Hypervalent molecule
Encyclopedia
A hypervalent molecule is a molecule
that contains one or more main group element
s formally bearing more than eight electron
s in their valence shells. Phosphorus pentachloride (PCl5), sulfur hexafluoride
(SF6), the phosphate
(PO43−) ion, chlorine trifluoride
(ClF3) and the triiodide
(I3−) ion are examples of hypervalent molecules.
other than the lowest.
Several specific classes of hypervalent molecules exist:
Examples of N-X-L nomenclature include:
and Irving Langmuir
and the debate over the nature of the chemical bond in the 1920s. Lewis maintained the importance of the two-center two-electron (2c-2e) bond in describing hypervalence, thus allowing for expanded octets. Langmuir, on the other hand, upheld the dominance of the octet rule and preferred the use of ionic bonds to account for hypervalence without violating the rule (e.g. SF42+, F22−).
In the late 1920s and 1930s, Sugden argued for the existence of a two-center one-electron (2c-1e) bond and thus rationalized bonding in hypervalent molecules without the need for expanded octets or ionic bond character; this was poorly accepted at the time. In the 1940s and 1950s, Rundle and Pimentel popularized the idea of the three-center four-electron bond
, which is essentially the same concept which Sugden attempted to advance decades earlier; the three-center four-electron bond can be alternatively viewed as consisting of two collinear two-center one-electron bonds, with the remaining two nonbonding electrons localized to the ligands.
The attempt to actually prepare hypervalent organic molecules began with Hermann Staudinger
and Georg Wittig
in the first half of the twentieth century, who sought to challenge the extant valence theory and successfully prepare nitrogen and phosphorus-centered hypervalent molecules. The theoretical basis for hypervalency was not delineated until J.I. Musher's work in 1969.
In 1990, Magnusson published a seminal work definitively excluding the role of d-orbital hybridization in bonding in hypervalent compounds of second-row elements. This had long been a point of contention and confusion in describing these molecules using molecular orbital theory. Part of the confusion here originates from the fact that one must include d-functions in the basis sets used to describe these compounds (or else unreasonably high energies and distorted geometries result), and the contribution of the d-function to the molecular wavefunction is large. These facts were historically interpreted to mean that d-orbitals must be involved in bonding. However, Magnusson concludes in his work that d-orbital involvement is not implicated in hypervalency.
proposed the replacement of 'hypervalency' with use of the term hypercoordination because this term does not imply any mode of chemical bonding and the question could thus be avoided altogether.
The concept itself has been criticized by Ronald Gillespie
who, based on an analysis of electron localization functions, wrote in 2002 that "as there is no fundamental difference between the bonds in hypervalent and non-hypervalent (Lewis octet) molecules there is no reason to continue to use the term hypervalent."
For hypercoordinated molecules with electronegative ligands such as PF5 it has been demonstrated that the ligands can pull away enough electron density from the central atom so that its net content is again 8 electrons or fewer. Consistent with this alternative view is the finding that hypercoordinated molecules based on fluorine ligands, for example PF5 do not have hydride
counterparts e.g. phosphorane
PH5 which is an unstable molecule.
Even an ionic model holds up well in thermochemical
calculations. It predicts favorable exothermic
formation of PF4+F− from phosphorus trifluoride
PF3 and fluorine
F2 whereas a similar reaction forming PH4+H− is not favorable.
, several alternative models have been proposed.
In the 1950s molecular orbital treatment of hypervalent bonding was adduced to explain the molecular architecture. According to MO theory, the central atom of penta- and hexacoordinated molecules would be sp3d and sp3d2 hybridized
, which requires the promotion of central atom electrons to unoccupied d-orbitals. However, advances in the study of ab initio
calculations have revealed that the contribution of d-orbitals to hypervalent bonding is too small to describe the bonding properties, and this hybrid orbital description is now regarded as much less important. It was shown that in the case of hexacoordinated SF6, d-orbitals are not involved in S-F bond formation, but charge transfer between the sulfur and fluorine atoms and the apposite resonance structures were able to account for the hypervalency.
Additional modifications to the octet rule have been attempted to involve ionic characteristics in hypervalent bonding. As one of these modifications, in 1951, the concept of the 3-center-4-electron (3c-4e) bond, which described hypervalent bonding with a qualitative molecular orbital
, was proposed. The 3c-4e bond is described as three molecular orbitals given by the combination of a p orbital on the central atom and two ligand
orbitals leading to an occupied non-bonding orbital (HOMO
), and an unoccupied anti-bonding orbital (LUMO
). This model in which the octet rule is preserved was also advocated by Musher.
