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. 2023 Jul 14;14(33):8785-8791.
doi: 10.1039/d2sc05939h. eCollection 2023 Aug 23.

Planar pentacoordinate s-block metals

Meng-Hui Wang  1 , Amlan J Kalita  2 , Mesías Orozco-Ic  3 , Gai-Ru Yan  1 , Chen Chen  1 , Bing Yan  1 , Gabriela Castillo-Toraya  4 , William Tiznado  5 , Ankur K Guha  2 , Sudip Pan  1 , Gabriel Merino  4 , Zhong-Hua Cui  1   6
Affiliations

Planar pentacoordinate s-block metals

Meng-Hui Wang et al. Chem Sci. .

Abstract

The presence of a delocalized π-bond is often considered an essential criterion for achieving planar hypercoordination. Herein, we show that σ-delocalization could be sufficient to make the planar configuration the most stable isomer in a series of planar pentacoordinate s-block metals. High-level ab initio computations reveal that the global minimum of a series of interalkali and interalkali-alkaline earth clusters (LiNa5, Li5Mg+, Na5Mg+, K5Ca+, CaRb5+, Rb5Sr+, and SrCs5+) adopts a singlet D5h structure with a planar pentacoordinate lithium or alkaline earth metal (AE = Mg, Ca, Sr). These clusters are unusual combinations to stabilize a planar pentacoordinate atom, as all their constituents are electropositive. Despite the absence of π-electrons, Hückel's rule is fulfilled by the six σ-electrons. Furthermore, the systems exhibit a diatropic ring current in response to an external magnetic field and a strong magnetic shielding, so they might be classified as σ-aromatic. Therefore, multicenter σ-bonds and the resulting σ-delocalization stabilize these clusters, even though they lack π-aromaticity.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. PBE0-D3/def2-QZVPP geometries of the low-lying energy isomers of LiM5 (M = Na/K/Rb). Relative energies in kcal mol−1 were computed at the CCSD(T)/def2-QZVPP//PBE0-D3/def2-QZVPP level, including the zero-point energy computed at the DFT level. Point groups and spectroscopic states are given in parentheses.
Fig. 2
Fig. 2. PBE0-D3/def2-QZVPP geometries of the low-lying energy isomers of M5Mg+ (M = Li/Na). Relative energies in kcal mol−1 were computed at the CCSD(T)/def2-QZVPP//PBE0-D3/def2-QZVPP level, including the zero-point energy calculated at the DFT level. Point groups and spectroscopic states are given in parentheses. For Li5Mg+, AE6 converges to AE2.
Fig. 3
Fig. 3. In the top panel: the Bindz isolines plotted in the molecular plane (left) and a transversal slide (right). In the bottom panel: Jind maps plotted near the molecular plane. The arrows indicate the direction of the current density. The |Jind| scale is in atomic units (1 au = 100.63 nA T−1 Å−2). The external magnetic field is oriented parallel to the z-axis, perpendicular to the molecular plane.
See this image and copyright information in PMC

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