Editing Hypervalent molecule (section)

====Silicon====
{|class="wikitable sortable" align=right
|+Observed third-order [[reaction rate constant]]s<br />for hydrolysis (displacement of chloride from silicon)<ref name= Corriu1978/>
|-
! Chlorosilane
! Nucleophile
! ''k''<sub>obs</sub> (M<sup>−2</sup>s<sup>−1</sup>)<br />at 20&nbsp;°C in anisole
|-
||[[Phenyl group|Ph]]<sub>3</sub>SiCl || [[Hexamethylphosphoramide|HMPT]] || 1200
|-
||Ph<sub>3</sub>SiCl || [[Dimethyl sulfoxide|DMSO]] || 50
|-
||Ph<sub>3</sub>SiCl || [[Dimethylformamide|DMF]]  || 6
|-
||[[Methyl group|Me]]Ph<sub>2</sub>SiCl || HMPT || 2000
|-
||MePh<sub>2</sub>SiCl || DMSO || 360
|-
||MePh<sub>2</sub>SiCl || DMF  || 80
|-
||Me(1-[[Naphthyl|Np]])PhSiCl || HMPT || 3500
|-
||Me(1-Np)PhSiCl || DMSO || 180
|-
||Me(1-Np)PhSiCl || DMF  || 40
|-
||(1-Np)Ph([[vinyl group|vinyl]])SiCl || HMPT || 2200
|-
||(1-Np)Ph(vinyl)SiCl || DMSO || 90
|-
||(1-Np)(''m''-[[Trifluoromethyl|CF<sub>3</sub>]]Ph)HSiCl || DMSO || 1800
|-
||(1-Np)(''m''-CF<sub>3</sub>Ph)HSiCl || DMF || 300
|-
|}

Corriu and coworkers performed early work characterizing reactions thought to proceed through a hypervalent transition state.<ref name= Corriu1978>{{cite journal | doi = 10.1016/S0022-328X(00)85545-X | author1 = Corriu, RJP | title = Mécanisme de l'hydrolyse des chlorosilanes, catalysée par un nucléophile: étude cinétique et mise en evidence d'un intermediaire hexacoordonné| journal = [[J. Organomet. Chem.]]| year = 1978|volume = 150|pages = 27–38 | last2 = Dabosi | first2 = G. | last3 = Martineau | first3 = M.}}</ref>  Measurements of the [[reaction rate]]s of hydrolysis of tetravalent chlorosilanes incubated with catalytic amounts of water returned a rate that is [[Order of reaction|first order]] in chlorosilane and second order in water.  This indicated that two water molecules interacted with the silane during hydrolysis and from this a binucleophilic reaction mechanism was proposed. Corriu and coworkers then measured the rates of hydrolysis in the presence of nucleophilic catalyst HMPT, DMSO or DMF. It was shown that the rate of hydrolysis was again first order in chlorosilane, first order in catalyst and now first order in water.  Appropriately, the rates of hydrolysis also exhibited a dependence on the magnitude of charge on the oxygen of the nucleophile.

Taken together this led the group to propose a reaction mechanism in which there is a pre-rate determining nucleophilic attack of the tetracoordinated silane by the nucleophile (or water) in which a hypervalent pentacoordinated silane is formed.  This is followed by a nucleophilic attack of the intermediate by water in a rate determining step leading to hexacoordinated species that quickly decomposes giving the hydroxysilane.

Silane hydrolysis was further investigated by Holmes and coworkers <ref name= Johnson1989>{{cite journal | doi= 10.1021/ja00191a023 | author1= Johnson, SE|author2=Deiters, JA|author3=Day, RO|author4=Holmes, RR | title= Pentacoordinated molecules. 76. Novel hydrolysis pathways of dimesityldifluorosilane via an anionic five-coordinated silicate and a hydrogen-bonded bisilonate. Model intermediates in the sol-gel process| journal = [[J. Am. Chem. Soc.]]| year = 1989|volume = 111|pages = 3250 | issue= 9| bibcode= 1989JAChS.111.3250J}}</ref> in which tetracoordinated {{chem|Mes|2|SiF|2}} (Mes = [[mesitylene|mesityl]]) and pentacoordinated {{chem|Mes|2|SiF|3|-}} were reacted with two equivalents of water. Following twenty-four hours, almost no hydrolysis of the tetracoordinated silane was observed, while the pentacoordinated silane was completely hydrolyzed after fifteen minutes.  Additionally, X-ray diffraction data collected for the tetraethylammonium salts of the fluorosilanes showed the formation of hydrogen bisilonate lattice supporting a hexacoordinated intermediate from which {{chem|HF|2|-}} is quickly displaced leading to the hydroxylated product. This reaction and crystallographic data support the mechanism proposed by Corriu ''et al.''.

[[image:Hydrolysis Silane Xray structure.png|thumb|500px | center | Mechanism of silane hydrolysis and structure of the hydrogen bisilonate lattice]]

The apparent increased reactivity of hypervalent molecules, contrasted with tetravalent analogues, has also been observed for Grignard reactions.  The Corriu group measured<ref name= Corriu1988>{{cite journal | doi = 10.1021/om00091a038 | author = Corriu, RJP | title = Pentacoordinated silicon anions: reactivity toward strong nucleophiles| journal = [[Organometallics]]| year = 1988|volume = 7|pages = 237–8 | last2 = Guerin | first2 = Christian. | last3 = Henner | first3 = Bernard J. L. | last4 = Wong Chi Man | first4 = W. W. C.}}</ref> Grignard reaction half-times by NMR for related 18-crown-6 potassium salts of a variety of tetra- and pentacoordinated fluorosilanes in the presence of catalytic amounts of nucleophile.

Though the half reaction method is imprecise, the magnitudinal differences in reactions rates allowed for a proposed reaction scheme wherein, a pre-rate determining attack of the tetravalent silane by the nucleophile results in an equilibrium between the neutral tetracoordinated species and the anionic pentavalent compound.  This is followed by nucleophilic coordination by two Grignard reagents as normally seen, forming a hexacoordinated [[transition state]] and yielding the expected product.
[[image:Hypercoordinated Silane Grignard.png|thumb|500px | center | Grignard reaction mechanism for tetracoordinate silanes and the analogous hypervalent pentacoordinated silanes]]

The mechanistic implications of this are extended to a hexacoordinated silicon species that is thought to be active as a transition state in some reactions. The reaction of [[allyl]]- or [[crotyl]]-trifluorosilanes with aldehydes and ketones only precedes with fluoride activation to give a pentacoordinated silicon. This intermediate then acts as a [[Lewis acid]] to coordinate with the carbonyl oxygen atom. The further weakening of the silicon–carbon bond as the silicon becomes hexacoordinate helps drive this reaction.<ref name=abinitio3>{{cite journal | title = Regiospecific and highly stereoselective allylation of aldehydes with allyltrifluorosilane activated by fluoride ions | journal = [[Tetrahedron Letters]] | year = 1987 | volume = 28 | pages = 4081–4084 | doi = 10.1016/S0040-4039(01)83867-3 | author1 = Kira, M | author2 = Kobayashi, M. | author3 = Sakurai, H. | issue = 35}}</ref>

[[File:Aldehyde crotylation with hypervalent silicon.png|thumb|500px|center]]