The complexes of the heptadentate receptor N,N'-bis(benzimidazol-2-ylmethyl)-1,10-diaza-15-crown-5 (L-2) with Mn-II, Co-II, Ni-II, Cu-II, and Zr-II are reported. The X-ray crystal structures of the Zn-II and Ni-II complexes show that whereas the Zn-II ion is seven -coordinated in a (distolrled) pentagonal-bipyramidal coordination environment, the N-II ion is only six-coordinated in a distorted octahedral coordination environment. Theoretical calculations on the [M(L-2)](2+) systems (M = Mn, Go, Ni, Cu, or Zn) performed at the density functional theory (DFT; B3LYP) level have been used to obtain information about the structure and electronic properties of these complexes, as well as to rationalize their preferences for a pentagonal-bipyramidal or an octahedral coordination. We have found that for the Mn-II, Co-II, Cu-II, and Zn-II complexes, geometry optimizations lead systematically to pentagonal -bipyramidal coordination environments around the metal ions. However, for the Ni-II complex, two minimum-energy conformations were obtained, with the metal ion teing in octahedral (o-[Ni(L-2)](2+)) or pentagonal-bipyramidal (pb-[Ni(L-2)](2+)) coordination. The stabilization of the octahedral geometry in the Ni-II complex can be considered as the result of the Jahn-Teller effect operating in pentagonal-bipyramidal geometry, which in an extreme case leads to an octahedral coordination. Spectrophotometric t trations carried out in dimethyl sulfoxicle (DMSO) and CH3CN/DMSO (9:1) solutions indicate the following stability sequence for the complexes of L-2: Co-II approximate to Ni-II > Zn-II >> Mn-II. The variations in the geometry and stability of the complexes may be rationalized in terms of the different occupations of the frontier molecular orbitals along the first-row transition-metal series. Finally, a time-dependent DFT approach was used to investigate the absorption spectrum of the [Cu(L-2)](2+) Complex based on the optimized geometries at the B3LYP level, also confirming a pentagonal-bipyramidal coordination in solution for this compound.