The electronic states of the tetrahedral iron clusters of iron–sulphur proteins. A theoretical model

Abstract

A simple molecular-orbital model is presented of the delocalised d-electron system of the polynuclear iron–sulphur clusters, [Fe₄S₄(S–R)₄]^n– where n= 1, 2, or 3 and R = alkyl or aryl group, which are known to exist in certain proteins and as synthetic model compounds. The model separates, using symmetry arguments, the iron–sulphur and the iron–iron interactions leaving the final energy-level ordering to be established by experiment. It is shown that this model provides a reasonable interpretation of measured ground-state g factors of the paramagnetic oxidation states, where n= 1 or 3, of the cluster. The state where n= 2 is diamagnetic at low temperature. A closed-shell configuration is assigned to this oxidation level. This conclusion is at variance with that obtained from a SCF-Xα-SW calculation. Arguments are presented which reveal the inadequacy of the level scheme suggested by the Xα calculation.