An example of this is the hexacoordinated SF6, which has been proposed to be composed of three 3c-4e bonds. In this model each bond is equivalent, linear and orthogonal with one lying along each of x, y and z axes. These interactions are F(p1)-S(3px2)-F(p1), F(p1)-S(3py2)-F(p1), and F(p1)-S(3pz2)-F(p1). Together these data account for both the octahedral symmetry of the molecule as well as observed molecular structure
A more complete description of hypervalent molecules arises from consideration of molecular orbital theory through quantum mechanical methods. A LCAO in, for example, sulfur hexafluoride, taking a basis set of the one sulfur 3s-orbital, the three sulfur 3p-orbitals, and six octahedral geometry symmetry-adapted linear combinations (SALCs) of fluorine orbitals, a total of ten molecular orbitals are obtained (four fully occupied bonding MOs of the lowest energy, two fully occupied intermediate energy non-bonding MOs and four vacant antibonding MOs with the highest energy) providing room for all 12 valence electrons. This is a stable configuration only for SX6 molecules containing electronegative ligand atoms like fluorine, which explains why SH6 doesn't form. Invoking the sulfur 3d-orbitals in the bonding model, the two non-bonding MOs (1eg) are further stabilized to lower-energy bonding MOs because of overlap with the two degenerate 3d orbitals having the proper symmetry (eg). However, the extent of d-orbital participation is thought to be minimal.
and relative discrepancy between the axial and equatorial bond lengths. The axial bonds may be represented as two half-bonds (the ‘average’ of the symmetrical resonance forms) or a single 3c-4e bond. However, the magnitude of the discrepancy between the axial and equatorial bond lengths is substantially smaller than this structural model predicts.
, each of the six bonds is the same length. The rationalization described above can be applied to generate resonance structures each with two covalent bonds and two 3c-4e bonds, such that the 3c-4e bond character is distributed across each of the sulfur-fluorine bonds.
molecules involving nitrogen, oxygen, or sulfur ligands provide examples of Lewis acid-Lewis base hexacoordination. For the two similar complexes shown below, the length of the C-P bond increases with decreasing length of the N-P bond; the strength of the C-P bond decreases with increasing strength of the N-P Lewis acid-Lewis base interaction.
examples shown below.
Interestingly, complexes such as these provide a model for the SN2 transition state
; the Si-O bonds range from close to the expected van der Waals value in A (a weak bond, representing an early SN2 transition state) almost to the expected covalent single bond value in C (a strong bond, representing a late SN2 transition state).
s of hydrolysis of tetravalent chlorosilanes incubated with catalytic amounts of water returned a rate that is 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 in which tetracoordinated Mes2SiF2 (Mes = mesityl) and pentacoordinated Mes2SiF3- 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 HF2- is quickly displaced leading to the hydroxylated product. This reaction and crystallographic data support the mechanism proposed by Corriu et al..
The apparent increased reactivity of hypervalent molecules, contrasted with tetravalent analogues, has also been observed for Grignard reactions. The Corriu group measured Grignard reaction half-times by NMR for related 18-crown-6 potassium salts of a variety of tetra- and pentacoordinated methylphenylfluorosilanes 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.
Hydrolysis of phosphoranes and oxyphosphoranes have been studied 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 hydroxlyated product.
Alchoholysis 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.
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.
A software program for ab initio calculations, Gaussian 86
, was used by Dieters and coworkers to compare tetracoordinated silicon and phosphorus to their pentacoordinate analogues. This ab initio
approach is used as a supplement to determine why reactivity improves in nucleophilic reactions with pentacoordinated compounds. For silicon, the 6-31+G* basis set
was used because of its pentacoordinated anionic character and for phosphorus, the 6-31G* basis set
was used.
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.
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. 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.
In addition Dieters and coworkers 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.
The energies for addition and removal of a fluoride ion in silicon and phosphorus species were calculated.
As the table shows, tetracoordinated species have much higher energy requirements for ligand removal than do pentacoordinated species. Overall, silicon species have lower energy requirements for ligand removal than do phosphorus species, which is an indication of weaker bonds in silicon.
