Major differences between preferred tetracarbagallane and tetracarbalane structures

Abstract

The complete series of tetracarbagallanes C4Gan–4Men (6 ≤ n ≤ 14) has been examined by density functional theory. The lowest-energy C4Ge6Me10 structure is a very symmetrical Td tetracapped octahedral structure with an adamantane-like surface very similar to the experimentally known R'4C4Ga6R6 structures and to the lowest energy structure for the tetracarbalane C4Al6Me10. This structure is the only lowest energy C4Gan–4Men (n = 6 to 14) structure containing isolated carbon atoms. The remaining low-energy tetracarbagallane structures consist of Gan–4 subpolyhedra having faces capped by aggregates of two to four carbon atoms. The most commonly encountered partitions of the four carbon atoms in the C4Gan–4Men structures are C3 + C or 2 × C2. Such capping often leads to significant distortion of the Gan–4 subpolyhedron including lengthening of some of the original edges beyond bonding distances. Many of the low-energy 6- to 9-vertex C4Gan–4Men structures are recognized as nido structures in accord with their 2n + 4 skeletal electrons determined by the Wade-Mingos procedure. Such structures can be derived from an (n + 1)-vertex deltahedron by removal of a degree 5 or 6 vertex and thus have a pentagonal or hexagonal open face. The low-energy structures for the 6- and 7-vertex tetracarbagallanes C4Gan–4Men (n = 6, 7) resemble those of their aluminum analogues.