Boron is an interesting element with unusual polymorphism. While three-dimensional (3D) structural motifs are prevalent in bulk boron, atomic boron clusters are found to have planar or quasi-planar structures, stabilized by localized two-center-two-electron (2c-2e) σ bonds on the periphery and delocalized multicenter-two-electron (nc-2e) bonds in both σ and π frameworks. Electron delocalization is a result of boron's electron deficiency and leads to fluxional behavior, which has been observed in B13(+) and B19(-). A unique capability of the in-plane rotation of the inner atoms against the periphery of the cluster in a chosen direction by employing circularly polarized infrared radiation has been suggested. Such fluxional behaviors in boron clusters are interesting and have been proposed as molecular Wankel motors. The concepts of aromaticity and antiaromaticity have been extended beyond organic chemistry to planar boron clusters. The validity of these concepts in understanding the electronic structures of boron clusters is evident in the striking similarities of the π-systems of planar boron clusters to those of polycyclic aromatic hydrocarbons, such as benzene, naphthalene, coronene, anthracene, or phenanthrene. Chemical bonding models developed for boron clusters not only allowed the rationalization of the stability of boron clusters but also lead to the design of novel metal-centered boron wheels with a record-setting planar coordination number of 10. The unprecedented highly coordinated borometallic molecular wheels provide insights into the interactions between transition metals and boron and expand the frontier of boron chemistry. Another interesting feature discovered through cluster studies is boron transmutation. Even though it is well-known that B(-), formed by adding one electron to boron, is isoelectronic to carbon, cluster studies have considerably expanded the possibilities of new structures and new materials using the B(-)/C analogy. It is believed that the electronic transmutation concept will be effective and valuable in aiding the design of new boride materials with predictable properties. The study of boron clusters with intermediate properties between those of individual atoms and bulk solids has given rise to a unique opportunity to broaden the frontier of boron chemistry. Understanding boron clusters has spurred experimentalists and theoreticians to find new boron-based nanomaterials, such as boron fullerenes, nanotubes, two-dimensional boron, and new compounds containing boron clusters as building blocks. Here, a brief and timely overview is presented addressing the recent progress made on boron clusters and the approaches used in the authors' laboratories to determine the structure, stability, and chemical bonding of size-selected boron clusters by joint photoelectron spectroscopy and theoretical studies. Specifically, key findings on all-boron hydrocarbon analogues, metal-centered boron wheels, and electronic transmutation in boron clusters are summarized.

Two-dimensional (2D) materials with planar hypercoordinate motifs are extremely rare due to the difficulty in stabilizing the planar hypercoordinate configurations in extended systems. Furthermore, such exotic motifs are often unstable. We predict a novel Cu2Si 2D monolayer featuring planar hexacoordinate copper and planar hexacoordinate silicon. This is a global minimum in 2D space which displays reduced dimensionality and rule-breaking chemical bonding. This system has been studied with density functional theory, including molecular dynamics simulations and electronic structure calculations. Bond order analysis and partitioning reveals 4c-2e σ bonds that stabilize the two-dimensional structure. We find that the system is quite stable during short annealing simulations up to 900 K, and predict that it is a nonmagnetic metal. This work opens up a new branch of hypercoordinate two-dimensional materials for study.

Recent developments in crystal structure prediction, in particular, the powerful evolutionary algorithm USPEX [1,2], enable reliable prediction of stable compounds formed by given elements. At normal conditions such calculations produce the well-known stable compounds: e.g., NaCl as the only compound of Na and Cl, or MgO as the only stable compound of Mg and O. At high pressures and in low-dimensional materials, unexpected phenomena have been predicted-then experimentally verified. I will discuss several recent examples: 1. Discovery of two new stable high-pressure compounds of helium, Na2He and Na2HeO (Na2He has been synthesized experimentally) [3]. This discovery has implications for both fundamental chemistry and planetary sciences. 2. Formation of new stable sodium chlorides: Na3Cl, Na2Cl, Na3Cl2, NaCl3, NaCl7 [4], Na4Cl3 [5], and a large number of new stable potassium chlorides [6]. These predictions were verified experimentally [5,6] and are still not fully understood. 3. New stable magnesium oxides: Mg3O2 and MgO2 [7] and MgO3 [8], and silicon oxides SiO and SiO3 [8]. Among these predictions, stability of MgO2 has already been experimentally confirmed [9]. These predictions may have implications for planetary chemistry. 4. USPEX-based prediction of the Cui group [10] and experimental verification of Eremets group [11] of a new high-temperature superconductor-cubic H3S. This discovery opens new hopes for room-temperature superconductivity. 5. Prediction [12] that dominant silicon oxide nanoparticles at normal conditions (ambient P-T, and normal air) will be oxygen-enriched and magnetic: e.g. Si7O19. This may explain well-documented carcinogenic activity of fine silica dust. Future avenues for explanation and generalization of these phenomena will be discussed. 1] Oganov A.R. et al, J.

