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Molecules and ModelsThe molecular structures of main group element compounds$
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Arne Haaland

Print publication date: 2008

Print ISBN-13: 9780199235353

Published to Oxford Scholarship Online: May 2008

DOI: 10.1093/acprof:oso/9780199235353.001.0001

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Electron deficient molecules: three-center, two-electron bonds

Electron deficient molecules: three-center, two-electron bonds

(p.177) Chapter 12 Electron deficient molecules: three-center, two-electron bonds
Molecules and Models

Arne Haaland

Oxford University Press

Molecules containing less than two valence electrons per bond are referred to as electron deficient. The simplest such molecule is the trigonal trihydrogen cation (H3 +). The bonding in this molecule is interpreted in terms of a symmetric three-center (3c) bonding molecular orbital containing the two valence electrons. Addition of another electron to an asymmetric MO leads to a breakdown of the trigonal symmetry. This chapter describes the structure of diborane and interprets it in terms of two 3c molecular orbitals, each formed by combination of one sp 3 hybrid orbital from each B atom with the 1s AO on a bridging H atom. The analogoues structure of hexamethyl dialane (Al2(CH3)6) is described. The variable-temperature NMR spectra of this compound are interpreted in terms of rapid exchange of methyl groups between bridging and terminal positions. Other electron deficient structures that are discussed include tetrahedral B4Cl4, icosahedral B12H12 -2, crystalline Be(CH3)2, and Li4(CH3)4.

Keywords:   electron deficient molecules, the trihydrogen cation, three-center molecular orbitals, Jahn-Teller distortion, hexamethyl dialane, variable-temperature spectroscopy, bridge-terminal exchange, closoboranes, tetrameric methyl lithium

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