta-NiL2, orthorhombic Pbca, a = 19.921(1) angstrom, b = 11.1 12(2) angstrom, c = 21.549(3) angstrom, Z = 8, R = 0.0476, R(W) = 0.0476.