Alkali-Metal-Supported Be-X (X = C, N, and B) Quadruple Orbital Overlapping Interactions

The formation of quadruple bonds in main-group chemistry has long been constrained by fundamental electronic limitations, notably the electron deficiency and inactive d orbitals of early second-row elements. Herein, we propose a series of alkali-metal-supported Be-XLi_n ^z (X = B, C, and N; n = 2 and 3; z = +1, 0, and -1) complexes. In particular, the quadruple orbital interaction patterns in octet valence electron structures such as BeBLi₃, BeCLi₂, BeCLi₃ ⁺, and BeNLi₂ ⁺ have rarely been reported. Lithium atoms effectively mitigate the electron deficiency of the core Be-X unit, enabling eight- or ten-valence electron systems that feature distinct Be-X quadruple orbital interactions with coexisting polar and delocalized characters. Comprehensive bonding analyses, including orbital composition, EDA-NOCV, adaptive natural density partitioning (AdNDP), atoms in molecules (AIM), and Electron Localization Function (ELF), confirm that this multiple bond consists of a highly polar Be → XLi_n ^z σ component and three reverse-dative Be ← XLi_n ^z interactions, which are formed by one σ and two π bonding modes. This study not only further extends the system of quadruple bonding interactions between Be and second-row elements (B, C, and N) but also presents a series of eight-valence electron environments in BeBLi₃, BeCLi₂, BeCLi₃ ⁺, and BeNLi₂ ⁺ clusters with fewer electrons, deepening the understanding of the nature of multiple bonds in main-group elements.