A covalent heterodinuclear magnetic exchange interaction formula has been established and the antiferromagneticcoupling phenomena of both heterodinuclear MnIIIFeIII and homodinuclear FeIIFeII spin-clusters in complex moleculeshave been studied. It is demonstrated that the antiferromagnetic phenomenon of transition-metal ion-pairs in moleculesshould be attributed to the combined effect of the direct-exchange interaction, the kinetic exchange interaction and thecovalent effect; the strong antiferromagnetic characteristic of MnIIIFeIII cluster in [L0MnIII

A theoretical model of the ground state and the magnetic moment of spin crossover of Fe(III) in a cu- bic ligand field has been established by diagonalizing the complete matrices for electron-electron-repulsion, ligand-field, spin-orbit-coupling, and Zeeman interactions. It is shown that the temperature dependence or unusual magnetic moment of Fe(III) in molecules is due to a large zero-field splitting and an asymmetrically high-order Zeeman effect of the electronic states between the high-spin 6A1 and the low-spin 2T2 states.

The EPR spectra of octahedral Fe3+ center in lutetium aluminum garnet have been studied by means of the complete energy matricesfor a d5 configuration ion in a trigonal ligand–field. It is manifested that the local lattice structure around an octahedrally coordinatedFe3+ center has an expansion distortion from that of undistorted host Al3+ ion. The expansion distortion can be ascribed to the fact thatthe radius of Fe3+ ion is bigger than that of Al3+ion, and consequently the Fe3+ ion pushes the oxygen ligands outwards. Simultaneously,for the octahedral Fe3+ center in the crystal, the local lattice structure parameters, R=(2.007±0.022)A andh=(52.708 ± 0.032), which reflect the interactions between impurity and crystal lattice, are determined from our calculation, andEPR parameters a, D, and (a-F) also get a unified explanation.

It is shown that Low's conjecture, that the Stark level 4TI(G) in MgO:Mn2+ would be higher than 14000 cm-l from the ground state, is in accord with EPR experiments. Furthermore, possi-ble reasons for the difference between our results and previous ones are given.

The complete energy matrix including all the spin states is constructed for d4ions in tetragonal symmetry within a strong-field representation. The excitationlevels, fine-structure splitting, and EPR parameters in Rb2CrCl4 and CrF2 arestudied with the energy matrix. The contributions of the spin singlets to zerofieldsplitting (ZFS) are investigated for the first time. The results show thatthe spin-singlet contribution to D is negligible, but the contributions to a and Fcannot be neglected.