A novel chiral and terdentate ligand, 7-amino-4,6(7)-dimethyl-5-aza-3-hepten-2-one (abbreviated ApnH3) has been synthesized. In keeping with the occurrence of two possible locations for the methyl substituent grafted on the diamino chain, the chirality of the substituted carbon atom, the conformation of the five-membered chelate ring and, possibly, the chirality of the second ligand, the related nickel(II) complexes, (ApnH2)NiN3 and [(ApnH2)NiPy]ClO4 may exist as a variety of geometrical and optical isomers. Solid-state (X-ray diffraction) and solution (NMR in achiral and chiral media, electronic and circular dichroism spectroscopies) studies show that no more than two isomers do exist. In both cases the methyl substituent is remote from the imino nitrogen and it adopts an equatorial orientation while an axial position is observed in the complexes of the related quadridentate Schiff base. In fact the two diasteroisomers display an enantiomeric relationship. The complex (ApnH2)NiN3 prepared from (rac)-diaminopropane crystallizes in the monoclinic space group P2(1)/c with four formula weights in a cell having the dimensions a = 9.117(1) angstrom, b = 8.738(1) angstrom, c = 14.160(2) angstrom, and beta = 99.73(2)-degrees.