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Abstract: The series of hexacarbalanes C6Aln–6Men (n = 7–11) represent a progression from localized organoaluminum structures to delocalized polyhedral structures en route to experimentally known 13‐ and 14‐vertex hexacarbalanes such as (AlMe)8(CCH2Ph)5(µ4H), (AlMe)8(CCH2Ph)5(CCPh), [R4N+]2[(AlH)8(CR)6], and (AlNMe3)2(AlR)6(CR)6. In this connection, the lowest energy seven‐vertex C6AlMe7 structure has a tetrahapto benzene ring with the four AlC(cage) bonding interactions required to give the aluminum the favored octet configuration. Related eight‐vertex C6Al2Me8 structures are found with a benzene ring bound to an Al2 unit with a short AlAl distance of ∼2.55 Å suggesting a formal double bond. However, the lowest energy C6Al2Me8 structure has a dialuminacyclobutene unit fused to a tricyclohexane unit through an Al2 edge. Other relatively low‐energy C6AlMe7 and C6Al2Me8 structures consist of a six‐carbon hexatriene chain either forming a seven‐membered C6Al ring in the seven‐vertex structure or acting as a “flyover” between an Al2 unit. The lowest energy nine‐vertex hexacarbalane C6Al3Me9 has two separate C3 units bridged by both an Al2 pair and a single aluminum atom. Higher energy C6Al3Me9 hexacarbalanes contain a pentadienyl chain and an isolated carbon atom with an imbedded bonded Al3 triangle. The low‐energy 10‐vertex C6Al4Me10 structures have a central Al4 butterfly with nonbonding distances between the wingtips ranging from 3.35 to 3.91 Å. The lowest energy 11‐vertex C6Al5Me11 structure has a central Al4 quadrilateral with a diagonal bridged by the fifth aluminum atom. Higher energy C6Al5Me11 structures have an edge rather than a diagonal of the central Al4 quadrilateral bridged by the fifth aluminum atom. [ABSTRACT FROM AUTHOR]

The CCSD(T)/cc‐pVTZ//CCSD/cc‐pVTZ method is used to determine the geometries and energetics of the isomers HXCY vs HY─CX (XN, P, As; YO, S) and their dimers from chain dimerizations and head‐to‐head or head‐to‐tail [2 + 2] cyclodimerizations. The HO─CX structures with CX triple bonds lie at energies at least 18.5 kcal/mol above their HXCO isomers. However, the energy differences between the HXCS and HS─CX isomers are found to be particularly small, especially in the [H,P,C,S] and [H,As,C,S] systems. For (HNCY)2, the lowest energy dimers are the chain isomers, which lie ~11 kcal/mol below the lowest energy cyclic dimers aNO containing a NCNC ring and cNS containing a NCSC ring. Formation of the remaining dimers through dimerization from two monomers is predicted to be endothermic and thus thermodynamically disfavored. However, the energies of the chain isomers in the other (HXCY)2 (XP, As; YO, S) series are higher than those of the corresponding isomeric lowest energy cyclodimers. For (HXCO)2 (XP, As), the lowest energy structures are the head‐to‐head dimers hPO and hAsO containing a C─C─XX ring. For (HXCS)2 (XP, As), the lowest energy structures are the head‐to‐head dimers gPS and gAsS with a CCXS ring. [ABSTRACT FROM AUTHOR]