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The antiferromagnetic phenomena of both mixed valent (FeFeII)-Fe-III spin clusters and homodinuclear (FeFeIII)-Fe-III clusters in complexes have been studied by means of our covalent magnetic exchange theoretical method. The sensitive relations between the exchange parameter J and the covalent factors N-A (for Fe-III) and N-B (for Fe-II) and the Fe...Fe separation R have been derived by use of our double-Slater function calculation procedure, the corresponding theoretical curve of J vs. NA, NB and R has been obtained. By taking the typical covalent factors of Fe-III and Fe-II, and using the observed Fe-O bond lengths: R-1 = 1.99 Angstrom, R-2 = 2.069 Angstrom for Fe-III-O-Fe-II cluster in model complex {[Fe(acacen)](2)ONa](2); R-1 = 1.79 Angstrom, R-2 = 1.88 Angstrom for Fe-III-O-Fe-III cluster in oxyhemerythrin, we obtained the theoretical values: -J(theore.) = 25.7 - 28.6 cm(-1) for Fe-III-O-Fe-II cluster and _J(theore.) = 75.4 - 81.8 cm(-1) for Fe-III-O-Fe-III cluster. These are in good agreement with the corresponding experimental findings -J(expt)=30cm(-1) in {[Fe(acacen)](2)ONa}(2) and -J(expt)=77cm(-1) in oxyhemerythrin, respectively. (C) 2003 Elsevier Ltd. All rights reserved.