d-p Hybridization Induced Open-Shell Planar Four-Membered Transition Metal Carbide Clusters with Double Möbius Aromaticity.

Aromaticity is one of the most classical concepts in the field of modern chemistry and has been employed to explain and design substances with special stability. Although the knowledge about Hückel's and Baird's rules has been well established, the understanding of Möbius aromaticity remains extremely limited. In this letter, by employing density functional theory (DFT) calculations, we demonstrated that the four-membered VIB transition metal (TM) carbide clusters possess a highly stable open-shell planar tetrameric structure and exhibit double Möbius aromaticity, which was evidenced by analyzing multiple aromaticity criteria, including the electronic, magnetic, and energetic indicators. Each cluster was characterized by four delocalized π electrons and four delocalized σ electrons, forming a novel class exhibiting double Möbius aromaticity. Intriguingly, the unexpected stability of these open-shell clusters was suggested to arise from the hybridization of d-p atomic orbitals, as revealed by analysis of the composition of delocalized orbitals. Our findings highlight the significance of hybridization between the d orbitals of transition metals and the p orbitals of main group elements in the creation of dual Möbius aromatic species, which offers new avenues for the design of single-molecule magnetic inorganic materials.