Pairing of carbon atoms in low-energy deltahedral dicarbagallane structures derived from vertex expansion of closo deltahedra

Abstract

The structures and energetics of the dicarbagallanes C2Gan−2Men (n = 6 to 14) have been investigated using density functional theory. In general, the low-energy structures for these dicarbagallanes have both carbon atoms at degree 4 vertices similar to the corresponding dicarbalanes C2Aln−2Men. However, the dicarbagallanes energetically prefer structures with adjacent carbon atoms to a much greater extent than the corresponding dicarbalanes. In most cases the lowest energy dicarbagallane structures, like the corresponding dicarbalane structures, are the most spherical closo deltahedra provided that two degree 4 vertices are available for the carbon atoms. However, the lowest energy structure of the 9-vertex C2Ga7Me9 is an isocloso 9-vertex deltahedron with the carbon atoms at adjacent degree 4 vertices rather than the tricapped trigonal prism where the degree 4 vertices are not adjacent to each other. In addition, the closo lowest energy structure of the 13-vertex dicarbagallane, C2Ga11Me13, has one of the carbon atoms at a degree 5 vertex adjacent to the other carbon atom at the unique degree 4 vertex of the 13-vertex closo deltahedron. The lowest energy structures of the 12- and 14-vertex dicarbagallanes adopt deltahedral arrangements providing two degree 4 vertices for the carbon atoms. The lowest energy structure for the 6-vertex dicarbagallane C2Ga4Me6 is a capped tetragonal pyramid with the two carbon atoms at adjacent degree 3 vertices of the tetragonal face. This contrasts with the lowest energy structure of the 6-vertex dicarbalane C2Al4Me6, which has a bicapped tetrahedral structure with the carbon atoms at the non-adjacent degree 3 capping vertices.