Editing Hypervalent molecule (section)

==Structure, reactivity, and kinetics==

===Structure===

====Hexacoordinated phosphorus====
Hexacoordinate [[phosphorus]] molecules involving nitrogen, oxygen, or sulfur ligands provide examples of Lewis acid-Lewis base hexacoordination.<ref name=Holmes>{{cite journal | title = Comparison of Phosphorus and Silicon: Hypervalency, Stereochemistry, and Reactivity | journal = [[Chem. Rev.]] | year = 1996 | volume = 96 | pages = 927–950 | doi = 10.1021/cr950243n | author1 = Holmes, R.R. | pmid=11848776 | issue = 3}}</ref>  For the two similar complexes shown below, the length of the C&ndash;P bond increases with decreasing length of the N&ndash;P bond; the strength of the C&ndash;P bond decreases with increasing strength of the N&ndash;P Lewis acid&ndash;Lewis base interaction.

[[image:hexa phos.png|thumb|300px | center | Relative bond strengths in hexacoordinated phosphorus compounds. In A, the N&ndash;P bond is 1.980&nbsp;Å long and the C&ndash;P is 1.833&nbsp;Å long, and in B, the N&ndash;P bond increases to 2.013&nbsp;Å as the C&ndash;P bond decreases to 1.814&nbsp;Å.<ref name="Holmes"/>]]

====Pentacoordinated silicon====
This trend is also generally true of pentacoordinated main-group elements with one or more lone-pair-containing ligand, including the oxygen-pentacoordinated [[silicon]] examples shown below.

[[image:penta sil oxy.png|thumb|500px | center | Relative bond strengths in pentacoordinated silicon compounds. In A, the Si-O bond length is 1.749Å and the Si-I bond length is 3.734Å; in B, the Si-O bond lengthens to 1.800Å and the Si-Br bond shortens to 3.122Å, and in C, the Si-O bond is the longest at 1.954Å and the Si-Cl bond the shortest at 2.307A.<ref name="Holmes"/>]]

The Si-halogen bonds range from close to the expected van der Waals value in A (a weak bond) almost to the expected covalent single bond value in C (a strong bond).<ref name="Holmes"/>

===Reactivity===

====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]]

====Phosphorus====
Similar reactivity has also been observed for other hypervalent structures such as the miscellany of phosphorus compounds, for which hexacoordinated transition states have been proposed.
Hydrolysis of phosphoranes and oxyphosphoranes have been studied <ref name= Bel>{{cite journal | author = Bel'Skii, VE| journal = [[J. Gen. Chem. USSR]]| year = 1979|volume = 49|pages = 298}}</ref> and shown to be second order in water. Bel'skii ''et al.''. have proposed a prerate determining nucleophilic attack by water resulting in an equilibrium between the penta- and hexacoordinated phosphorus species, which is followed by a proton transfer involving the second water molecule in a rate determining ring-opening step, leading to the hydroxylated product.  
[[image:Hydrolysis Pentacoordinated Phosphorous.png|thumb|500px | center | Mechanism of the hydrolysis of pentacoordinated phosphorus]]

Alcoholysis of pentacoordinated phosphorus compounds, such as trimethoxyphospholene with benzyl alcohol, have also been postulated to occur through a similar octahedral transition state, as in hydrolysis, however without ring opening.<ref name= Ramirez1968>{{cite journal | doi = 10.1021/ja01005a035 | author = Ramirez, F | title = Nucleophilic substitutions at pentavalent phosphorus. Reaction of 2,2,2-trialkoxy-2,2-dihydro-1,3,2-dioxaphospholenes with alcohols| journal = [[J. Am. Chem. Soc.]]| year = 1968|volume = 90|pages = 751 | last2 = Tasaka | first2 = K. | last3 = Desai | first3 = N. B. | last4 = Smith | first4 = Curtis Page. | issue = 3| bibcode = 1968JAChS..90..751R }}</ref>

[[image:Base Catalyzed Alcoholysis Pentacoordinated Phosphorous.png|thumb|500px | center | Mechanism of the base catalyzed alcoholysis of pentacoordinated phosphorus]]

It can be understood from these experiments that the increased reactivity observed for hypervalent molecules, contrasted with analogous nonhypervalent compounds, can be attributed to the congruence of these species to the hypercoordinated activated states normally formed during the course of the reaction.

