A covalent heterodinuclear magnetic exchange interaction formula has been established and the antiferromagnetic coupling phenomena of both heterodinuclear (MnFeIII)-Fe-III and homodinuclear (FeFeII)-Fe-II, spin-clusters in complex molecules have been studied. It is demonstrated that the antiferromagnetic phenomenon of transition-metal ion-pairs in molecules should be attributed to the combined effect of the direct-exchange interaction, the kinetic exchange interaction and the covalent effect; the strong antiferromagnetic characteristic of (MnFeIII)-Fe-III cluster in [L'Mn-III (mu-O)(mu-CH3CO2)(2)Fe-III L'] is mainly due to the d(xz)-d(xz) and d(yz)-d(yz) orbital exchange interactions, while the weak antiferromagnetic coupling in deoxyhemerythrin should be attributed to a mu-(OH-FeFeII)-Fe-II cluster with a long Fe-O bond length in its active site. The theoretical results agree well with the experimental findings. (C) 2002 Published by Elsevier Science B.V.