[Fe(trans-diammac)](2+) (trans-diammac) = exo-6,13-diamino-6, 13-dimethyl- 1,4,8,11-tetraazatetradecane) is one of the very few fully characterized examples of a low-spin iron(II) compound with saturated amine ligands. The crystal structure analysis (monoclinic, P2(1)/c; a = 9.547(8), b = 14.631(13), c = 16.91(2) Angstrom; beta = 98.92(7)degrees; Z = 4) defines the iron(II) coordination geometry as distorted octahedral with very short in-plane Fe-N distances (average of 2.01 Angstrom) to the secondary amines of the macrocyclic ligand and slightly longer distances to the pendant primary amine donors (2.03 Angstrom); there is a considerable tilt of the vector involving the axial donors with respect to the plane defined by the secondary amines and the metal center (theta = 11.5 degrees). The metal-donor distances are shorter than those for other low-spin iron(II) hexaamines, and consequently, the redox potential (Fe3+/2+) is very small (0.45 V vs SHE) and the ligand field splitting is very large (Dq = 1785 cm(-1)). The structural, magnetic, and spectroscopic properties are discussed on the basis of the experimental data in comparison with model studies.