The Chemistry of Hypervalent Molecules

Abstract

dynamic interaction between ions and water molecules, the migrating ions in A and K elements transport water from the dilute to the concentrated salt solution. Negative osmosis is therefore to be expected in mosaic membranes. The reflection coefficient for a symmetrical mosaic membrane is theoretically given by the equation [lo21 As expected, 5 is always negative. The approximation cs w -X/2zc can be used at high fixed-ion concentrations ( X > c). Thus pressure differences many times greater than the equilibrium pressure difference Arc (eqs. (11) and (12)) should occur in osmotic experiments with mosaic membranes. According to equation (1 3), the reflection coefficient 5 also determines the filtration behavior of the membrane; for G < 0, the solute is enriched in the filtrate.On filtration of a dilute salt solution through a mosaic membrane having a high fixed-ion concentration, the filtrate should be a highly concentrated salt solution. This interesting effect has not yet been verified experimentally.Molecules formed from atoms of Groups V-VIII of the periodic table in their higher valences are studied and a general theory for their electronic and geometric structure is presented. These molecules, which we call hypervalent molecules, are unique in that a donor atom can possess two identical monovalent ligands which are bonded by digerent types of chemical bonds. The present study provides a simple theoretical description of such molecules using doubly-occupied lone-pair orbitals to form hypervalent bonds, and these bonds are divided into two classes as to whether they involve p electrons alone or utilize s electrons as well. While the theoretical classiJication is of some interest in itself, there are even more interesting problems concerning the physico-chemical properties of these molecules and the syntheses of new molecules andpolymers. We thus present a brief description of the energetics, possibilities for intermolecular inversion and site-exchange, and synthetic paths for the preparation of hypervalent molecules, as well as indicate a number of new molecules which can probably be synthesized using these ideas.

Structural Trends in Silicon Atranes

“…Note however from Table 2 In order to achieve good 3c4e bonding, electronegative axial groups and an electropositive central atom are needed. 60 Thus, we observe from Table 1 a steady shortening of the SiN bond from R = CH3 to R = F and again from R = SiH3 to R = Cl.…”

Hypervalent Adducts of Chalcogen-Containing peri-Substituted Naphthalenes; Reactions of Sulfur, Selenium, and Tellurium with Dihalogens

“…The partial negative charge acting on the terminal Br(2) atom could be sufficient to act as a donor towards a second (acceptor) molecule of Br 2 . Delocalization of negative charge onto the terminal Br(2) atom would strengthen the Se(1)-Br(1) bond to the extent that the Br(1)-Br(2) bond is cleaved forming the cation [RSe-Br] + and a quasi-linear hypervalent 3c-4e Br 3 -counteranion. 15 Figure 3 The molecular structure of the ionic C.-T. "extended spoke" adduct 2 formed from the reaction of D2 with dibromine (compound 3 adopts a similar structure, see the supporting information).…”

“…(6)- (5)- (10)- (1) 176.1(10) -177.4(12) 179.3(8) -172.7(7) 176.7(6) C:(4)- (5)- (10)- (9) 176. Central naphthalene ring torsion angles C:(6)- (5)- (10) (2) 3.6956(7) 3.8730(9) 3.8527(13) E-E-X and E-X-X bond angles 3 …”

“…7,9,11,14,15 In 1969 Musher classified "hypervalent" molecules as those "formed by elements in Groups V-VIII of the periodic table in any of their valences other than their lowest stable chemical valence of 3, 2, 1, and 0 respectively." 3 The pertinence of the term "hypervalence" has recently been under dispute following the advancement of quantum mechanical calculations which reinforce the 3c-4e theory proposed by Rundle and Pimentel and conform to the octet rule. 2,7,11,13,16 As a consequence the use of octet expansion of the heavier main group elements has diminished from current bonding descriptions.…”

Fluorotrimethylsilane affinities of anionic nucleophiles: A study of fluoride-induced desilylation

“…However, because third row and lower main-group elements (e.g., P, S, Cl) readily undergo hypervalent bonding [24], the pentavalent siliconate is a stable intermediate in the reaction and can, in many cases, be observed [25][26][27][28]. Because of their stability, siliconates are often used in fundamental studies of hypervalency [26, 29 -32], as well as in numerous theoretical [33,34] and experimental [7,29,[35][36][37][38][39][40] studies.…”