Theoretical Study of Complexes of Closo-Borane, Alane, and Gallane Anions with Cations of Light Metals Inside and Outside of Icosahedral Clusters [A₁₂H₁₂^2-] (A = B, Al, and Ga)

Abstract

Systematic B3LYP/6-31G* and B3LYP/6-311+G* calculations have been performed for families of closo-borane, alane, and gallane clusters consisting of A 12 H 12 2- (A = B, Al, and Ga) cages with endohedral or exohedral L n + metal cations (L n + = Li +, Na +, Cu +, Be 2+, Mg 2+, Zn 2+, Al +, Al 3+, Ga +, and Ga 3+ ). Exohedral structure 1, with tridentate cation coordination at an icosahedron face, is a global minimum for most species; bidentate coordination (structure 2 ) is the transition state for cation migration around the dianion exterior. Migrational barriers ( h migr ), which range from 3 to 5 kcal/mol for monocations and 10−15 kcal/mol for dications, increase with increased cation charge and increased cationic radius. L n + @B 12 H 12 2- (L n + = Li +, Be 2+, Na +, Mg 2+, Al 3+ ), L n + @Al 12 H 12 2- (L n + = Li +, Na +, Mg 2+, Al + ), and L n + @Ga 12 H 12 2- (L n + = Li +, Na +, Mg 2+, Ga 3+ ) endohedral clusters, with their cations located at the A 12 H 12 2- cage centers, are local I h minima ( 3 ). Endohedral−exohedral isomer relative energies, E rel ( 3 / 1 ), which are very high for the boranes, decrease rapidly down the borane−alane−gallane group and decrease along the He−Li + −Be 2+ −B 3+ and Ne−Na + −Mg 2+ −Al 3+ isoelectronic series. Endohedral isomers of gallane clusters with heavy multicharged cations are predicted to be most favorable in energy. Two types of transition structures for a 3 → 1 endohedral−exohedral rearrangement exist: cation exit through an edge and L n + exit via a ruptured pentagonal “neck” of the cage. Li + and Be 2+ borane salts prefer the former pathway, whereas Li +, Na +, and Mg 2+ alane and gallane salts favor the latter mechanism. Cation exit barriers, h rear, range from ∼15−55 kcal/mol and in an isoelectronic series decrease with increasing cation charge and increasing atomic mass. Endohedral L n + @A 12 H 12 2- clusters show significant charge transfer from the anion to the cation; the hydrogen shell [H] 12 donates electrons to L n + via the internal [A] 12 shell, which in many cases serves as an electron “conductor”.