Chemical Bonding of Low-Valent Radical Magnesium Species and Computational Design of Low-Valent Radical Calcium Complexes

The synthesis of mononuclear low-valent calcium species remains a significant challenge. In this study, we initially investigated the electronic structure and bonding nature of experimentally synthesized mononuclear low-valent radical magnesium species (Am)Mg(CAAC). It reveals that the stability of this system arises from the CAAC ligand’s strong π-electron-accepting and σ-electron-donating properties, combined with the inductive effect of the Am ligand, which enhances electron transfer to the CAAC ligand. Building on these findings, we explore the computational design of stable mononuclear low-valent radical calcium complexes. However, a straightforward substitution of Mg with Ca does not inherently guarantee stability due to Ca’s larger atomic radius and distinct electronic configuration. To overcome this challenge, we introduce six-membered cyclic β-diketiminate (BDI) (BDI2–BDI6) ligands, which possess computational potential to stabilize the mononuclear low-valent radical calcium complexes by modulating electron flow toward the CAAC ligand and enhancing the involvement of Ca’s 3d orbitals in bonding. These computational insights, combined with our computational design strategies, provide a theoretical foundation for the development of potential mononuclear low-valent radical calcium complexes.