title: Bonding, aromaticity, and planar tetracoordinated carbon in Si2CH 2 and Ge 2CH 2.
abstract: Natural bond orbital (NBO) analyses and dissected nucleus-independent chemical shifts (NICS π z z ) were computed to evaluate the bonding (bond type, electron occupation, hybridization) and aromatic character of the three lowest-lying Si2CH2 (1-Si, 2-Si, 3-Si) and Ge2CH2 (1-Ge, 2-Ge, 3-Ge) isomers. While their carbon C3H2 analogs favor classical alkene, allene, and alkyne type bonding, these Si and Ge derivatives are more polarizable and can favor "highly electron delocalized"? and "non-classical"? structures. The lowest energy Si 2CH2 and Ge 2CH2 isomers, 1-Si and 1-Ge, exhibit two sets of 3-center 2-electron (3c-2e) bonding; a π-3c-2e bond involving the heavy atoms (C-Si-Si and C-Ge-Ge), and a σ-3c-2e bond (Si-H-Si, Ge-H-Ge). Both 3-Si and 3-Ge exhibit π and σ-3c-2e bonding involving a planar tetracoordinated carbon (ptC) center. Despite their highly electron delocalized nature, all of the Si2CH2 and Ge2CH2 isomers considered display only modest two π electron aromatic character (NICS(0) π z z =--6.2 to -8.9 ppm, computed at the heavy atom ring center) compared to the cyclic-C 3H2 (-13.3 ppm). Graphical Abstract The three lowest Si2CH2 and Ge2CH2 isomers.
authors.affiliation: Facultad de Química, Pontifícia Universidad Católica de Chile, Santiago, Chile, savogt@uc.cl.
authors.forename: Stefan
authors.initials: S
authors.lastname: Vogt-Geisse
authors.forename: Judy I-Chia
authors.initials: JI
authors.lastname: Wu
authors.forename: Paul v R
authors.initials: Pv
authors.lastname: Schleyer
authors.forename: Henry F
authors.initials: HF
authors.lastname: Schaefer