Bonding in Classical and Nonclassical Transition Metal Carbonyls: The Interacting Quantum Atoms Perspective

On the physical role of exchange in the formation of an intramolecular bond path between two electronegative atoms

In this paper, we present a detailed energetic decomposition of intramolecular O···X interactions (X being O, S, or a halogen atom) based on the interacting quantum atoms approach of Pendás and co-workers. The nature of these interactions (repulsive or attractive, more or less electrostatic) is discussed in the framework of Bader's atoms in molecules theory, a particular emphasis being put on delocalization (measured by delocalization indexes and in terms of the source function) and on the exchange contributions. Notably, the concept of exchange channels introduced by Pendás and collaborators provides means of rationalizing and predicting the presence of bond critical points, enhancing the physical meaning of bond paths.

Domain-averaged exchange-correlation energies as a physical underpinning for chemical graphs

A novel solution to the problem of assigning a molecular graph to a collection of nuclei (i.e. how to draw a molecular structure) is presented. Molecules are universally understood as a set of nuclei linked by bonds, but establishing which nuclei are bonded and which are not is still an empirical matter. Our approach borrows techniques from quantum chemical topology, which showed for the first time the construction of chemical graphs from wave functions, shifting the focus on energetics. This new focus resolves issues surrounding previous topological analyses, in which domain-averaged exchange-correlation energies (V(xc)), quantities defined in real space between each possible atom pair, hold the key. Exponential decay of V(xc) in non-metallic systems as the intercenter distance increases guarantees a well-defined hierarchy for all possible V(xc) values in a molecule. Herein, we show that extracting the set of atom pairs that display the largest V(xc) values in the hierarchy is equivalent to retrieving the molecular graph itself. Notably, domain-averaged exchange-correlation energies are transferable, and they can be used to calculate bond strengths. Fine-grained details resulted to be related to simple stereoelectronic effects. These ideas are demonstrated in a set of simple pilot molecules.

Platinum(II)-catalyzed cyclization sequence of aryl alkynes via C(sp3)-H activation: a DFT study

The mechanism and intermediates of hydroalkylation of aryl alkynes via C(sp(3))-H activation through a platinum(II)-centered catalyst are investigated with density functional theory at the B3LYP/[6-31G(d) for H, O, C; 6-31+G(d,p) for F, Cl; SDD for Pt] level of theory. Solvent effects on reactions were explored using calculations that included a polarizable continuum model for the solvent (THF). Free energy diagrams for three suggested mechanisms were computed: (a) one that leads to formation of a Pt(II) vinyl carbenoid (Mechanism A), (b) another where the transition state implies a directed 1,4-hydrogen shift (Mechanism B), and (c) one with a Pt-aided 1,4-hydrogen migration (Mechanism C). Results suggest that the insertion reaction pathway of Mechanism A is reasonable. Through 4,5-hydrogen transfer, the Pt(II) vinyl carbenoid is formed. Thus, the stepwise insertion mechanism is favored while the electrocyclization mechanism is implausible. Electron-withdrawing/electron-donating groups substituted at the phenyl and benzyl sp(3) C atoms slightly change the thermodynamic properties of the first half of Mechanism A, but electronic effects cause a substantial shift in relative energies for the second half of Mechanism A. The rate-limiting step can be varied between the 4,5-hydrogen shift process and the 1,5-hydrogen shift step by altering electron-withdrawing/electron-donating groups on the benzyl C atom. Additionally, NBO and AIM analyses are applied to further investigate electronic structure changes during the mechanism.

Bonding between strongly repulsive metal atoms: an oxymoron made real in a confined space of endohedral metallofullerenes

Endohedral metallofullerenes (EMFs) are able to encapsulate up to four metal atoms. In EMFs, metal atoms are positively charged because of the electron transfer from the endohedral metal atoms to the carbon cage. It results in the strong Coulomb repulsion between the positively charged ions trapped in the confined inner space of the fullerene. At the same time, in many EMFs, such as Lu(2)@C(76), Y(2)@C(79)N, M(2)@C(82) (M = Sc, Y, Lu, etc.), Y(3)@C(80), or Sc(4)O(2)@C(80), metals do not adopt their highest oxidation states, thus yielding a possibility of the covalent metal-metal bonding. In some other EMFs (e.g., La(2)@C(80)), metal-metal bonding evolves as the result of the electrochemical or chemical reduction, which leads to the population of the metal-based LUMO with pronounced metal-metal bonding character. This article highlights different aspects of the metal-metal bonding in EMFs. It is concluded that the valence state of the metal atoms in dimetallofullerenes is not dependent on their third ionization potential, but is determined by their ns(2)(n- 1)d(1)→ns(1)(n- 1)d(2) excitation energies. Peculiarities of the metal-metal bonding in EMFs are described in terms of molecular orbital analysis as well as topological approaches such as Quantum Theory of Atoms in Molecules and Electron Localization Function. Interplay of Coulomb repulsion and covalent bonding is analyzed in the framework of the Interacting Quantum Atom approach.

Meaningful structural descriptors from charge density

This paper provides a short introduction to the basics of electron density investigations. The two predominant approaches for the modelling and various interpretations of electron density distributions are presented. Their potential translations into chemical concepts are explained. The focus of the article lies on the deduction of chemical properties from charge density studies in some selected main group compounds. The relationship between the obtained numerical data and commonly accepted simple chemical concepts unfortunately is not always straightforward, and often the chemist relies on heuristic connections rather than rigorously defined ones. This article tries to demonstrate how charge density analyses can shed light on aspects of chemical bonding and reactivity resulting from the determined bonding situation. Sometimes this helps to identify misconceptions and sets the scene for new unconventional synthetic approaches.