The condensation of 2-pyridinecarboxaldehyde N-oxide and triethylenetetramine yields a product with two imidazolidine rings, as proven by a solid-state X-ray structure analysis as well as by NMR solution spectra. This ligand, L-1, undergoes a ring-opening reaction on complex formation with Cu(II), yielding [CuL2](2+) where L-2 functions as a pentadentate ligand, containing only one imidazolidine ring. On complexation with Zn(II) and Fe(III), both rings are opened and the complexes [ZnL3](2+) and [FeL3](3+) with a hexadentate L-3 ligand are formed. The recrystallization of [ZnL3](2+) from DMSO solution results in the complex [ZnL1(DMSO)(2)](2+) in which L-1 behaves as a tetradentate ligand. Thus L-1, L-2, and L-3 are structural isomers with two, one, or no imidazolidine rings, as confirmed by X-ray structure analyses. The intramolecular ring formation is the result of the nucleophilic addition of the N(amino) group to the electrophilic sp(2)-hybridized -HCdelta+=N site. Owing to the absence of the chelate effect on the sp(3)-hybridized carbon atom belonging to the imidazolidine ring, the ring opening is facilitated and readily observed upon complex formation with Cu(II), Zn(II), and Fe(III).