Bonding Nature of B2L2-SiR2 (L = NHC, Silylene; R = H, Cl): Metallomimetic, Silylene, or Diboryne?

Low-valent main-group compounds represent a highly active frontier field, owing to their unique bonding characteristics and transition-metal-like reactivity in activating inert small molecules. As prototypical systems, diborynes and silylenes have attracted considerable attention, yet the fundamental nature of their bonding remains largely elusive. Herein, we report a systematic theoretical study of the bonding nature and substituent effects in B2L2-SiR2 complexes (L = NHC, silylene; R = H, Cl) using density functional theory (DFT) and the block-localized wave function energy decomposition (BLW-ED) method at the M06-2X-D3/6-311+G(d) level. Our calculations reveal that SiH2 undergoes a HOMO-LUMO inversion induced by Pauli repulsion upon approaching diborynes, thereby enabling a synergistic Dewar-Chatt-Duncanson (DCD)-type orbital interaction. In this mode, it is the silylene that behaves like a metallomimetic center with π//-back-donation as the dominant charge-transfer contributor. By comparison, transition-metal silylene complexes are governed by electrostatic attraction, and SiH2 simply acts as a classical σ-donor ligand with negligible π-back-donation. Chlorine substitution imposes divergent effects on the two types of interactions. These findings unveil the bonding behavior of silylenes and advance the fundamental understanding of metallomimetic chemistry in low-valent main-group systems, offering valuable theoretical insights for the rational design of novel main-group complexes.