Valence delocalization

Fig. 3. Ground state spin (S) and valence delocalization schemes for the known oxidation states of [Fe3S4] clusters. Discrete [Fe3S4] clusters have not been observed in siny protein, but they have been identified as fragments in heterometallic cubane clusters. Reduction of the [Fe3S4]+ cluster by three electrons, to yield a putative aU-ferrous cluster, occurs with the concomitant addition of three protons. Key S , grey Fe +, black Fe +, white Fe, white with central black dot.

A major breakthrough in assigning the VTMCD spectra of [Fe3S4] clusters came with the discovery of S = valence-delocalized [Fe2S2l clusters in the alkaline forms of the C56S and C60S mutants of C. 

Fig. 14. Comparison of the VTMCD spectra ( 1.6 K and 6 T) of the valence-delocalized S = 9/2 [Fe2S2] cluster in the aUtEdine form of the C60S mutemt of C. pasteurianum 2Fe Fd with those of the [ZnFesSJ Emd [Fe3S4] clusters in P. furiosus Fd. Magnetization and temperature dependence studies show that the bands marked with an asterisk in the valence-delocEdized S = 9/2 [Fe2S2] spectra result from a valence-localized S = 1/2 component.

A schematic molecular orbital diagram for the Fe-Fe interaction in an S = I valence-delocalized Fe Fe pair based on effective C v symmetry at the Fe sites and the observed electronic transitions for the valance-delocalized [Fe2S2l cluster is shown in Fig. 15. The dominant interaction (responsible for the S = ground state) is the a overlap between the pair of orbitals, with progressively smaller tt interactions between pairs of d z and dyz orbitals and S interactions between pairs of d y amd / orbitals. The three highest energy tran-... 

Fig. 15. Schematic MO diagram and proposed electronic assignments for the Fe-Fe interaction in a valence-delocalized S = 9/2 [Fe2S2] unit. Modified with permission from Ref. (207).

IR VTMCD band in each of the [Fe3S4] spectra, therefore, corresponds to the uniaxial a- a transition of Fe-Fe interaction and the resonance delocalization energy, (3, is determined to be 4290 25 cm for Type 1 clusters and 4350 25 cm for Type 2 clusters. Hence, the Fe-Fe interactions within the valence-delocalized pair are very similar in both locations. 

A major discrepancy that remains unresolved in the excited-state properties of the [Fe384]° cluster in D. gigas Fdll concerns the existence of a low-lying, fully valence-delocalized state that becomes populated at temperatures above 25 K. 8uch a state is clearly apparent in the temperature-dependent Mossbauer studies of reduced D. gigas Fdll (29) and P. furiosus 3Fe Fd (198) and is represented by one quad-rupole doublet with AEq 0.9 mm/s and S = 0.45 mm/s. 8uch a... 

B. Polymetallic Systems Valence Delocalization Considerations on the Reduction Potentird Solution Structure... 

A priori, one might have expected a [3Fe-4S] center to give a particularly simple EPR spectrum. Contrary to what was suggested in Ref. (13), the electronic structure of this cluster, which possess three ferric sites, is not liable to be complicated by valence delocalization phenomena, so that the intersite interactions can be described by the Heisenberg Hamiltonian ... 

Fe-4S] clusters, the asymmetric electron distribution apparent in Fig. 8B results essentially from the interplay between valence delocalization and vibronic coupling effects 113). 

High-valent iron also occurs in -nitrido bridged dimers with linear [Fe °-N=Fe" ]" and [Fe =N=Fe ] " cores [209, 210] (and references therein). Such compounds have been prepared first by thermolysis [247] or photolysis [248] of iron(III)-porphyrin complexes with an azide ligand, (N3). Mixed-valent iron-nitrido porphyrin dimers exhibit valence delocalization as can be inferred from the... 

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