Variation of a bond angle can tune the reactivity of a chemical compound. To exemplify this concept, the nature of the siloxane linkage (Si-O-Si), the most abundant chemical bond in the earth's crust, was examined using theoretical calculations on the molecular model compounds H(3)SiOSiH(3), (H(3)Si)(2)OHOH, and (H(3)Si)(2)OHOSiH(3) and high-resolution synchrotron X-ray diffraction experiments on 5-dimethylhydroxysilyl-1,3-dihydro-1,1,3,3-tetramethyl-2,1,3-benzoxadisilole (1), a molecular compound that gives rise to the formation of very rare intermolecular hydrogen bonds between the silanol groups and the siloxane linkages. For theoretical calculations and experiment, electronic descriptors were derived from a topological analysis of the electron density (ED) distribution and the electron localization function (ELF). The topological analysis of an experimentally obtained ELF is a newly developed methodology. These descriptors reveal that the Si-O bond character and the basicity of the siloxane linkage strongly depend on the Si-O-Si angle. While the ionic bond character is dominant for Si-O bonds, covalent bond contributions become more significant and the basicity increases when the Si-O-Si angle is reduced from linearity to values near the tetrahedral angle. Thus, the existence of the exceptional intermolecular hydrogen bond observed for 1 can be explained by its very small strained Si-O-Si angle that adopts nearly a tetrahedral angle.

Covalency and ionicity are orthogonal rather than antipodal concepts. We demonstrate for the case of siloxane systems [R Si-(O-SiR ) -O-SiR ] that both covalency and ionicity of the Si-O bonds impact on the basicity of the Si-O-Si linkage. The relationship between the siloxane basicity and the Si-O bond character has been under debate since previous studies have presented conflicting explanations. It has been shown with natural bond orbital methods that increased hyperconjugative interactions of LP(O)→σ*(Si-R) type, that is, increased orbital overlap and hence covalency, are responsible for the low siloxane basicity at large Si-O-Si angles. On the other hand, increased ionicity towards larger Si-O-Si angles has been revealed with real-space bonding indicators. To resolve this ostensible contradiction, we perform a complementary bonding analysis, which combines orbital-space, real-space, and bond-index considerations. We analyze the isolated disiloxane molecule H SiOSiH with varying Si-O-Si angles, and n-membered cyclic siloxane systems Si H O(CH ) . All methods from quite different realms show that both covalent and ionic interactions increase simultaneously towards larger Si-O-Si angles. In addition, we present highly accurate absolute hydrogen-bond interaction energies of the investigated siloxane molecules with water and silanol as donors. It is found that intermolecular hydrogen bonding is significant at small Si-O-Si angles and weakens as the Si-O-Si angle increases until no stable hydrogen-bond complexes are obtained beyond φ =168°, angles typically displayed by minerals or polymers. The maximum hydrogen-bond interaction energy, which is obtained at an angle of 105°, is 11.05 kJ mol for the siloxane-water complex and 18.40 kJ mol for the siloxane-silanol complex.

Closely following the procedure for the preparation of the base-stabilized dichlorosilylene complex NHC ⋅SiCl reported by Roesky, Stalke, and co-workers (Angew. Chem. Int. Ed. 2009, 48, 5683-5686), a few crystals of the salt [NHC -H⋅⋅⋅Cl⋅⋅⋅H-NHC ]Si(SiCl ) were isolated, aside from the reported byproduct [NHC -H ⋅⋅⋅Cl ], and characterized by X-ray crystallography (NHC =N,N-di(2,6-diisopropylphenyl)imidazo-2-ylidene). They contain the weakly coordinating anion Si(SiCl ) , which was also obtained in high yields upon deprotonation of the conjugate Brønsted acid HSi(SiCl ) with NHC or PMP (PMP=1,2,2,6,6-pentamethylpiperidine). The acidity of HSi(SiCl ) was estimated by DFT calculations to be substantially higher than those of other H-silanes. Further DFT studies on the electronic structure of Si(SiCl ) , including the electrostatic potential and the electron localizability, confirmed its low basicity and nucleophilicity compared with other silyl anions.

In Anlehnung an die Vorschrift von Roeskyu nd Stalke et al. (Angew.C hem. 2009,1 21, 5793-5796) zur Herstellung des basenstabilisierten Dichlorsilylen-Komplexes NHC Dipp ·SiCl 2 wurden neben dem beschriebenen Nebenprodukt [NHC Dipp -H + ···Cl -]wenige Kristalle des Salzes [NHC Dipp -H···Cl···H-NHC Dipp ]Si(SiCl 3 ) 3 isoliert und kristallographisch charakterisiert (NHC Dipp = N,N-Bis(2,6-diisopropylphenyl)imidazo-2-yliden). Sie enthalten das schwach koordinierende Si(SiCl 3 ) 3 À -Anion, das systematischinhohen Ausbeuten durch die Deprotonierung der konjugierten Brønsted-Säure HSi-(SiCl 3 ) 3 mit NHC Dipp oder 1,2,2,6,6-Pentamethylpiperidin erhalten wurde.D ie durch DFT-Rechnungen abgeschätzte Aciditätv on HSi(SiCl 3 ) 3 ist bedeutend hçher als die anderer H-Silane.D FT-Rechnungen zur elektronischen Struktur von Si-(SiCl 3 ) 3 À ,e inschließlich des elektrostatischen Potentials und der Elektronenlokalisierbarkeit, bestätigen die geringe Basizitätu nd Nukleophilie im Vergleichz ua nderen Silylanionen.