A theoretical investigation on geometry and electronic structure of small FemSn nanoclusters (1 ⩽ m, n ⩽ 4)

► The influence of S doping on the electronic structure of Fen and FenSm−1 complexes. ► A closed-shell metal-ligand interaction, intermediate between values for pure covalent and pure ionic bonds are evident. ► The energy liberated upon addition of atomic sulfur appears to increase with increasing Fe/S ratio.