A new tridentate cyclopentane-bridged iminophosphorane ligand, N-(2-diisopropylphosphinophenyl)-P,P-diisopropyl-P-(2-(2,6-diisopropylphenylamido)cyclopent-1-enyl)phosphoranimine (NpNPiPr), was synthesized and used in the preparation of a diiron dinitrogen complex. The reaction of the iron complex FeBr(NpNPiPr) with KC8 under dinitrogen yielded the dinudear dinitrogen Fe complex [Fe(NpNPiPr)](2)(mu-N-2), which was characterized by X-ray analysis and resonance Raman and NMR spectroscopies. The X-ray analysis revealed a diiron complex bridged by the dinitrogen molecule, with each metal center coordinated by an NpNPiPr ligand and dinitrogen in a trigonalmonopyramidal geometry. The N N bond length is 1.184(6) angstrom, and resonance Raman spectra indicate that the N-N stretching mode nu(N-14(2)/N-15(2)) is 1755/1700 cm(-1). The magnetic moment of [Fe(NpNPiPr)](2)(mu-N-2) in benzene-d(6) solution, as measured by H-1 NMR spectroscopy by the Evans method, is 6.91 mu(B) (S = 3). The Mossbauer spectrum at 78 K showed delta = 0.73 mm/s and Delta E-Q= 1.83 mm/s. These findings suggest that the iron ions divalent with a high-spin configuration and that the N2 molecule has (N=N)' character. Density functional theory calculations performed on [Fe(NpNPiPr](2)(mu-N-2) also suggested that the iron is in a high-spin divalent state and that the coordinated dinitrogen molecule is effectively activated by in back-donation from the two iron ions (d pi) to the dinitrogen molecule (pi(x)* and pi(y)*). This is supported by cooperation between a large negative charge on the iminophosphorane ligand and strong electron donation and effective orbital overlap between the iron dir orbitals and N-2 pi* orbitals supplied by the phosphine ligand.