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Hypervalent molecule
 
 
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A hypervalent molecule is a molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
 that contains one or more typical elements (group
Periodic table group

In chemistry, a group is a vertical column in the periodic table of the chemical elements. The name family is derived from the fact that the elements share similar characteristics and traits, just as members of any human family would....
 1, 2, 13-18) formally bearing more than eight electron
Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
s in their valence shells. Phosphorus pentachloride (PCl5), sulfur hexafluoride
Sulfur hexafluoride

Sulfur hexafluoride is an inorganic compound with the formula . It is a colorless, odorless, non-toxicity and non-flammable gas . has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom....
 (SF6), the phosphate
Phosphate

A phosphate, an inorganic chemical, is a Salt of phosphoric acid. Inorganic phosphates are mining to obtain phosphorus for use in agriculture and industry....
 (PO43-) ion, Chlorine trifluoride
Chlorine trifluoride

Chlorine trifluoride is the chemical 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 ....
 and the triiodide
Triiodide

In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3-, a polyatomic ion anion composed of three iodine atoms....
 (I3-) ion are examples of hypervalent molecules. Hypervalent molecules were first defined by Jeremy I.






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A hypervalent molecule is a molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
 that contains one or more typical elements (group
Periodic table group

In chemistry, a group is a vertical column in the periodic table of the chemical elements. The name family is derived from the fact that the elements share similar characteristics and traits, just as members of any human family would....
 1, 2, 13-18) formally bearing more than eight electron
Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
s in their valence shells. Phosphorus pentachloride (PCl5), sulfur hexafluoride
Sulfur hexafluoride

Sulfur hexafluoride is an inorganic compound with the formula . It is a colorless, odorless, non-toxicity and non-flammable gas . has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom....
 (SF6), the phosphate
Phosphate

A phosphate, an inorganic chemical, is a Salt of phosphoric acid. Inorganic phosphates are mining to obtain phosphorus for use in agriculture and industry....
 (PO43-) ion, Chlorine trifluoride
Chlorine trifluoride

Chlorine trifluoride is the chemical 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 ....
 and the triiodide
Triiodide

In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3-, a polyatomic ion anion composed of three iodine atoms....
 (I3-) ion are examples of hypervalent molecules. Hypervalent molecules were first defined by Jeremy I. Musher in 1969 as molecules of the elements of group 15-18 in any oxidation state
Oxidation 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 Electrical charge that an atom would have if all bonds to atoms of different elements were 100% Ionic bond....
 other than the lowest.

Several specific classes of hypervalent molecules exist:
  • Hypervalent iodine compounds are useful reagents in organic chemistry.
  • Sulfuranes and persulfuranes are hypervalent sulfur compounds.


Bonding in hypervalent molecules


Since the hypervalent molecules do not seem to obey the octet rule
Octet rule

The octet rule is a simple chemistry rule of thumb that states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electronic configuration as a noble gas....
, some models have been proposed to describe their bonding properties. Irving Langmuir
Irving Langmuir

Irving Langmuir was an United States chemistry and physics. His most noted publication was the famous 1919 article "The Arrangement of Electrons in Atoms and Molecules" in which, building on Gilbert N....
 in the 1920s held the view that the octet rule did prevail and that bonding was based on ionic interactions (e.g. SF42+F22-). His opponent in this era, Gilbert N. Lewis
Gilbert N. Lewis

Gilbert Newton Lewis was a famous American physical chemistry 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 bases, and his photochemical experiments....
 on the other hand believed in octet expansion.

Hypervalent bonding has also been believed to be described as sp3d and sp3d2 hybrid orbital
Orbital hybridisation

In chemistry, hybridisation or hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties....
s composed of s, p, and d-orbitals at higher energy levels. However, advances in the study of ab initio
Ab initio

The Latin term ab initio means from the beginning and is used in several contexts:* when describing literature: told from the beginning as opposed to in medias res ...
 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.

As another description of hypervalent molecules, modifications of 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 that describes 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....
, was proposed. The 3c-4e bond is described as three molecular orbitals given by the combination of a p orbital on a central atom and two ligand
Ligand

In chemistry, a ligand is either an atom, ion, or molecule that bonds to a central metal, generally involving formal donation of one or more of its electrons....
 orbitals: an occupied bonding orbital, an occupied non-bonding orbital (HOMO
Homo

Homo may refer to:In science:* Homo , the genus including modern humans and closely related species such as Neanderthals* Highest Occupied Molecular Orbital, in chemistry: see HOMO/LUMO...
), 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 Congo during the 1994 Rwandan genocide....
). This model in which the octet rule is preserved was also advocated by Musher.

N-X-L notation

The 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 involved in bonding
  • X is the chemical symbol of the central atom
  • L the number of ligands to the central atom
Examples are
  • XeF2
    Xenon difluoride

    Preperation:- Xe + F2 = XeF2Properties:-1. It is a colourless solids & its melting point is 127 C.Xenon difluoride is a powerful fluorinating agent, with the chemical formula , is one of the most stable xenon compounds....
    , 10-Xe-2
  • PCl5, 10-P-5
  • SF6
    Sulfur hexafluoride

    Sulfur hexafluoride is an inorganic compound with the formula . It is a colorless, odorless, non-toxicity and non-flammable gas . has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom....
    , 12-S-6
  • IF7
    Iodine heptafluoride

    Iodine heptafluoride, also known as iodine fluoride or even iodine fluoride, is the compound iodinefluoride7. It has an unusual Pentagonal bipyramid molecular geometry structure, as predicted by VSEPR theory....
    , 14-I-7


Criticism of the term

As a replacement for the term hypervalency, Paul von Ragué Schleyer
Paul von Rague Schleyer

Paul von Ragu? Schleyer is an organic chemistry physical chemist of substantial significance whose research has been cited with great frequency....
 in 1984 proposed hypercoordination because this term does not imply any mode of chemical bonding.

The concept of hypervalency has been criticized by Ronald Gillespie
Ronald Gillespie

Ronald J. Gillespie, Order of Canada , a chemistry professor at McMaster University, specializes in the field of Molecular Geometry in Chemistry....
 who, based on an analysis of electron localization functions, concluded 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

Hydride is the name given to the Electric charge ion of hydrogen, H-. Although this ion does not exist except in extraordinary conditions, the term hydride is widely applied to describe Chemical compound of hydrogen with other chemical element, particularly those of Periodic table group 1–16....
 counterparts e.g. phosphorane
Phosphorane

A phosphorane is a functional group in chemistry with pentavalent phosphorus. It has the general structure PR5. The parent compound is the non-stable phosphoran PH5 or ?5-Phosphan according to IUPAC nomenclature....
 PH5 which is an unstable molecule.

Even an ionic model holds up well in thermochemical
Thermochemistry

In thermodynamics and physical chemistry, thermochemistry is the study of the energy evolved or absorbed in chemical reactions and any physical transformations, such as melting and boiling....
 calculations. It predicts favorable exothermic
Exothermic

File:Explosion1.JPG In thermodynamics, the term exothermic describes a process or reaction that releases energy usually in the form of heat, but also in form of light , electricity , or sound....
 formation of PF4+F- from phosphorus trifluoride
Phosphorus trifluoride

Phosphorus trifluoride...
 PF3 and fluorine
Fluorine

Fluorine is the chemical element with the symbol F and atomic number 9. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F2 molecule....
 F2 whereas a similar reaction forming PH4+H- is not favorable.