Hexacoordinated hypervalent compounds

Keywords hexacoordinate, hypervalent compounds, model Berry pseudorotation, dative-bond dissociation... 

Akiba and coworkers have established the existence of hexacoordinate hypervalent compounds, that is, 12-C-6 species with 12 formally assignable electrons in the valence shell of carbon with six ligands (substituents). In 12-C-6 compounds, carbon is sp hybridized, bears two positive charges, and attracts four nucleophiles to form two 3c-Ae bonds. Geometry optimization at the B3LYP/cc-pVDZ level has been performed, with dications having the... 

Hexacoordinate, hypervalent sulfur fluorides have an octahedral geometry that is symmetrical for SF6, which appears as a sharp singlet at +56 ppm, but which has magnetically nonequivalent (axial and equatorial, ab4 system) fluorines for compounds of the structure R-SF5. Compounds with the general structure R-SF4-X can exist as cis- and trans-isomers, the former having three types of fluorine, and the latter only one (Scheme 7.17). 

A special feature of anions 475 in complexes with different counterions (entries 4-16) is a lengthening of the N — Sn or O — Sn coordinative bonds up to 2.28-2.32 and 2.17-2.27 A, respectively, compared to the mean values for penta- and hexacoordinated tin compounds (2.12 and 2.15 A)9. Accordingly, the Sn—Cl bond length trans to N or O, at 2.36-2.42 A, is somewhat shorter than that in hypervalent compounds (2.45 A), and is generally shorter in comparison with the other Sn—Cl bonds in these complexes. 

Pentacoordinated (lO-B-5) and hexacoordinated (12-B-6) boron compounds are available from the reaction of BCI3 with the dilithio salt of the bis addition product of HFA and pyridine (265). These hypervalent compounds show signals in the B NMR spectrum at very high field (173a). 

By using a sum of the bond order of 4 in the hypervalent O GeCCl2—Cl fragment and a value of 1 for the axial bonds, values of 0.92 and 0.87, respectively, are obtained for the coefficient c. These and the previously calculated value of 0.80 for penta- and hexacoordinate Ge compounds" are lower than c for tin (1.20" ). This demonstrates a decreased tendency toward expanding the coordination sphere of germanium atom compared to that of... 

Hypervalent compounds of organic germanium, tin and lead derivatives 1149 IX. HEXACOORDINATE ANIONIC COMPOUNDS A. Intermolecular Complexes... 

Recently, Furukawa et al. have reported the stable penta- and hexacoordinate hypervalent sulfur compounds, [10-S-5(C2N2C1)] 98 and [12-S-6(C2N2C12)] 97, which are stabilized by the transannular interaction between nitrogen and sulfur atoms, as shown in Scheme 39 [88]. 

Hypervalent silicon compounds B. Hexacoordinate Silicon Compounds... 

The coordination tendencies of phosphorus to form hypervalent compounds have been especially studied due to the fact that penta- and hexacoordinated phosphoranes are involved in numerous biological processes such as hydrolyses of RNA, or phosphoryl transfer reactions. Therefore considerable attention has been given to the synthesis, chemical transformations, structures and configurational stability of hypervalent or-ganophosphorus compounds. 

It is well known that the coordination ability of phosphorus to form hypervalent compound, mainly penta- and hexacoordinated, is the driving force in describing the mechanistic action of phosphoryl transfer enzymes. On the other hand, organophosphorus compounds play also a fundamental role in inorganic, organic and applied chemistry as a key species, reaction intermediates or final products. Therefore, the utility of hypervalent phosphorus compounds in many chemical processess is indisputable and in some cases facilitate the outcome of the reaction to be defined. Recently, some achievements on the role of hypervalent phosphoranes in various chemical processess have been described. 

In the absence of electron-withdrawing ligands, disilanes possess only a little Lewis acidity consequently, they show a limited propensity to form hypervalent complexes. Starting from the symmetric methylchlorodisilanes ClMe2SiSiMe2Cl and Cl2MeSiSiMeCl2 as well as carboxylic acids and thiocarboxylic acids, Kawashima et al. have prepared rare examples of penta- and hexacoordinated disilene, namely compounds 886-889 (Scheme 125).855 856... 

Hypervalent bonds in hexacoordinate compounds are formed in a similar fashion the four equatorial M—X bonds (two 3c-4e bonding) in Oh are longer and more polar than the two axial bonds. 

In connection with the use of the hypervalent concept as an explanation of mutual cis/trans-influence in penta- and hexacoordinated compounds of group 14 elements, the experimental24-29 and theoretical30,31 works and references cited therein should be consulted. 

Reaction between Me2N(CH2)3Sn(OPh)Ph2 (239e) and TEAA proceeds smoothly due to the high reactivity of hypervalently activated organotin compounds and gives stanna-trane 432 (R = (CH2)3NMe2)644. Its 5119Sn NMR appears between those of stannatranes with penta- and hexacoordinated tin atoms. This fact was interpreted as a result of a fast equilibrium on the NMR time scale. 

Hypervalent bonds in compounds of hexacoordinated silicon are formed in a similar fashion. Thus, in the anions of the SiF and SiFg type there are two three-center four-electron bonds ... 

In this context, the existence of stable penta- and hexacoordinate silicon derivatives and their structure have elicited considerable interest. Isomerization processes of these compounds are also of importance, since penta- or hexacoor-dinate intermediates are implicated in the substitution and racemization of tetracoordinate silicon derivatives. For all these reasons, we include here (i) some available structural data for hypervalent silicon compounds (the review is not intended to be exhaustive), (ii) recent reports concerning the nucleophilically induced substitution and racemization reactions at silicon, and (ii) a criti-... 

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