The crystal structures of two compounds, CuL(2) and LiNCS.HL [HL = 4'-[2-(tosylamino)benzylideneamino]-2,3-benzo-15-crown-5], have been determined from synchrotron powder diffraction data. Both compounds crystallize in the monoclinic space group P2(1)/c and with one molecule in the asymmetric unit. In CuL(2) the four N atoms of two bidentate L ligands coordinate the Cu(II) ion in a distorted tetrahedral geometry with Cu-N distances of 1.98 (5)-2.05 (5) A, while two O atoms from two sulfoxide groups complete the distorted octahedral Cu coordination [Cu-O 2.64 (4), 2.74 (4) A]. In LiNCS.HL, lithium is coordinated by all five ether O atoms with Li-O distances of 2.03 (3)-2.50 (3) A and an N atom from the thiocyanate moiety [Li-N 1.98 (3) A] in a distorted pentagonal pyramidal geometry. Preliminary potentiometric selectivity measurements for ion-selective electrodes (ISEs) based on CuL(2) and ZnL(2) demonstrated significant differences in their selectivity. In order to find a possible reason for this, theoretical calculations at the DFT (B3LYP) level were performed. These calculations used the crystal structures of CuL(2), LiNCS.HL, ZnL(2) and HL as input geometries for the minimum energy optimization in vacuo. The results indicate that in ML(2) complexes (M = Cu, Zn) the electronic structure of the metal ion determines the spatial orientation of benzo-15-crown-5 macrocycles, and their different orientation in CuL(2) and ZnL(2) results in different potentiometric selectivities of ISEs based on these compounds.