===Ab initio calculations===
The enhanced reactivity at pentacoordinated silicon is not fully understood. Corriu and coworkers suggested that greater electropositive character at the pentavalent silicon atom may be responsible for its increased reactivity.<ref name=abinitio2>{{cite journal | doi = 10.1021/om00157a016 | title = Pentacoordinated silicon anions: Synthesis and reactivity | year = 1990 | last1 = Brefort | first1 = Jean Louis | last2 = Corriu | first2 = Robert J. P. | last3 = Guerin | first3 = Christian | last4 = Henner | first4 = Bernard J. L. | last5 = Wong Chi Man | first5 = Wong Wee Choy | journal = [[Organometallics]] | volume = 9 | issue = 7 | pages = 2080 }}</ref> Preliminary ab initio calculations supported this hypothesis to some degree, but used a small basis set.<ref name=abinitio1>{{cite journal | title = Enhanced Reactivity of Pentacoordinated Silicon Species. An ab Initio Approach | journal = [[J. Am. Chem. Soc.]] | year = 1990 | volume = 112 | pages = 7197–7202 | doi = 10.1021/ja00176a018 | author1 = Dieters, J. A. | author2 = Holmes, R. R. | issue = 20}}</ref>

A software program for ab initio calculations, [[Gaussian (software)|Gaussian 86]], was used by Dieters and coworkers to compare tetracoordinated silicon and phosphorus to their pentacoordinate analogues. This [[ab initio quantum chemistry methods|ab initio]] approach is used as a supplement to determine why reactivity improves in nucleophilic reactions with pentacoordinated compounds.  For silicon, the [[Basis set (chemistry)|6-31+G* basis set]] was used because of its pentacoordinated anionic character and for phosphorus, the [[Basis set (chemistry)|6-31G* basis set]] was used.<ref name=abinitio1 />

Pentacoordinated compounds should theoretically be less electrophilic than tetracoordinated analogues due to steric hindrance and greater electron density from the ligands, yet experimentally show greater reactivity with nucleophiles than their tetracoordinated analogues. Advanced ab initio calculations were performed on series of tetracoordinated and pentacoordinated species to further understand this reactivity phenomenon. Each series varied by degree of fluorination. Bond lengths and charge densities are shown as functions of how many hydride ligands are on the central atoms. For every new hydride, there is one less fluoride.<ref name="abinitio1"/>

For silicon and phosphorus bond lengths, charge densities, and Mulliken bond overlap, populations were calculated for tetra and pentacoordinated species by this ab initio approach.<ref name="abinitio1"/> Addition of a fluoride ion to tetracoordinated silicon shows an overall average increase of 0.1 electron charge, which is considered insignificant.  In general, bond lengths in trigonal bipyramidal pentacoordinate species are longer than those in tetracoordinate analogues. Si-F bonds and Si-H bonds both increase in length upon pentacoordination and related effects are seen in phosphorus species, but to a lesser degree. The reason for the greater magnitude in bond length change for silicon species over phosphorus species is the increased effective nuclear charge at phosphorus. Therefore, silicon is concluded to be more loosely bound to its ligands.{{multiple image | align = center | direction = horizontal | header = Effects of fluorine substitution on positive charge density | width = 500 | image1 = charge densities - silicon.png | caption1 = Comparison of Charge Densities with Degree of Fluorination for Tetra and Pentacoordinated Silicon}}

In addition Dieters and coworkers <ref name="abinitio1"/> show an inverse correlation between bond length and bond overlap for all series. Pentacoordinated species are concluded to be more reactive because of their looser bonds as trigonal-bipyramidal structures.{{multiple image  | align = center  | direction = horizontal  | header    = Calculated bond length and bond overlap with degree of fluorination | width = 500| image1 = Si-F bond lengths.png  | caption1  = Comparison of Bond Lengths with Degree of Fluorination for Tetra and Pentacoordinated Silicon| image2 = bond lengths - phosphorus.png  | caption2  = Comparison of Bond Lengths with Degree of Fluorination for Tetra and Pentacoordinated Phosphorus}}

By calculating the energies for the addition and removal of a fluoride ion in various silicon and phosphorus species, several trends were found. In particular, the tetracoordinated species have much higher energy requirements for ligand removal than do pentacoordinated species. Further, silicon species have lower energy requirements for ligand removal than do phosphorus species, which is an indication of weaker bonds in silicon.