Iron-sulfur proteins are involved in various biological electron transport processes, such as photosynthesis and cellular respiration, which require rapid electron transfer to sustain the energy or biochemical needs of the organism. To serve their various biological roles, iron-sulfur proteins effect rapid electron transfers and span the whole range of physiological redox potentials from -600 mV to +460 mV.
Fe-SR bonds have unusually high covalency which is expected. When comparing the covalency of Fe with the covalency of Fe, Fe has almost double the covalency of Fe (20% to 38.4%). Fe is also much more stabilized than Fe. Hard ions like Fe normally have low covalency because of the energy mismatch of the metal lowest unoccupied molecular orbital with the ligand highest occupied molecular orbital.
External water molecules positioned close to the iron-sulfur active site reduces covalency; this can be shown by lyophilization experiments where water is removed from the protein. This reduction is because external water hydrogen bonds with cysteine S, decreasing the latter's lone pair electron donation to the Fe by pulling away S electrons. Since covalency stabilizes Fe more than Fe, Fe is more destabilized by the HOH-S hydrogen-bonding.
The Fe 3d orbital energies follow the "inverted" bonding scheme which fortuitously has the Fe d-orbitals closely matched in energy with the sulfur 3p orbitals, giving high covalency in the resulting bonding molecular orbital. This high covalency lowers the inner sphere reorganization energy and ultimately contributes to a rapid electron transfer.