Bridging a Knowledge Gap from Siloxanes to Germoxanes and Stannoxanes. A Theoretical Natural Bond Orbital Study

Abstract

Explaining the nature of the E-O chemical bond (E = Si, Ge, Sn) has been a great challenge for theoretical chemists during the last decades. Among the large number of models used for this purpose, the one based on hyperconjugative interactions sheds more light on the nature of chemical bonding in siloxanes. Starting from this concept, this study aimed to evaluate the impact of siloxane type hyperconjugative effects on the structural features of germoxanic and stannoxanic species and in addition to assess if p-d-like back-bonding interactions can also play important roles in determining the particular structures of these heavier analogues of ethers. Natural bond orbital deletion (NBO DEL) optimizations, carried out at the DFT level of theory, revealed that hyperconjugative effects dictate to a large extent the structural behavior of these species. Furthermore, this study points out that p-d back-bonding interactions also influence the equilibrium geometry of these species, although acting as a secondary electronic effect within the E-O-E moieties (E = Si, Ge, Sn).

Reactivity of Bis(sulfonyl) O,C,O‐Chelated Metallylenes in Cycloaddition with ortho‐Benzoquinone: An Experimental and Computational Study

“…These short lengths can be explained in terms of hyperconjugative interactions (LP O →σ* Ge–R with R = C, O, Cl; LP = lone pair), as already pointed out in previous studies performed for similar bonding patterns. The calculated lengths of the corresponding Sn–O bonds in 4_I (2.018 and 2.033 Å, Table S2) were also shorter than their sum of covalent radii (2.05 Å); however, the differences were smaller than those identified for the Ge–O bonds, as a consequence of the decreasing hyperconjugative effect down the group …”

“…For isomer 3_I (the most stable germanium-benzoquinone adduct according to the calculations and also the adduct identified in the solid state), short distances between the Ge atom and the O atoms contained in the quinone unit were identified, with calculated values of 1.807 and 1.805 Å (Table S2) which are shorter than typical Ge-O bonds or than their sum of covalent radii (1.86 Å). [13] These short lengths can be explained in terms of hyperconjugative interactions (LP O →σ* Ge-R with R = C, O, Cl; LP = lone pair), as already pointed out in previous studies [22] performed for similar bonding patterns. The calculated lengths of the corresponding Sn-O bonds in 4_I (2.018 and 2.033 Å, Table S2) were also shorter than their sum of covalent radii (2.05 Å); however, the differences were smaller than those identified for the Ge-O bonds, as a consequence of the decreasing hyperconjugative effect down the group.…”

“…The calculated lengths of the corresponding Sn-O bonds in 4_I (2.018 and 2.033 Å, Table S2) were also shorter than their sum of covalent radii (2.05 Å); however, the differences were smaller than those identified for the Ge-O bonds, as a consequence of the decreasing hyperconjugative effect down the group. [22] Natural bond orbital (NBO) analysis carried out on the investigated compounds further revealed the occurrence of hyperconjugative effects involving the Cl atom. Yet, the total amount of energy corresponding to these interactions seemed to decrease significantly from germanium derivative 3 to tin derivative 4; for instance, the total hyperconjugation energy corresponding to the LP Cl →σ* M-C (M = Ge, Sn) interactions was calculated to be 17.1 kcal/mol in compound 3_I and only 3.2 kcal/ mol in compound 4_I.…”

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” .…”

Me3SiBr/InCl3 catalyzed allylation of alcohols: Identifying the combined Lewis structure and investigating the reaction mechanism

“…Natural population analysis (NPA) was performed to clarify the charge population and results are shown in Table 1. [50] Positive charge is concentrated onto Me 3 Si + , and negative charge is located on the bridge atom (Cl for INT_0 and Br for INT_1) as well as In-halide moiety. The overall charge of InCl 3 is −0.191 for INT_0, and InBrCl 2 carries −0.171 in INT_1 as shown in Figure 2.…”