A Fe2S2 cluster with unprecedented CysSS(-) (cysteinepersulfide) coordination has been observed crystallographically in the AdoMet-dependent hydrogenase maturase enzyme HydE. Geometry-optimized density functional theo gamma calculations are used to develop an electronic structure description of this unusual cluster. The results indicate that the CysSS(-) ligand is unique because it can act as a donor as well as an acceptor ligand. This is due to the presence of S-S pi* (occupied) End S-S sigma* (unoccupied) orbitals in this ligand. Extensive back-bonding is observed between the cluster and the S-S sigma* orbital. The back-bonding is significantly higher in the reduced state, which is calculated to shift the reduction potential of this Fe2S2 cluster by +400 mV in the gas phase relative to a CysS(-)-coordinated Fe2S2 cluster model of BioB.