Nonspherical Deltahedra in Low-Energy Dicarbalane Structures Testing the Wade–Mingos Rules: The Regular Icosahedron Is Not Favored for the 12-Vertex Dicarbalane

Theoretical studies on the dicarbalanes C2Aln–2Men (n = 7–14; Me = methyl) predict both carbon atoms to be located at degree 4 vertices of a central C2Aln–2 deltahedron in the lowest energy structures. As a consequence, deltahedra having two degree 4 vertices, two degree 6 vertices, and eight degree 5 vertices rather than the regular icosahedron having exclusively degree 5 vertices are found for the 12-vertex dicarbalane C2Al10Me12. However, the lowest energy C2Aln–2Men (n = 7–11) structures are based on the same most spherical (closo) deltahedra as the corresponding deltahedral boranes. The lowest energy structures for the 13- and 14-vertex systems C2Aln–2Men (n = 13 and 14) are also deltahedra having exactly two degree 4 vertices for the carbon atoms. The six-vertex C2Al4Me6 system is exceptional since bicapped tetrahedral and capped square pyramidal structures with degree 3 vertices for the carbon atoms are energetically preferred over the octahedral structure suggested by the Wade–Mingos rules.