In conclusion, it has been shown that charge density changes are insignificant in accounting for enhanced reactivity with nucleophiles in hypercoordinated silicon and phosphorus. On the other hand, enhanced reactivity is due to weaker bonds, particularly in the axial positions, of pentacoordinated species.
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
that contains one or more main group element
Main group element
In chemistry and atomic physics, main group elements are elements in groups whose lightest members are represented by helium, lithium,...
s formally bearing more than eight electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s in their valence shells. Phosphorus pentachloride (PCl5), sulfur hexafluoride
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...
(SF6), the phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
(PO43−) ion, chlorine trifluoride
Chlorine trifluoride
Chlorine trifluoride is an interhalogen compound with the formula ClF3. This colourless, poisonous, corrosive and very reactive gas condenses to a pale-greenish yellow liquid, the form in which it is most often sold...
(ClF3) and the triiodide
Triiodide
In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...
(I3−) ion are examples of hypervalent molecules.
Definitions and nomenclature
Hypervalent molecules were first formally defined by Jeremy I. Musher in 1969 as molecules having central atoms of group 15-18 in any oxidation stateOxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
other than the lowest.
Several specific classes of hypervalent molecules exist:
- Hypervalent iodine compounds are useful reagents in organic chemistry (e.g. Dess-Martin periodinaneDess-Martin periodinaneDess–Martin periodinane is a chemical reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. This periodinane has several advantages over chromium- and DMSO-based oxidants that include milder conditions , shorter reaction times, higher yields, simplified workups,...
) - Tetra-, penta- and hexacoordinated phosphorus, silicon, and sulfur compounds (ex. PCl5, PF5, SF6, sulfuranes and persulfuranes)
- Noble gas compounds (ex. xenon tetrafluoride, XeF4)
- Halogen polyfluorides (ex. ClF5)
- Non-classical carbocations (ex. Norbornyl cation)
- Many common acids (ex. chloric acidChloric acidChloric acid, HClO3, is an oxoacid of chlorine, and the formal precursor of chlorate salts. It is a strong acid and oxidizing agent....
, phosphoric acidPhosphoric acidPhosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...
, and sulfuric acidSulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
)
N-X-L notation
N-X-L nomenclature, introduced in 1980, is often used to classify hypervalent compounds of main group elements, where:- N represents the number of valence electrons
- X is the chemical symbol of the central atom
- L the number of ligands to the central atom
Examples of N-X-L nomenclature include:
- XeF2Xenon difluorideXenon difluoride is a powerful fluorinating agent with the chemical formula , and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture sensitive. It decomposes on contact with light or water vapour. Xenon difluoride is a dense, white crystalline solid. It...
, 10-Xe-2 - PCl5, 10-P-5
- SF6Sulfur hexafluorideSulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...
, 12-S-6 - IF7Iodine heptafluorideIodine heptafluoride, also known as iodine fluoride or even iodine fluoride, is an interhalogen compound with chemical formula IF7. It has an unusual pentagonal bipyramidal structure, as predicted by VSEPR theory...
, 14-I-7
History and controversy
The debate over the nature and classification of hypervalent molecules goes back to Gilbert N. LewisGilbert N. Lewis
Gilbert Newton Lewis was an American physical chemist known for the discovery of the covalent bond , his purification of heavy water, his reformulation of chemical thermodynamics in a mathematically rigorous manner accessible to ordinary chemists, his theory of Lewis acids and...
and Irving Langmuir
Irving Langmuir
Irving Langmuir was an American chemist and physicist. His most noted publication was the famous 1919 article "The Arrangement of Electrons in Atoms and Molecules" in which, building on Gilbert N. Lewis's cubical atom theory and Walther Kossel's chemical bonding theory, he outlined his...
and the debate over the nature of the chemical bond in the 1920s. Lewis maintained the importance of the two-center two-electron (2c-2e) bond in describing hypervalence, thus allowing for expanded octets. Langmuir, on the other hand, upheld the dominance of the octet rule and preferred the use of ionic bonds to account for hypervalence without violating the rule (e.g. SF42+, F22−).
In the late 1920s and 1930s, Sugden argued for the existence of a two-center one-electron (2c-1e) bond and thus rationalized bonding in hypervalent molecules without the need for expanded octets or ionic bond character; this was poorly accepted at the time. In the 1940s and 1950s, Rundle and Pimentel popularized the idea of the three-center four-electron bond
Three-center four-electron bond
The 3-center 4-electron bond is a model used to explain bonding in hypervalent molecules such as phosphorus pentafluoride, sulfur hexafluoride, the xenon fluorides, and the bifluoride ion. It is also known as the Pimentel-Rundle three-center model after the work published by George C. Pimentel in...
