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This study examines systems containing planar tetracoordinate group 13–15 atoms (E) within pentagonal C4H2E rings bridged by Si or Ge atoms. A detailed chemical bonding analysis of eleven candidates shows that true tetracoordination is achieved only in the C4H2NGe2+ system. [ABSTRACT FROM AUTHOR]

Planar boron clusters have often been regarded as "π‐analogous" to aromatic arenes because of their similar delocalized π‐bonding. However, unlike arenes such as C5H5− and C6H6, boron clusters have not previously shown the ability to form sandwich complexes. In this study, we present the first sandwich complex involving beryllium and boron, B7Be6B7. The global minimum of this combination adopts a unique architecture having a D6h geometry, featuring an unprecedented monocyclic Be6 ring sandwiched between two quasi‐planar B7 motifs. The thermochemical and kinetic stability of B7Be6B7 can be attributed to strong electrostatic and covalent interactions between the fragments. Chemical bonding analysis shows that B7Be6B7 can be considered as a [B7]3−[Be6]6+[B7]3− complex. Moreover, there is a significant electron delocalization within this cluster, supported by the local diatropic contributions of the B7 and Be6 fragments. [ABSTRACT FROM AUTHOR]

Here we show that substituting the ten protons in the dianion of a bispentalene derivative (C18H102−) by six Si2+ dications produces a minimum energy structure with two planar tetracoordinate carbons (ptC). In Si6C18, the ptCs are embedded in the terminal C5 pentagonal rings and participate in a three‐center, two‐electron (3c‐2e) Si‐ptC‐Si σ‐bond. Our exploration of the potential energy surface identifies a triphenylene derivative as the putative global minimum. Nevertheless, robustness to Born–Oppenheimer molecular dynamics (BOMD) simulations at 900 and 1500 K supports bispentalene derivative kinetic stability. Chemical bonding analysis reveals ten delocalized π‐bonds, which, according to Hückel's 4n + 2 π‐electron rule, would classify it as an aromatic system. Magnetically induced current density analysis reveals the presence of intense local paratropic currents and a weakly global diatropic current, the latter agreeing with the possible global aromatic character of this specie. [ABSTRACT FROM AUTHOR]

Here, it is shown that the M3B12 (M = Cu-Au) clusters' global minima consist of an elongated planar B12 fragment connected by an in-plane linear M3 fragment. This result is striking since this B12 planar structure is not favored in the bare cluster, nor when one or two metals are added. The minimum energy structures were revealed by screening the potential energy surface using genetic algorithms and density functional theory calculations. Chemical bonding analysis shows that the strong electrostatic interactions with the metal compensate for the high energy spent in the M3 and B12 fragment distortion. Furthermore, metals participate in the delocalized π-bonds, which infers an aromatic character to these species. [ABSTRACT FROM AUTHOR]

Recently, we reported a series of global minima whose structures consist of carbon rings decorated with heavier group 14 elements. Interestingly, these structures feature planar tetracoordinate carbons (ptCs) and result from the replacement of five or six protons (H+) from the cyclopentadienyl anion (C5H5−) or the pentalene dianion (C8H62−) by three or four E2+ dications (E = Si–Pb), respectively. The silicon derivatives of these series are the Si3C5 and Si4C8 clusters. Here we show that ptC persists in some clusters with an equivalent number of C and Si atoms, i.e., Si5C5, Si8C8, and Si9C9. In all these species, the ptC is embedded in a pentagonal C5 ring and participates in a three-center, two-electron (3c-2e) Si-ptC-Si σ-bond. Furthermore, these clusters are π-aromatic species according to chemical bonding analysis and magnetic criteria. [ABSTRACT FROM AUTHOR]

After exploring the potential energy surfaces of MmCE2p (E=S−Te, M=Li−Cs, m=2, 3 and p=m‐2) and MnCE3q (E=S−Te, M=Li−Cs, n=1, 2, q=n‐2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V‐shaped CE22− and Y‐shaped CE32− dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals. [ABSTRACT FROM AUTHOR]

Herein, the first global minima containing a planar hexacoordinate carbon (phC) atom are reported. The fifteen structures belong to the CE3M3+ (E=S–Te and M=Li–Cs) series and satisfy both geometric and electronic criteria to be considered as a true phC. The design strategy consisted of replacing oxygen in the D3h CO3Li3+ structure with heavy and less electronegative chalcogens, inducing a negative charge on the C atom and an attractive electrostatic interaction between C and the alkali‐metal cations. The chemical bonding analyses indicate that carbon is covalently bonded to three chalcogens and ionically connected to the three alkali metals. [ABSTRACT FROM AUTHOR]

A simple and chemically intuitive approach is used to design ptC-containing E–C clusters (E = Si–Pb). This approach consists in replacing three or two consecutive protons from an aromatic hydrocarbon by one E24+ or one E2+ fragment, respectively. In the model, electrons from E are removed from the pz orbitals, emptying them. Si–Pb favors the formation of a 3c-2e (E–C–E) σ-bond, which involves the ptC. Additionally, the π-electronic cloud is delocalized through the E-pz orbitals allowing the E atoms to effectively take part in the electronic delocalization, preserving the 4n + 2 Hückel's rule from the parent hydrocarbon. Two aromatic monocycles and one aromatic bicycle – benzene (C6H6), cyclopentadienyl anion (C5H5−) and pentalene dianion (C8H62−) – have been transformed into C–E systems. After an extensive exploration of their potential energy surfaces, four new global minima with ptC are identified, resulting from the substitution of the protons by Si and Ge cations in C5H5− and C8H62− (E3C5 and E4C8). The analysis of both the chemical bonding and the magnetic response to an external magnetic field confirms the aromatic character of these species. [ABSTRACT FROM AUTHOR]

The nature of the chemical bonds in the EC3+, EC4+, EC5+, and EC6+ species (E = Sc, Y and La) has been examined in depth using a combination of different theoretical strategies. Firstly, exhaustive explorations of the relevant potential energy surfaces using a combination of stochastic and gradient-based search procedures provide us with fan-like structures in singlet and triplet spin states as the most energetically stable configurations. Secondly, the chemical bonding analysis, using natural bond orbital theory, energy decomposition analysis, quantum theory of atoms in molecules, and the adaptive natural density partitioning analysis, indicate that metal–carbon interaction in these species is best described as an intermediate point between ionic and covalent. [ABSTRACT FROM AUTHOR]

A series of small planar boron clusters has extensively been studied in the past using different theoretical approximations, and their chemical bonding has been rationalized in terms of aromaticity, antiaromaticity and conflicting aromaticity. Here, we study these systems by means of our recently proposed orbital localization procedure based on the partitioning of the space according to the electron localization function (ELF) topology. The results are compared with those obtained from the adaptive natural density partitioning (AdNDP) method, which is a most extensively tested orbital localization procedure. Minor discrepancies have been found, especially in large clusters. In those cases, an alternative set of localized AdNDP orbitals recovered the representation obtained by ELF localization procedure. These results support the need for multicenter bonding incorporation into the localization models for rationalizing chemical bonding in atomic clusters. Additionally, the aromatic character of the clusters, when it is present, is adequately supported by the more classical treatment based on the ELF topological analysis. [ABSTRACT FROM AUTHOR]