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.

A theoretical method for studying the inter-relation between electronic and molecular structure has been proposed by diagonalizing the complete energy matrices for a d5 configuration ion in a trigonal ligand fieldand considering the second-order and fourth-order EPR parameters D and (a - F) simultaneously. As forZnSiF6

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

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.

A theoretical method for describing magnetic-exchange-interactions in covalent molecular systems is established byintroducing covalent effect into Girerd-Journaux-Kahn’s (GJK) magnetic formula and adopting our Double-Slater-Function (DSF) calculation method. The calculations show that the exchange coupling parameter J depends sensitivelyon the covalent factor N; the stronger the covalent effect the stronger the antiferromagnetic exchange interaction; thedirect-and kinetic-exchange interactions are both important. The calculation for MnIII-O-MnIII spin-clusters inMn2Oð5-NO2saldienÞ2 reveals that about 30% of the antiferromagnetic contribution comes from covalent effect, andthat the theoretical values J ¼111:02 to138:85 cm 1 agree well with the experimental findings ðJðexpt:Þ ¼ 120 cm 1Þ.