ORIGINAL PAPER Structure, electronic properties, and NBO and TD-DFT analyses of nickel(II), zinc(II), and palladium(II) complexes based on Schiff-base ligands Amina Guelai 1 & Houari Brahim 1 & Abdelkrim Guendouzi 1 & Mostefa Boumediene 1 & Sefia Brahim 1 Received: 26 March 2018 /Accepted: 18 September 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract In this work we studied the structural and electronic properties of the metal–Schiff base complexes NiL 2 2 (1), PdL 1 2 (2), ZnL 2 2 (3), and NiL 1 2 (4), where L 1 and L 2 are Schiff bases synthesized from salicylaldehyde and 2-hydroxy-5-methylbenzaldehyde, respectively. Natural bond analysis showed that in complexes 1 and 2, the metal ion coordinates to the ligands through electron donation from lone pairs on ligand nitrogen and oxygen atoms to s and d orbitals on the metal ion. In complex 3, metal–N and metal–O bonds are formed through charge transfer from the lone pairs on nitrogen and oxygen atoms to an s orbital of Zn. Dimethylation of the phenolate rings in the ligands decreases the energy gap and redshifts the spectrum of the nickel complex. The main absorptions observed were assigned on the basis of singlet-state transitions. The simulated spectra of the two complexes 1 and 2 are characterized by excited states with ligand-to-ligand charge-transfer (LLCT), metal-to-ligand charge-transfer (MLCT), ligand-to-metal charge-transfer (LMCT), and metal-centered (MC) character. Keywords TD-DFT . Schiff base . Complexes . Absorption spectrum . Nickel(II) . Zinc(II) . Palladium(II) . Excited states . NBO Introduction Schiff-base ligands are characterized by their ability to coor- dinate readily with various transition metal ions to form stable complexes that are easy to synthesize [1–3]. Often, the Schiff- base ligand has two or more electron-donating functional groups and can be coordinated with metal ions in different modes (monodentate, bidentate, tridentate, etc) via donor–ac- ceptor interactions between the functional groups of the ligand and the metal [4]. Because of these remarkable properties, Schiff bases have become popular in a number of scientific fields, such as biological science and medicine [5]; indeed, metal–Schiff base complexes have been widely used as anti- inflammatory and antibacterial agents [6–8]. To improve their biological activities, several studies aimed at synthesizing new allylamine-derived Schiff-base ligands and their respective complexes (usually with first-row metal ions) have been car- ried out [9–11]. Recently, a new class of such complexes has been synthesized: NiL 2 2 (1), PdL 1 2 (2), and ZnL 2 2 (3) (Fig. 1), where L 1 and L 2 are Schiff bases (L 1 = 2-allyliminomethyl phenolate and L 2 = 2-allyliminomethyl-4-methyl phenolate) synthesized from salicylaldehyde and 2-hydroxy-5- methylbenzaldehyde, respectively, as well as allylamine [12]. These three complexes have been fully characterized using different techniques and various spectroscopic methods of analysis. X-ray data show that complexes 1 and 2 adopt a square-planar geometry around the metallic ion and that com- plex 3 is nonplanar. In the UV-visible region, the three com- plexes are characterized by a weak band in the visible region and intense absorptions in the UV-C region. However, the geometric structures of the three complexes have not been well studied, particularly the first coordination sphere of the metal ion (or more precisely, how the metal ion coordinates with oxygen and nitrogen). In order to assign the main absorp- tions observed experimentally, the electronic absorption spec- trum must be simulated. Numerous theoretical studies have Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00894-018-3839-9) contains supplementary material, which is available to authorized users. * Houari Brahim brahim.h@outlook.com 1 Department of Chemistry, University of Saida—Docteur Moulay Tahar, 20000 Saida, Algeria Journal of Molecular Modeling (2018) 24:301 https://doi.org/10.1007/s00894-018-3839-9