Density functional theory calculations have been performed on Fe-only hydrogenase analogues compounds [(i-PDT)Fe₂(CO)₄(CN)₂]^2- (compound I) and [Fe₂(CO)₄MeSCH₂C(Me)-(CH₂S)₂}(CN)]^- (compound II). Results using BPW91/6-311+G(d,p) show electron affinity and trends in molecular orbitals consistent with photoelectron spectroscopy data. The HOMO orbitals show familiar Fe-Fe bonding characteristics while the LUMO orbitals show Fe-Fe anti-bonding characteristics. To compare the effects of a second CN versus a terminal sulfur ligand, results for compound I after one electron is removed (compound I—1e) were compared with those of compound II. It was found that compound I—1e has a much lower HOMO. Mulliken charge analysis also shows that the terminal sulfur has a stronger electron donating effect than a CN ligand. From the standpoint of computational materials design, we therefore think that similarities in electron affinity, 2Fe charge density and HOMO/LUMO characteristics to the biologically present active site should be bases for predicting catalytic activity of Fe-only hydrogenase analogues. [DOI: 10.1380/ejssnt.2007.148]