Crystal structure, configurational and density functional theory analysis of nickel(II) complexes with pentadentate 1,3-pd3a-type ligands

The O–O–N–N–O-type pentadentate ligands H31,3-pd3a and H31,3-pd2ap (H31,3-pd3a stands for 1,3-propanediamine-N,N,N′-triacetic acid; H31,3-pd2ap stands for 1,3-propanediamine-N,N′-diacetic-N-3-propionic acid) and the corresponding novel octahedral nickel(II) complexes have been prepared and characterized. H31,3-pd3a and H31,3-pd2ap ligands coordinate to nickel(II) ion via five donor atoms (three deprotonated carboxylate atoms and two amine nitrogens) affording octahedral geometry in case of all investigated Ni(II) complexes. A sixth place within octahedra has been occupied by the molecule of water. A six coordinate, octahedral geometry has been established crystallographically for the K[Ni(1,3-pd3a)(H2O)]·3H2O complex. Structural data correlating similar chelate Ni(II) complexes have been used for an extensive strain analysis. This is discussed in relation to information obtained for similar complexes. The infra-red and electronic absorption spectra of the complexes are interpreted and compared with related complexes of known geometries. Density functional theory (DFT) has been used to model the most stable geometry isomer and Natural Energy Decomposition Analysis (NEDA) to reveal the energetic relationship of these compounds. The results from density functional studies have been compared with X-ray data. NEDA has been done for the [LNi]− and [H2O] units.