We present a DFT study of the reaction mechanism on electrocatalytic oxidation of formate by a family of [Ni((P2N2R')-N-R)(2)](2+) complexes ((P2N2R')-N-R = 1,5-diR'-3,7-diR derivative of 1,5-diaza-3,7-diphosphacyclooctane, where R and R' are aryl or alkyl groups). [Ni((P2N2Me)-N-Ph)(2)](2+) complex 1 was used as a model complex to mimic a family of [Ni((P2N2R')-N-R)(2)](2+) complexes. Our calculated results show that the decarboxylation step (corresponding to TS3) is the rate-determining step for the electrocatalytic oxidation of formate and that a NiII-H intermediate is involved in the reaction mechanism. The pendant amine plays an important role in the deprotonation of the nickel hydride complex generated in the decarboxylation step. In addition, our study indicates that the choice of external bases is important for removing the proton (H+) from the nitrogen-protonated nickel(0) complexes. For the electrocatalytic oxidation of formate using the catalytically inactive [Ni(depe)(2)](2+) (depe = 1,2-bis(diethylphosphino)ethane) complex, calculations on 1-depe have also been carried out for comparison.