This review compiles species that are known or potential intermediates in the catalytic formation of H₂ by diiron dithiolate complexes inspired by the active site of the [FeFe] hydrogenases. The data collection emphasizes spectroscopic characteristics (NMR, IR, UV-Vis, EPR) of protonated and reduced derivatives of the iron complexes that could provide reference data to the identification of intermediates in mechanistic studies.