The electron deficiency and strong bonding capacity of boron have led to a vast variety of molecular structures in chemistry and materials science. Here we report the observation of highly symmetric cobalt-centered boron drum-like structures of CoB16−, characterized by photoelectron spectroscopy and ab initio calculations. The photoelectron spectra display a relatively simple spectral pattern, suggesting a high symmetry structure. Two nearly degenerate isomers with D8d (I) and C4v (II) symmetries are found computationally to compete for the global minimum. These drum-like structures consist of two B8 rings sandwiching a cobalt atom, which has the highest coordination number known heretofore in chemistry. We show that doping of boron clusters with a transition metal atom induces an earlier two-dimensional to three-dimensional structural transition. The CoB16− cluster is tested as a building block in a triple-decker sandwich, suggesting a promising route for its realization in the solid state.

We analyze the chemical bonding in graphene using a fragmental approach, the adaptive natural density partitioning method, electron sharing indices, and nucleus-independent chemical shift indices. We prove that graphene is aromatic, but its aromaticity is different from the aromaticity in benzene, coronene, or circumcoronene. Aromaticity in graphene is local with two π-electrons delocalized over every hexagon ring. We believe that the chemical bonding picture developed for graphene will be helpful for understanding chemical bonding in defects such as point defects, single-, double-, and multiple vacancies, carbon adatoms, foreign adatoms, substitutional impurities, and new materials that are derivatives of graphene.

Small boron clusters are known to be planar, and may be used as ligands to form novel coordination complexes with transition metals. Here we report a combined photoelectron spectroscopy and ab initio study of CoB12(-) and RhB12(-). Photoelectron spectra of the two doped-B12 clusters show similar spectral patterns, suggesting they have similar structures. Global minimum searches reveal that both CoB12(-) and RhB12(-) possess half-sandwich-type structures with the quasi-planar B12 moiety coordinating to the metal atom. The B12 ligand is found to have similar structure as the bare B12 cluster with C3v symmetry. Structures with Co or Rh inserted into the quasi-planar boron framework are found to be much higher in energy. Chemical bonding analyses of the two B12 half sandwiches reveal two sets of σ bonds on the boron unit: nine classical two-center-two-electron (2c-2e) σ bonds on the periphery of the B12 unit and four 3c-2e σ bonds within the boron unit. Both σ and π bonds are found between the metal and the B12 ligand: three M-B single σ bonds and one delocalized 4c-2e π bond. The exposed metal sites in these complexes can be further coordinated by other ligands or become reaction centers as model catalysts.

Synthetic strategies to yield molecular complexes of high-valent lanthanides, other than the ubiquitous Ce 4+ ion, are exceptionally rare, and thorough, detailed characterization in these systems is limited by complex lifetime and reaction and isolation conditions. The synthesis of high-symmetry complexes in high purity with significant lifetimes in solution and the solid state is essential for determining the role of ligand-field splitting, multiconfigurational behavior, and covalency in governing the reactivity and physical properties of these potentially technologically transformative tetravalent ions. We report the synthesis and physical characterization of an S 4 symmetric, fourcoordinate tetravalent terbium complex, [Tb(NP(1,2-bis-t Budiamidoethane)(NEt 2 )) 4 ] (where Et is ethyl and t Bu is tert-butyl). The ligand field in this complex is weak and the metal− ligand bonds sufficiently covalent so that the tetravalent terbium ion is stable and accessible via a mild oxidant from the anionic, trivalent, terbium precursor, [(Et 2 O)K][Tb(NP(1,2-bis-t Bu-diamidoethane)(NEt 2 )) 4 ]. The significant stability of the tetravalent complex enables its thorough characterization. The stepwise development of the supporting ligand points to key ligand control elements for further extending the known tetravalent lanthanide ions in molecular complexes. Magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, X-ray absorption near-edge spectroscopy (XANES), and density functional theory studies indicate a 4f 7 ground state for [Tb(NP(1,2-bis-t Bu-diamidoethane)(NEt 2 )) 4 ] with considerable zero-field splitting, demonstrating that magnetic, tetravalent lanthanide ions engage in covalent metal−ligand bonds. This result has significant implications for the use of tetravalent lanthanide ions in magnetic applications since the observed zero-field splitting is intermediate between that observed for the trivalent lanthanides and for the transition metals. The similarity of the multiconfigurational behavior in the ground state of [Tb(NP(1,2-bis-t Bu-diamidoethane)(NEt 2 )) 4 ] (measured by Tb L 3 -edge XAS) to that observed in TbO 2 implicates ligand control of multiconfigurational behavior as a key component of the stability of the complex.

We predict a two-dimensional (2D) antiferromagnetic (AFM) boron (designated as M -boron) by using ab initio evolutionary methodology. M -boron is entirely composed of B20 clusters in a hexagonal arrangement. Most strikingly, the highest valence band of M -boron is isolated, strongly localized, and quite flat, which induces spin polarization on either cap of the B20 cluster. This flat band originates from the unpaired electrons of the capping atoms, and is responsible for magnetism. M -boron is thermodynamically metastable and is the first magnetic 2D form of elemental boron.