Nickel (I) complexes of 1,5-diaza-3,7-diphosphacyclooctane (P2N2R)-N-Ph ligands were obtained from precursor Ni(II) by exhaustive electrolysis of [Ni((P2N2R)-N-Ph)(2)](2+) precursors at the Ni(II/I) couple potential and characterized by several techniques. Paramagnetic Ni(I) complexes were investigated by cyclic voltammetry, spectroelectrochemistry UV/Vis and ESR methods at room temperature and in a frozen solution. Catalytic behavior of Ni(I) complex in different ligand environment for hydrogen evolution from acid depends from the ligand nature. The two possible catalytic cycles were proposed depending on the nitrogen substituents in 1,5-diaza-3,7-diphosphacyclooctane. (C) 2016 Elsevier Ltd. All rights reserved.