Aufbau Principle for Diffuse Electrons of Double-Shell Metal Ammonia Complexes: The Case of M(NH3)4@12NH3, M = Li, Be+, B2.

Positively charged or neutral metal ammonia complexes can form molecular species called solvated electron precursors (SEPs) that accommodate peripheral electrons in approximately hydrogenic diffuse orbitals. This work expands the notion of SEPs to metal ammonia complexes wherein a second coordination shell with 12 ammonia molecules is attached to M(NH3)4 (M = Li, Be+, B2+) SEPs via hydrogen bonding. In such complexes, denoted M(NH3)4@12NH3, the 12 outer ammonia molecules displace the peripheral electrons even further away from the first shell of ammonia molecules. We have benchmarked several density functional methods against CCSD(T) results and found that CAM-B3LYP provides the best M(NH3)4@12NH3 structures. The electron attachment energies of the closed-shell cores calculated with electron-propagator methods and the corresponding Dyson orbitals reveal the Aufbau principle for the ground and excited states of M(NH3)4@12NH3 to be 1s, 1p, 1d, 1f, 2s, 2p, 1g, 2d. These orbitals are diffuse and delocalized over the periphery of the second solvation shell.