Theoretical observation of hexaatomic molecules containing pentacoordinate planar carbon

Structure and Bonding in Planar Hypercoordinate Carbon Compounds

The term hypercoordination refers to the extent of the coordination of an element by its normal value. In the hypercoordination sphere, the element can achieve planar and/or non-planar molecular shape. Hence, planar hypercoordinate carbon species violate two structural rules: (i) The highest coordination number of carbon is four and (ii) the tetrahedral orientation by the connected elements and/or groups. The unusual planar orientations are mostly stabilized by the electronic interactions of the central atom with the surrounding ligands. In this review article, we will talk about the current progress in the theoretical prediction of viable planar hypercoordinate carbon compounds. Primary knowledge of the planar hypercoordinate chemistry will lead to its forthcoming expansion. Experimental and theoretical interests in planar tetracoordinate carbon (ptC), planar pentacoordinate carbon (ppC), and planar hexacoordinate carbon (phC) are continued. The proposed electronic and mechanical strategies are helpful for the designing of the ptC compounds. Moreover, the 18-valence electron rule can guide the design of new ptC clusters computationally as well as experimentally. However, the counting of 18-valence electrons is not a requisite condition to contain a ptC in a cluster. Furthermore, this ptC idea is expanded to the probability of a greater coordination number of carbon in planar orientations. Unfortunately, until now, there are no such logical approaches to designing ppC, phC, or higher-coordinate carbon molecules/ions. There exist a few global minimum structures of phC clusters identified computationally, but none have been detected experimentally. All planar hypercoordinate carbon species in the global minima may be feasible in the gas phase.

Binary mono-anions with unprecedented anti-aromatic planar tetracoordinate carbon and nitrogen atoms

Global minima (CBe₄^- and NBe₄^-) with planar tetracoordinate carbon and nitrogen atoms are stabilized by multicentric bonds. Their unusual anti-aromaticity is due to the strong localized Be-Be bonds preventing the full delocalization of the electrons in the CBe₄/NBe₄ skeleton. These binary mono-anions are expected to be experimentally realized in the gas phase.

σ-Aromaticity Planar Pentacoordinate Beryllium Atoms

Six-valence-electron planar pentacoordinate beryllium (ppBe) is explored herein as a global minimum, which is only constructed by s-block metals in BeM₅⁺ (M = Cu, Ag, Au). The bonding in ppBe can be regarded as the excited-stated Be with a 2p_x¹2p_y¹ electronic configuration, forming electron sharing with doublet M₅⁺ motifs followed by two sets of Be(p_∥) → [M₅⁺] σ donations and one Be(s) ← [M₅⁺] σ back-donation. Thus, the σ aromaticity originating from three delocalized σ orbitals gives rise to the whole stability of the high D_5h-symmetry ppBe and strongly enriches s-block planar hypercoordinate bonding.

Avoided spin coupling: an unexpected σ-σ diradical in global planar pentacoordinate carbon

We present a global planar pentacoordinate carbon (ppC) featuring a hitherto unreported σ-σ diradical characteristic. Using the multi-reference approach combined with the CCSD(T)/aug-cc-pVTZ method, the ppC C₃Li₃^- was found to be an intriguing triplet ground state, in which the unpaired density is mostly located at three Li ligands. Chemical bonding analysis reveals that the 2p_zπ electrons of C₃Li₃^- are fully located at the C₃ ring formed by C-C multiple bonds, in contrast to the perfect 2p_zπ-delocalization found in the well-known ppCs.

Ternary 12-electron CBe3X3+ (X = H, Li, Na, Cu, Ag) clusters: planar tetracoordinate carbons and superalkali cations

Molecules with planar tetracoordinate carbons (ptCs) are exotic in chemical bonding, and they are normally designed according to the 18-electron rule. Here we report on the viability of ptC clusters with as few as 12 valence electrons, which represent the lower limit in terms of electron counting. Specifically, we have computationally designed a class of ternary 12-electron ptC clusters, CBe3X3+ (X = H, Li, Na, Cu, Ag), based on a rhombic CBe32- unit. Computer structural searches reveal that the ptC species are global minima, whose C center is coordinated in-plane by three Be atoms and a terminal X atom via robust C-Be/C-X bonding, either covalent or ionic. The other two X atoms are on the periphery and each bridge two Be atoms. Bonding analyses show that the ptC core is governed by delocalized 2π/6σ bonding, that is, double π/σ aromaticity, which collectively conforms to the 8-electron counting. Additional 4 electrons contribute to peripheral Be-X-Be and Be-Be σ bonding. The delocalized 2π/6σ frameworks appear to be universal for all ptC clusters, ranging from 18-electron down to 12-electron systems. In other words, the ptC species are dictated entirely by the 8-electron counting. Predicted vertical electron affinities of these ptC clusters range from 3.13 to 5.48 eV, indicative of superalkali or pseudoalkali cations.

Stabilization of beryllium-containing planar pentacoordinate carbon species through attaching hydrogen atoms

The diagonal relationship between beryllium and aluminum and the isoelectronic relationship between BeH unit and Al atom were utilized to design nine new planar and quasi-planar pentacoordinate carbon (ppC) species CAl n Be m H x q (n + m = 5, q = 0, ±1, x = q + m - 1) (1a-9a) by attaching H atoms onto the Be atoms in CAl4Be, CAl3Be2 -, CAl2Be3 2-, and CAlBe4 3-. These ppC species are σ and π double aromatic. In comparison with their parents, these H-attached molecules are more stable electronically, as can be reflected by the more favourable alternative negative-positive-negative charge-arranging pattern and the less dispersed peripheral orbitals. Remarkably, seven of these nine molecules are global energy minima, in which four of them are kinetically stable, including CAl3Be2H (2a), CAl2Be3H- (4a), CAl2Be3H2 (5a), and CAlBe4H4 + (9a). They are the promising target for the experimental realization of species with a ppC.

Planar pentacoordinate carbons in CBe54− derivatives

Ab initio computations show that the global minimum structure of the CBe₅Li_nⁿ⁻⁴ clusters (n = 1 to 5) contains a planar pentacoordinate carbon atom.