, which is essentially the same concept which Sugden attempted to advance decades earlier; the three-center four-electron bond can be alternatively viewed as consisting of two collinear two-center one-electron bonds, with the remaining two nonbonding electrons localized to the ligands.
The attempt to actually prepare hypervalent organic molecules began with Hermann Staudinger
Hermann Staudinger
- External links :* Staudinger's * Staudinger's Nobel Lecture *....
and Georg Wittig
Georg Wittig
Georg Wittig was a German chemist who reported a method for synthesis of alkenes from aldehydes and ketones using compounds called phosphonium ylides in the Wittig reaction. He shared the Nobel Prize in Chemistry with Herbert C...
in the first half of the twentieth century, who sought to challenge the extant valence theory and successfully prepare nitrogen and phosphorus-centered hypervalent molecules. The theoretical basis for hypervalency was not delineated until J.I. Musher's work in 1969.
In 1990, Magnusson published a seminal work definitively excluding the role of d-orbital hybridization in bonding in hypervalent compounds of second-row elements. This had long been a point of contention and confusion in describing these molecules using molecular orbital theory. Part of the confusion here originates from the fact that one must include d-functions in the basis sets used to describe these compounds (or else unreasonably high energies and distorted geometries result), and the contribution of the d-function to the molecular wavefunction is large. These facts were historically interpreted to mean that d-orbitals must be involved in bonding. However, Magnusson concludes in his work that d-orbital involvement is not implicated in hypervalency.
Criticism
Both the term and concept of hypervalency still fall under criticism. In 1984, in response to this general controversy, Paul von Ragué SchleyerPaul von Rague Schleyer
Paul von Ragué Schleyer is an organic physical chemist of substantial significance whose research has been cited with great frequency. A 1997 survey indicated that Dr. Schleyer was, at the time, the world's third most cited chemist, with over 1100 technical papers produced...
proposed the replacement of 'hypervalency' with use of the term hypercoordination because this term does not imply any mode of chemical bonding and the question could thus be avoided altogether.
The concept itself has been criticized by Ronald Gillespie
Ronald Gillespie
Ronald James Gillespie, CM , a chemistry professor at McMaster University, specializes in the field of Molecular Geometry in Chemistry. In 2007 he was awarded the Order of Canada....
who, based on an analysis of electron localization functions, wrote in 2002 that "as there is no fundamental difference between the bonds in hypervalent and non-hypervalent (Lewis octet) molecules there is no reason to continue to use the term hypervalent."
For hypercoordinated molecules with electronegative ligands such as PF5 it has been demonstrated that the ligands can pull away enough electron density from the central atom so that its net content is again 8 electrons or fewer. Consistent with this alternative view is the finding that hypercoordinated molecules based on fluorine ligands, for example PF5 do not have hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
counterparts e.g. phosphorane
Phosphorane
A phosphorane is a functional group in organophosphorus chemistry with pentavalent phosphorus. It has the general formula PR5. The parent hydride compound is the unstable molecule PH5...
PH5 which is an unstable molecule.
Even an ionic model holds up well in thermochemical
Thermochemistry
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...
calculations. It predicts favorable exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
formation of PF4+F− from phosphorus trifluoride
Phosphorus trifluoride
Phosphorus trifluoride , is a colorless and odorless gas. It is highly toxic and it reacts slowly with water. Its main use is as a ligand in metal complexes...
PF3 and fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
F2 whereas a similar reaction forming PH4+H− is not favorable.
Bonding in hypervalent molecules
Early considerations of the structure of hypervalent molecules, returned familiar arrangements that were well explained by the VSEPR model for atomic bonding. Accordingly, AB5 and AB6 type molecules would possess a trigonal bi-pyramidal and octahedral geometry, respectively. However in order to account for the observed bond angles, bond lengths and apparent violation of the Lewis octet ruleOctet rule
The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...
, several alternative models have been proposed.
In the 1950s molecular orbital treatment of hypervalent bonding was adduced to explain the molecular architecture. According to MO theory, the central atom of penta- and hexacoordinated molecules would be sp3d and sp3d2 hybridized
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...
