A covalent chemical bond carries the connotation of overlap of atomic orbitals between bonded atoms, leading to a buildup of the electron density in the internuclear region. Stabilization of the valence 5f orbitals as the actinide series is crossed leads, in compounds of the minor actinides americium and curium, to their becoming approximately degenerate with the highest occupied ligand levels and hence to the unusual situation in which the resultant valence molecular orbitals have significant contributions from both actinide and the ligand yet in which there is little atomic orbital overlap. In such cases, the traditional quantum-chemical tools for assessing the covalency, e.g., population analysis and spin densities, predict significant metal–ligand covalency, although whether this orbital mixing is really covalency in the generally accepted chemical view is an interesting question. This review discusses our recent analyses of the bonding in AnCp3 and AnCp4 (An = Th–Cm; Cp = η5-C5H5) using both the trad...

Does covalency increase or decrease across the actinide series? Implications for minor actinide partitioning.

Metal-cage and intracluster bonding was studied in detail by quantum theory of atoms in molecules (QTAIM) for the four major classes of endohedral metallofullerenes (EMFs), including monometallofullerenes Ca@C(72), La@C(72), M@C(82) (M=Ca, Sc, Y, La), dimetallofullerenes Sc(2)@C(76), Y(2)@C(82), Y(2)@C(79)N, La(2)@C(78), La(2)@C(80), metal nitride clusterfullerenes Sc(3)N@C(2n) (2n=68, 70, 78, 80), Y(3)N@C(2n) (2n=78, 80, 82, 84, 86, 88), La(3)N@C(2n) (2n=88, 92, 96), metal carbide clusterfullerenes Sc(2)C(2)@C(68), Sc(2)C(2)@C(82), Sc(2)C(2)@C(84), Ti(2)C(2)@C(78), Y(2)C(2)@C(82), Sc(3)C(2)@C(80), as well as Sc(3)CH@C(80) and Sc(4)O(x)@C(80) (x=2, 3), that is, 42 EMF molecules and ions in total. Analysis of the delocalization indices and bond critical point (BCP) indicators such as G(bcp)/rho(bcp), H(bcp)/rho(bcp), and |V(bcp)|/G(bcp), revealed that all types of bonding in EMFs exhibit a high degree of covalency, and the ionic model is reasonable only for the Ca-based EMFs. Metal-metal bonds with negative values of the electron-density Laplacian were found in Y(2)@C(82), Y(2)@C(79)N, Sc(4)O(2)@C(80), and anionic forms of La(2)@C(80). A delocalized nature of the metal-cage bonding results in a topological instability of the electron density in EMFs with an unpredictable number of metal-cage bond paths and large elipticity values.

Bonding in Endohedral Metallofullerenes as Studied by Quantum Theory of Atoms in Molecules

In addition to the underlying basic concepts and early recognition of halogen bonding, this paper reviews the conflicting views that consistently appear in the area of noncovalent interactions and the ability of covalently bonded halogen atoms in molecules to participate in noncovalent interactions that contribute to packing in the solid-state. It may be relatively straightforward to identify Type-II halogen bonding between atoms using the conceptual framework of σ-hole theory, especially when the interaction is linear and is formed between the axial positive region (σ-hole) on the halogen in one monomer and a negative site on a second interacting monomer. A σ-hole is an electron density deficient region on the halogen atom X opposite to the R–X covalent bond, where R is the remainder part of the molecule. However, it is not trivial to do so when secondary interactions are involved as the directionality of the interaction is significantly affected. We show, by providing some specific examples, that halogen bonds do not always follow the strict Type-II topology, and the occurrence of Type-I and -III halogen-centered contacts in crystals is very difficult to predict. In many instances, Type-I halogen-centered contacts appear simultaneously with Type-II halogen bonds. We employed the Independent Gradient Model, a recently proposed electron density approach for probing strong and weak interactions in molecular domains, to show that this is a very useful tool in unraveling the chemistry of halogen-assisted noncovalent interactions, especially in the weak bonding regime. Wherever possible, we have attempted to connect some of these results with those reported previously. Though useful for studying interactions of reasonable strength, IUPAC’s proposed “less than the sum of the van der Waals radii” criterion should not always be assumed as a necessary and sufficient feature to reveal weakly bound interactions, since in many crystals the attractive interaction happens to occur between the midpoint of a bond, or the junction region, and a positive or negative site.

Halogen Bonding: A Halogen-Centered Noncovalent Interaction Yet to Be Understood

Topological analysis of the metal-metal bond: A tutorial review

Dispersion Makes the Difference: Bisligated Transition States Found for the Oxidative Addition of Pd(PtBu3)2 to Ar-OSO2R and Dispersion-Controlled Chemoselectivity in Reactions with Pd[P(iPr)(tBu2)]2

Experimental charge densities for (C5H5)Mn(CO)3 (2), (η6-C6H6)Cr(CO)3 (3), and (E)-{(η5-C5H4)CF═CF(η5-C5H4)}(η5-C5H5)2Fe2 (4) have been obtained by multipole refinement of high-resolution X-ray diffraction data at 100 K. The resultant densities were analyzed using the quantum theory of atoms in molecules (QTAIM). The electronic structures of these and related π-hydrocarbyl complexes have also been studied by ab initio density functional theory calculations, and a generally good agreement between theory and experiment with respect to the topological parameters was observed. The topological parameters indicate significant metal−ring covalency. A consistent area of disagreement concerns the topology of the metal−ring interactions. It is shown that because of the shared-shell bonding between the metal and the ring carbons, an annulus of very flat density ρ and very small ▽ρ is formed, which leads to topologically unstable structures close to catastrophe points. This in turn leads to unpredictable numbers of m...

The QTAIM approach to chemical bonding between transition metals and carbocyclic rings: a combined experimental and theoretical study of (η5-C5H5)Mn(CO)3, (η6-C6H6)Cr(CO)3, and (E)-{(η5-C5H4)CF=CF(η5- C5H4)}(η5-C5H5)2Fe2

Max Roemer

Papers

The QTAIM approach to chemical bonding between transition metals and carbocyclic rings: a combined experimental and theoretical study of (η5-C5H5)Mn(CO)3, (η6-C6H6)Cr(CO)3, and (E)-{(η5-C5H4)CF=CF(η5- C5H4)}(η5-C5H5)2Fe2