Isoelectronic and isolobal O, CH₂, CH₃⁺and BH₃as electron pairs; similarities between molecular and solid-state chemistry

Abstract

A topological analysis of the electron localization function (ELF) of a molecule of hexamethyldisiloxane, (H 3 C) 3 -Si-OSi-(CH 3 ) 3 , has been carried out, drawing a consistent picture of Si-O-Si bonding both in the linear and angular geometries. The ELF analysis confirms the idea that the O atom, in the linear geometry of (H 3 C) 3 -Si-O-Si-(CH 3 ) 3 , is isolobal with the isoelectronic -CH þ 3 -and -BH 3 -groups, the bonding in the Si-O-Si group being described as a two-electron, three-center (2e, 3c) bond. At the same time, the three oxygen lone pairs mirror the three C-H and B-H bonds, respectively. On the contrary, in the angular geometry the same O atoms form two Si-O bonds and its lone pairs mimic the geometry of the -CH 2 -group. In this model the O atoms would play the same role as the formally present O 2À anions in the 'so-called' ionic solids, such as in the skeletons of aluminate and silicate polyanions, thereby connecting molecular and solid-state chemistry as formulated by the 'fragment formalism' or the 'molecular unit-cell approach'. This unifying concept as well as the calculations we have carried out fully agree and also give support to earlier ideas developed by Bragg and Bent, among other authors. Bonding in the series of compounds P 4 , P 4 O 6 , P 4 O 10 , N 4 (CH 2 ) 6 (hexamethylenetetramine) and (CH) 4 (CH 2 ) 6 (adamantane) is discussed in the context of the isolobal model.

Covalency and Ionicity Do Not Oppose Each Other—Relationship Between Si−O Bond Character and Basicity of Siloxanes

“…The question has been addressed by West and Gibbs and their respective co‐workers from 1960 on . The case was reopened in 2009 with an experimental electron density study of a siloxanol molecule, which triggered recent theoretical investigations . Still, diverging viewpoints are present and unreconciled: although some authors ascribe a highly ionic character to the Si−O bond, others state that it has a “substantial covalent character” .…”

An alternative approach to rationalizing the structures of the cyclotrisilicates: Rb10[Si6O17], Cs8[Si6O16] and Na3Y[Si6O15] by viewing them in light of the extended Zintl–Klemm concept

“…Note that here each C-H group is equivalent to an N atom not only because they are isolectronic but also because the six C-H bonds are mimicking the six lone pairs of the trigonal N 6 , as has recently been reported (Vegas et al, 2013). Both molecules N 6 S 5 and C 6 H 6 are represented in Fig.…”

Interplay of thermochemistry and structural chemistry, the journal (volume 24, 2013, issues 3–4) and the discipline

“…The author showed that two macrocycles lie on parallel planes; it can be reasonably speculated that this finding can be extended to all l-oxo dimers of porphyrins and phthalocyanines of trivalent metals. Recently, the oxygen of the loxo species (SiH 3 ) 2 O was described as participating in 3 center/4 electron bonding [68]. We question whether this latter oxygen-bonding description also applies to (AlP) 2 O and how the understanding of the energetics of general species with singly bonded oxygen, any species with a -Ogroup, is affected by assuming 3 center/4 electron bonding instead of two oxygen-containing r bonds.…”