It has been proposed that late transition metals with low coordination numbers (square planar or linear) can act as nucleophiles and participate in σ-hole interactions as electron donors. This is due to the existence, in this type of metal complexes, of a pair of electrons located at high energy d-orbitals (dz2 or dx2-y2), which are adequate for interacting with antibonding σ-orbitals [σ*(X–Y)] where Y is usually an electron withdrawing element and X an element of the p-block. This type of d[M]→σ*(X–Y) interaction has been reported for metals of groups 9–11 in oxidation states +1 and +2 (d8 and d10) as electron donors and σ-holes located in halogen and chalcogen atoms as electron acceptors. To our knowledge, it has not been described for σ-holes located in pnictogen atoms. In this manuscript, evidence for the existence of pnictogen bonding involving the square planar Pt(II) metal as the electron donor and Sb as the electron acceptor is provided by using an X-ray structure retrieved from the Cambridge Structural Database (CSD) and theoretical calculations. In particular, the quantum theory of atoms in molecules (QTAIM), the noncovalent interaction plot (NCIPlot) and molecular electrostatic potential (MEP) methods were used. Moreover, to further confirm the nature of the Sb···Pt(II) contact, a recently developed method was used where the electron density (ED) and electrostatic potential (ESP) distribution were compared along the Sb···Pt(II) bond path.

Long range substituent effects in regium bonding interactions involving Au(I) linear complexes are investigated for the first time. The Au(I) atom is coordinated to two para-substituted pyridine ligands. The interaction energy (RI-MP2/def2-TZVP level of theory) of the π-hole regium bonding assemblies is affected by the pyridine substitution. The Hammett's plot representations for several sets of Lewis bases have been carried out and, in all cases, good regression plots have been obtained (interaction energies vs. Hammett's σ parameter). The Bader's theory of "atoms-in-molecules" has been used to evidence that the electron density computed at the bond critical point that connects the Au-atom to the electron donor can be used as a measure of bond order in regium bonding. Several X-ray structures retrieved from the Cambridge Structural Database (CSD) provide experimental support to the existence of π-hole regium bonding in [Au(Py) 2 ] + derivatives.

Noncovalent interactions involving metals as electron acceptors are continuously under investigation. The term “matere bond” has been proposed to identify noncovalent donor–acceptor interactions where elements of group 7 of the periodic table play the role of the electrophilic site. Most of the works on matere bonds involve rhenium atoms usually in +7 oxidation state. This work emphasizes for the first time their importance in technetium derivatives in several oxidation states (+7, +6, +5, and +3). The Cambridge Structural Database (CSD) in combination with density functional theory (DFT) calculations are used to demonstrate the structure directing role of matere bonds in X-ray structures, even involving anion···anion interactions. Further characterization of the matere bonds is provided using Molecular Electrostatic Potential (MEP) surface calculations, the “Quantum Theory of Atoms in Molecules” (QTAIM), and Natural Bond Orbital (NBO) analyses. It should be emphasized that some types of matere bonds reported herein have not been previously described in literature.

In this manuscript, we have examined the CSD (Cambridge Structural Database) to investigate the relative ability of Te and I (in practice, the heaviest chalcogen and halogen atoms) in di- and tri-iododiorganyltellurium(IV) derivatives to establish σ-hole interactions. The geometry around the Te(IV) in this type of compound is trigonal bipyramidal where the stereoactive lone pair at Te(IV) occupies one of the equatorial positions. In the solid state, Te(IV) tends to form pseudo-octahedral coordination by establishing strong noncovalent interactions opposite to the two covalent bonds of the equatorial plane. Such contacts can also be classified as chalcogen bonds following the recommendation of the International Union of Pure and Applied Chemistry (IUPAC). Such contacts have been analyzed energetically in this work using density functional theory (DFT) calculations, rationalized using molecular electrostatic potential (MEP) surface analysis and characterized using a combination of the quantum theory of atoms in molecules (QTAIM) and noncovalent interaction plot (NCIplot) computational tools. Finally, the observation of halogen bonds and type I halogen···halogen contacts is also emphasized and compared to the chalcogen bonds. Energy decomposition analysis has also been performed to compare the physical nature of chalcogen, halogen and type I interactions.

In this study, we provide crystallographic (Protein Data Bank (PDB) inspection) and theoretical (RI-MP2/def2-TZVP//PBE0-D3/def2-SVP level of theory) evidence of the involvement of nucleobases in Regium−π bonds (RgBs). This noncovalent interaction involves an electrophilic site located on an element of group 11 (Cu, Ag, and Au) and an electron-rich species (lone pair, LP donor, or π-system). Concretely, an initial PDB search revealed several examples where RgBs were undertaken involving DNA bases and Cu(II), Ag(I), and Au(I/ III) ions. While coordination positions (mainly at the N atoms of the base) are well known, the noncovalent binding force between these counterparts has been scarcely studied in the literature. In this regard, computational models shed light on the strength and directionality properties of the interaction, which was also further characterized from a charge-density perspective using Bader's "atoms in molecules" (AIM) theory, noncovalent interaction plot (NCIplot) visual index, and natural bonding orbital (NBO) analyses. As far as our knowledge extends, this is the first time that RgBs in metal−DNA complexes are systematically analyzed, and we believe the results might be useful for scientists working in the field of nucleic acid engineering and chemical biology as well as to increase the visibility of the interaction among the biological community.