Five dinitrogen-molybdenum complexes bearing bis(diphenylphosphino)amine derivative ligands (L-R) that form a unique 4-membered P-N-P chelate ring, trans-[Mo(N-2)(2)(L-R)(2)] (2(R): R = Ph, Xy, p-MeOPh, 3,5-iPr(2)Ph, iPr), were prepared for the purpose of binding a dinitrogen molecule. The corresponding two dichloride-molybdenum complexes, trans-[MoCl2(L-R)(2)] (1(R): R = Ph, Xy), were also prepared as comparisons. FT-IR spectra of 2(R) were measured and compared the nu(N N) values. Moreover, X-ray crystal structure determination of 1(R) (R = Ph, Xy) and 2(R) (R = Xy, 3,5-iPr(2)Ph) is performed. These experimental results indicated that the coordinated dinitrogen molecule gets easily influenced by the N-substitutent of diphosphinoamine ligand. In addition, to investigate the effect of the properties of the diphosphinoamine ligand for the dinitrogen molybdenum complexes, we performed DFT calculations that focused on the difference of N-substituent, the dihedral angle between P-N-P plane and N-substituent aryl group, and the P-N-P bite angle. This calculation revealed that the competition between the back-donation from metal to dinitrogen and that from metal to ligand was affected by P-N-P bite angle and the dihedral angle of N-substituent of ligand. In order to examine the reactivity with respect to conversion of dinitrogen to ammonia, protonation and trimethylsilylation reactions of the coordinated dinitrogens were carried out for 2(R).