This review examines the synthesis, properties, and acid reactivity of transition metal dinitrogen complexes bearing phosphine ligands containing pendant amine groups in the second coordination sphere. We have synthesized non-precious metal dinitrogen complexes containing bidentate, tridentate, and tetradentate phosphine ligands, some of which generate N-2 derived ammonia and/or hydrazine in the presence of excess acid. The metal identity, metal oxidation state, ligand denticity, and functional groups on phosphorus/nitrogen all play a role in controlling the site of protonation (at the metal, pendant amine, or coordinated dinitrogen) with the addition of exogenous acid. (C) 2016 The Authors. Published by Elsevier B.V. All rights reserved.