Triple Bonds of the Heavy Main‐Group Elements: Acetylene and Alkylidyne Analogues of Group 14

Abstract

Carbon copies? Compounds containing homo‐ and heteronuclear multiple bonds involving the heavier Group 14 elements have become available in the past few years. The most recent example is the cationic complex 1 (Mes=mesityl=2,4,6‐trimethylphenyl), for which the structural analysis and DFT calculations indicate a pronounced silylidene character and a nonclassical H⋅⋅⋅Si interaction.

Polyanionic Hexagons: X6n– (X = Si, Ge)

“…The synthesis of silicon-and germanium-containing hexagons with a cyclic X n C 6-n backbone (X = Si, Ge) have been reported only with n = 1 and 2 [33]. Only the hexasilaprismane derivative was synthesized as a hexasila-analogue of (CH) 6 [34], though five (CH) 6 isomers (benzene, benzvalene, Dewar benzene, prismane, and bicyclopropenyl) have been synthesized. Theoretical investigations supported the synthetic results, that is, hexasilaprismane is the most stable among the (SiH) 6 isomers [35][36][37].…”

“…The silicon and germanium -electron systems, however, differ remarkably in structure and stability from a carbon -electron system [3][4][5][6][7][8], although the electronic structures are analogues and formally the same hybridization occurs. The rings of hexasilabenzene (SiH) 6 and OPEN ACCESS hexagermabenzene (GeH) 6 are puckered as in a chair-like cyclohexane (Scheme Ia) and are different from planar D 6h -symmetric benzene (Scheme Ib) [7,8]. A common explanation for the difference in geometry between carbon and the heavier Group 14 elements was that the energy separation between valence s and p orbitals is small only in first-row elements, allowing efficient hybridization, and the energy separation in higher rows is much larger.…”

“…Nearly-planar hexagons consisting of Si and Ge were found in solid-state Zintl anions: Si 6 10-in Ba 4 Li 2 Si 6 and Ge 6 10-in Ba 4 Li 2 Ge 6 [10]. These species are geometrically similar to benzene, but are Hückel arenes with 10  electrons.…”

“…Planar hexasilylhexasilacyclohexene anions (Si 12 ) containing Si 6 -ring system of approximate D 6h symmetry was obtained in unusual Zintl phase (Ca 7 Mg 7.5± Si 14 ) with metallic conductivity [11], but the Si 6 ring is nonaromatic. The first flat aromatic D 6h -symmetric Si 6 and Ge 6 rings with six  electrons were computationally designed by us as polyanionic clusters [12,13].…”

“…In this review, we mainly consider our findings on the flat D 6h -symmetric Si 6 and Ge 6 , give the background to reach them, and propose a synthetically accessible molecule. The construction of the rest of the paper will be separated into sections discussing the following topics: Section 2, the synthetic and theoretical background of unsaturated silicon and germanium compounds; Section 3, Hückel arenes consisting of silicon and germanium found in Zintl phase of crystal; Section 4, the stability of polyanionic clusters based on the electron-counting rule (Wade's rule); Section 5, the aromaticity of polyanionic Si 6 and Ge 6 rings with D 6h symmetry; and Section 6, the design of synthetically accessible hexagons with the two-dimensional aromaticity.…”