, which requires the promotion of central atom electrons to unoccupied d-orbitals. However, advances in the study of ab initio
Ab initio
ab initio is a Latin term used in English, meaning from the beginning.ab initio may also refer to:* Ab Initio , a leading ETL Tool Software Company in the field of Data Warehousing.* ab initio quantum chemistry methods...
calculations have revealed that the contribution of d-orbitals to hypervalent bonding is too small to describe the bonding properties, and this hybrid orbital description is now regarded as much less important. It was shown that in the case of hexacoordinated SF6, d-orbitals are not involved in S-F bond formation, but charge transfer between the sulfur and fluorine atoms and the apposite resonance structures were able to account for the hypervalency.
Additional modifications to the octet rule have been attempted to involve ionic characteristics in hypervalent bonding. As one of these modifications, in 1951, the concept of the 3-center-4-electron (3c-4e) bond, which described hypervalent bonding with a qualitative molecular orbital
Molecular orbital
In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
, was proposed. The 3c-4e bond is described as three molecular orbitals given by the combination of a p orbital on the central atom and two ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
orbitals leading to an occupied non-bonding orbital (HOMO
Homo
Homo may refer to:*the Greek prefix ὅμο-, meaning "the same"*the Latin for man, human being*Homo, the taxonomical genus including modern humans...
), and an unoccupied anti-bonding orbital (LUMO
Lumo
Lumo is a 2007 documentary film about twenty-year-old Lumo Sinai, a woman who fell victim to "Africa's First World War." While returning home one day, Lumo and another woman were gang-raped by a group of soldiers fighting for control of the Democratic Republic of the Congo during the 1994 Rwandan...
). This model in which the octet rule is preserved was also advocated by Musher.
An example of this is the hexacoordinated SF6, which has been proposed to be composed of three 3c-4e bonds. In this model each bond is equivalent, linear and orthogonal with one lying along each of x, y and z axes. These interactions are F(p1)-S(3px2)-F(p1), F(p1)-S(3py2)-F(p1), and F(p1)-S(3pz2)-F(p1). Together these data account for both the octahedral symmetry of the molecule as well as observed molecular structure
A more complete description of hypervalent molecules arises from consideration of molecular orbital theory through quantum mechanical methods. A LCAO in, for example, sulfur hexafluoride, taking a basis set of the one sulfur 3s-orbital, the three sulfur 3p-orbitals, and six octahedral geometry symmetry-adapted linear combinations (SALCs) of fluorine orbitals, a total of ten molecular orbitals are obtained (four fully occupied bonding MOs of the lowest energy, two fully occupied intermediate energy non-bonding MOs and four vacant antibonding MOs with the highest energy) providing room for all 12 valence electrons. This is a stable configuration only for SX6 molecules containing electronegative ligand atoms like fluorine, which explains why SH6 doesn't form. Invoking the sulfur 3d-orbitals in the bonding model, the two non-bonding MOs (1eg) are further stabilized to lower-energy bonding MOs because of overlap with the two degenerate 3d orbitals having the proper symmetry (eg). However, the extent of d-orbital participation is thought to be minimal.
Pentacoordinated phosphorus
For hypervalent compounds in which the ligands are more electronegative than the central, hypervalent atom, resonance structures can be drawn with no more than four covalent electron pair bonds and completed with ionic bonds to obey the octet rule. For example, phosphorus pentafluoride’s three equatorial bonds can be formed from sp2-hybridized phosphorus orbitals. The axial bonds can then be described by two resonance forms each containing one ionic bond and one covalent bond, thus satisfying the octet rule and explaining both the observed molecular geometryMolecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...
and relative discrepancy between the axial and equatorial bond lengths. The axial bonds may be represented as two half-bonds (the ‘average’ of the symmetrical resonance forms) or a single 3c-4e bond. However, the magnitude of the discrepancy between the axial and equatorial bond lengths is substantially smaller than this structural model predicts.
Hexacoordinated sulfur
For a hexacoordinate molecule such as sulfur hexafluorideSulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...
, each of the six bonds is the same length. The rationalization described above can be applied to generate resonance structures each with two covalent bonds and two 3c-4e bonds, such that the 3c-4e bond character is distributed across each of the sulfur-fluorine bonds.
