Faculty of Chemical and Food Technology STU in Bratislava
The properties of molecules correlate with their electronic structure. The valence electron density distribution can be obtained experimentally from accurate single crystal diffraction data. Applying the results of the so-called multipole refinement to the properties of molecules in solution, on surfaces or in a living cell is not easy. In a crystal, interactions with neighboring molecules, so-called "non-bonding interactions", influence the electron density distribution in a certain way. In a different environment of the molecule, the resulting valence electron distribution may be altered and a direct correlation is not possible. The mediator for studying the correlation between valence electron distribution and compound properties is theoretical calculations.
Despite the fact that in transition metal complexes the ratio of the core to valence electrons is not favorable for the study of the experimental electronic structure, when the crystals are of good quality and the experiment is carried out with high redundancy, important results can be obtained. The distribution of valence electrons is a robust property, so the main features are easily recognized. The classical coordination bond was found in Cu-Cu and Cr-Cr dinuclear acetate complexes. Metal-metal interactions are discussed [1]. The titanium(IV) coordination compound with peroxo anion is a possible model structure of the reaction center for the theoretical study of hemoglobin. We have shown that the O—O bonding electron density is significantly shifted towards the central titanium atom. The O-O bond in the peroxide complex is weakened and, therefore, could be susceptible to a nucleophilic addition reaction [2]. The study of Ni(II) and Ni(III) complexes with the same ligand (3,6-dichlorobenzene-1,2-dithiolate) shows similar and typical square-planar coordination. In the experimental results, we see two combined effects. One effect is that the positive charge on the central atom is always lower than the formal oxidation state, and the other is that one part of electron density is shifted from the central atom to the non-innocent ligand. Thus, the non-innocent ligand can adapt to the requirements of the central atom [3]. In nitrosyl a μ3‑Oxido Trinuclear Diiron(III)-Ruthenium(II) complex we have studied whether such a complex can release NO photolytically [4]. We will discuss also tetrahedral Cu(I) and pseudo-octahedral Cu(II) complexes with biphenyldiimino dithioether as the blue copper protein model structures. The real challenge is a compound with the Cr-Cr distance of 1.8077(7) Å [5]. Is there a sufficient electron density for one σ, two π and two σ bonds?
I am grateful to the HPC center at the Slovak University of Technology in Bratislava, which is a part of the Slovak Infrastructure of High Performance Computing (SIVVP project, ITMS code 26230120002, funded by the European region development funds, ERDF), for the computational time and resources made available.