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