Hexacarbalane Structures with 2n + 8 Skeletal Electrons: Decorating an Aluminum Cube with Carbon Atoms

Density functional theory and coupled cluster calculations show the hexacarbalane dianions [C6Aln–6Men]2– (n = 12–14) having 2n + 8 skeletal electrons, including the highly symmetrical experimentally known Oh 14-vertex hexacapped cube dianion [C6Al8Me14]2–, to be highly favored structures, provided that they contain at least as many aluminum atoms as carbon atoms. In addition, the three isomeric hexacapped cube bis­(trimethylamine) adducts C6Al8Me12(NMe3)2 are highly favorable neutral species isoelectronic with the [C6Al8Me14]2– dianion. The low-energy structures for neutral C6Al8Me14, with only 2n + 6 skeletal electrons, have a distorted central Al8 cube with four faces capped by single carbon atoms and a fifth face capped by a C2 unit, leaving a sixth uncapped face. A related experimentally known neutral 13-vertex C5Al8Me13(μ4-H) structure with five faces of a central Al8 cube capped by carbon atoms and the sixth face capped by a hydrogen atom is also energetically favorable relative to other isomers. The lowest-energy 12- and 13-vertex hexacarbalane structures C6Aln–6Men and their dianions do not all have identifiable Aln–6 subpolyhedra, suggesting that they are the lower size limits for structures of this type. However, some of these structures have a central Al6 trigonal prism with at least one capping C2 unit. The chemical bonding topology for n-vertex hexacarbalanes with at least 12 vertex atoms having 2n + 8 skeletal electrons is analogous to that of the deltahedral boranes BnHn2– (n ≥ 6) having 2n + 2 skeletal electrons. However, the eight electrons for core bonding in these hexacarbalanes corresponds to a filled 1S + 1P shell of the approximately spherical C6Aln–6 polyhedron, as shown by the molecular orbital analysis of [C6Al8Me14]2–.