Hexacoordinated phosphorus
Hexacoordinate phosphorusPhosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
molecules involving nitrogen, oxygen, or sulfur ligands provide examples of Lewis acid-Lewis base hexacoordination. For the two similar complexes shown below, the length of the C-P bond increases with decreasing length of the N-P bond; the strength of the C-P bond decreases with increasing strength of the N-P Lewis acid-Lewis base interaction.
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 siliconSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
examples shown below.
Interestingly, complexes such as these provide a model for the SN2 transition state
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
; the Si-O bonds range from close to the expected van der Waals value in A (a weak bond, representing an early SN2 transition state) almost to the expected covalent single bond value in C (a strong bond, representing a late SN2 transition state).
Silicon
Corriu and coworkers performed early work characterizing reactions thought to proceed through a hypervalent transition state. Measurements of the reaction rateReaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
s of hydrolysis of tetravalent chlorosilanes incubated with catalytic amounts of water returned a rate that is 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 in which tetracoordinated Mes2SiF2 (Mes = mesityl) and pentacoordinated Mes2SiF3- 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 HF2- is quickly displaced leading to the hydroxylated product. This reaction and crystallographic data support the mechanism proposed by Corriu et al..
The apparent increased reactivity of hypervalent molecules, contrasted with tetravalent analogues, has also been observed for Grignard reactions. The Corriu group measured Grignard reaction half-times by NMR for related 18-crown-6 potassium salts of a variety of tetra- and pentacoordinated methylphenylfluorosilanes 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.
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 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 hydroxlyated product.
Alchoholysis 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.
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. Preliminary ab initio calculations supported this hypothesis to some degree, but used a small basis set.A software program for ab initio calculations, Gaussian 86
GAUSSIAN
Gaussian is a computational chemistry software program initially released in 1970 by John Pople and his research group at Carnegie-Mellon University as Gaussian 70. It has been continuously updated since then...
, was used by Dieters and coworkers to compare tetracoordinated silicon and phosphorus to their pentacoordinate analogues. This ab initio
Ab initio quantum chemistry methods
Ab initio quantum chemistry methods are computational chemistry methods based on quantum chemistry. The term ab initiowas first used in quantum chemistry by Robert Parr and coworkers, including David Craig in a semiempirical study on the excited states of benzene.The background is described by Parr...
approach is used as a supplement to determine why reactivity improves in nucleophilic reactions with pentacoordinated compounds. For silicon, the 6-31+G* basis set
Basis set (chemistry)
A basis set in chemistry is a set of functions used to create the molecular orbitals, which are expanded as a linear combination of such functions with the weights or coefficients to be determined. Usually these functions are atomic orbitals, in that they are centered on atoms. Otherwise, the...
was used because of its pentacoordinated anionic character and for phosphorus, the 6-31G* basis set
Basis set (chemistry)
A basis set in chemistry is a set of functions used to create the molecular orbitals, which are expanded as a linear combination of such functions with the weights or coefficients to be determined. Usually these functions are atomic orbitals, in that they are centered on atoms. Otherwise, the...
was used.
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.
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. 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.
In addition Dieters and coworkers 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.
The energies for addition and removal of a fluoride ion in silicon and phosphorus species were calculated.
As the table shows, tetracoordinated species have much higher energy requirements for ligand removal than do pentacoordinated species. Overall, silicon species have lower energy requirements for ligand removal than do phosphorus species, which is an indication of weaker bonds in silicon.
In conclusion, it has been shown that charge density changes are insignificant in accounting for enhanced reactivity with nucleophiles in hypercoordinated silicon and phosphorus. On the other hand, enhanced reactivity is due to weaker bonds, particularly in the axial positions, of pentacoordinated species.
Application
The mechanistic implications of this are extended to a hexacoordinated silicon species, which is thought to be active as a transition state in reactions such as the allylation of aldehydes with allyltrifluorosilane. The reaction only precedes with fluoride activation to the pentacoordinated state and weakening of the bond between silicon and carbon in the hexacoordinate state drives this reaction.See also
- VSEPR theoryVSEPR theoryValence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
- Molecular orbital theoryMolecular orbital theoryIn chemistry, molecular orbital theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule...
- Three-center four-electron bondThree-center four-electron bondThe 3-center 4-electron bond is a model used to explain bonding in hypervalent molecules such as phosphorus pentafluoride, sulfur hexafluoride, the xenon fluorides, and the bifluoride ion. It is also known as the Pimentel-Rundle three-center model after the work published by George C. Pimentel in...