We report a Na:−→B dative bond in the NaBH3− cluster, which was designed on the principle of minimum‐energy rupture, prepared by laser vaporization, and characterized by a synergy of anion photoelectron spectroscopy and electronic structure calculations. The global minimum of NaBH3− features a Na−B bond. Its preferred heterolytic dissociation conforms with the IUPAC definition of dative bond. The lone electron pair revealed on Na and the negative Laplacian of electron density at the bond critical point further confirm the dative nature of the Na−B bond. This study represents the first example of a Lewis adduct with an alkalide as the Lewis base.

Both hydrogen bonding (HB) and halogen bonding (XB) are essentially electrostatic interactions, but whereas hydrogen bonding has a well‐documented record of stabilizing unstable anions, little is known about halogen bonding's ability to do so. Herein, we present a combined anion photoelectron spectroscopic and density functional theory study of the halogen bond‐stabilization of the pyrazine (Pz) anion, an unstable anion in isolation due to its neutral counterpart having a negative electron affinity (EA). The halogen bond formed between the σ‐hole on bromobenzene (BrPh) and the lone pair(s) of Pz significantly lowers the energies of the Pz(BrPh)1− and Pz(BrPh)2− anions relative to the neutral molecule, resulting in the emergence of a positive EA for the neutral complexes. As seen through its charge distribution and electrostatic potential analyses, the negative charge on Pz− is diluted due to the XB. Thermodynamics reveals that the low temperature of the supersonic expansion plays a key role in forming these complexes.

We reply to the comment by S. Pan and G. Frenking who challenged our interpretation of the Na−:→BH3 dative bond in the recently synthesized NaBH3− cluster. Our conclusion remains the same as that in our original paper (https://doi.org/10.1002/anie.201907089 and https://doi.org/10.1002/ange.201907089). This conclusion is additionally supported by the energetic pathways and NBO charges calculated at UCCSD and CASMP2(4,4) levels of theory. We also discussed the suitability of the Laplacian of electron density (QTAIM) and Adaptive Natural Density Partitioning (AdNDP) method for bond type assignment. It seems that AdNDP yields more sensible results. This discussion reveals that the complex realm of bonding is full of semantic inconsistencies, and we invite experimentalists and theoreticians to elaborate this topic and find solutions incorporating different views on the dative bond.

Some transition metal borides are ultra hard. While not harder than diamond, they are easier to process and can be cheaper, sparking intense interest. However, we so far cannot predict which particular borides should be ultra hard. A striking example is the three structurally similar diborides of Ti, Re, and Os, among which only ReB 2 is ultra hard. For this trio, using a combination of theory and experiment done on both the solids and small cluster models, we show that the nature of the metal-boron bonds is the key to hardness, in contrast to the existing theory, which overlooks metal-boron interactions. Ti-B bonding is purely ionic in TiB 2 , and the material yields to shear stress like graphite. OsB 2 is highly covalent, with both bonding and antibonding Os-B backbonds present, which weaken the Bnetwork, and ease the OsB 2 yield to compression. ReB 2 has only the bonding Re-B σ-backbond, which strengthens the material against both shear and compression. A general strategy for ultra hard boride design is proposed.

The results of a combined experimental and computational study of the uranium atom are presented with the aim of determining its electron affinity. Experimentally, the electron affinity of uranium was measured via negative ion photoelectron spectroscopy of the uranium atomic anion, U−. Computationally, the electron affinities of both thorium and uranium were calculated by conducting relativistic coupled-cluster and multi-reference configuration interaction calculations. The experimentally determined value of the electron affinity of the uranium atom was determined to be 0.309 ± 0.025 eV. The computationally predicted electron affinity of uranium based on composite coupled cluster calculations and full four-component spin–orbit coupling was found to be 0.232 eV. Predominately due to a better convergence of the coupled cluster sequence for Th and Th−, the final calculated electron affinity of Th, 0.565 eV, was in much better agreement with the accurate experimental value of 0.608 eV. In both cases, the ground state of the anion corresponds to electron attachment to the 6d orbital.