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• 17 FeH, 14 RuH and 8 OsH complexes are prepared and characterized. • Their MIII/II redox potentials are measured. • These and literature data are used to obtain the Lever E L for the hydride. • E L (H_) is −0.65 V for FeH and RuH but −0.37 V for OsH. • These E L (H_) with Lever's equations allow the prediction of unknown E 1/2 (MIII/II). A series of over 30 iron, ruthenium, and osmium hydride phosphine complexes are reported, along with their MIII/II redox potentials. The complexes are of the type MH(PP) n (X) or [MH(PP) n (L)]+, where PP is one of the following bidentate phosphine ligands: dppe, dtpe, depe, and dtfpe, with n = 2; or the tetradentate phosphine ligand meso-tet-1, with n = 1. The electrochemical data of these complexes and those from the literature are used to determine the Lever E L parameter of −0.65 V for the hydride ligand for iron and ruthenium. For osmium, however, the E L value for the hydride ligand is found to be more positive at only −0.37 V, an increase which is attributed to Os-H σ bond strengthening due to relativistic effects. The correlation holds for irreversible oxidations as well as reversible ones. These E L values can now be used along with Lever's equations to predict redox potentials of other iron-group hydride complexes. [ABSTRACT FROM AUTHOR]