Electron-counting rules are promising for determination of chemically stable complexes for various elemental groups. While the 18-electron rule has been established for transition metals, recent experiments have discovered that alkaline-earth metals may follow a 20-electron rule when coordinated with CO or benzene. Herein, by employing first-principles calculations at the level of the generalized gradient approximation functional combined with the def2-SVP basis set, we theoretically predict a series of stable 20-electron exohedral alkaline-earth metallofullerenes, in which each alkaline-earth metal is η6-coordinated with three C60s, leading to stable trigonal pyramidal structures. Molecular orbital calculation and energy decomposition analysis indicate that not only (n)s orbitals but also (n − 1)d(n)p orbitals of the alkaline-earth metal atoms participate in bonding interactions with the fullerenes. Energy decomposition analysis reveals that the bonding between metal atoms and C60 can be divided into weak donation from C60 to metal d orbitals and strong back-donation from the metal d orbitals to C60, consistent with 20-electron metal complexes. Based on stable trigonal pyramidal building blocks, we designed a series of two-dimensional assembled materials with fullerenes forming a Kagome sublattice and metals forming a honeycomb sublattice, possessing topologically nontrivial flat bands with exotic quantum properties.

Link：20-electron exohedral alkaline-earth metallofullerenes M(C60)3 (M = Ca, Sr, and Ba) - Physical Chemistry Chemical Physics (RSC Publishing)