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which serves a s a representation of the protein site electronic and structural changes in a Fd /Fd electron transfer couple in the absence of%xtr&"sd,c constraints such a s might be imposed by protein structural features [2,3]. bfrksbauer, epr, magnetization and x-ray crystallographic measurements on analogues in both oxidation states have been used to characterize the properties of the Fe4S4 core [l -51. In the present paper we compare the results previously obtained in the two oxidation states and present additional experimental data on the trianion analogue in order to gain insight into the relative strengths of exchange interactions between the iron ions and the sensitivity of the Fe4S4 core structure to terminal ligand variations and crystal packing effects. In the dianion with formal electronic configuration 2FeZf + 2 ~ e 3 4 , various experimental measurements [l] show that the electrons a re highly delocalized resulting in four equivalent iron ions of intermediate valence ~ e ~ " ~ + , The iron spins a re strongly exchange coupled producing a singlet ground state. Furthermore, the electronic properties of the FeqSq core are relatively insensitive to terminal ligand substitution and crystal packing effects. In the trianion with formal electronic configuration 3Fe2+f 1Fe3', observed isomer shifts and quadrupole splittings indicate a high degree of electron delocalization [Z, 3,5]. However, in contrast to the dianion, the trianion is sensitive to changes in the nature of both the terminal ligand and quaternary ammonium counterion when studied in crystalline form, Mb'ssbauer and magnetization studies show that the trianions with different R groups assume one of two possible structures (I, 11) resulting in different ground state configurations [2,3]. Furthermore, the iron ions are exchange coupled resulting in a ground state of S = 1/2 for structure (I) (R = phenyl, 2-tolyl) and

MAGNETIC PROPERTIES OF [Fe4S4(SR)4]3- CLUSTERS

MAGNETIC PROPERTIES OF [Fe₄S₄(SR)₄]^3- CLUSTERS