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A hypervalent molecule is a molecule that contains one or more typical elements (group 1, 2, 13-18) formally bearing more than eight electrons in their valence shells. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by In Chemistry a group, also known as a family, is a vertical column in the Periodic table of the Chemical elements There are 18 groups in The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J An electron shell may be crudely thought of as an Orbit followed by Electrons around an Atom nucleus. Phosphorus pentachloride (PCl5), sulfur hexafluoride (SF6), the phosphate (PO43−) ion, Chlorine trifluoride and the triiodide (I3−) ion are examples of hypervalent molecules. Phosphorus pentachloride is the Chemical compound with the formula PCl5 Sulfur hexafluoride is an Inorganic compound with the formula. A phosphate, an Inorganic chemical, is a salt of Phosphoric acid. Chlorine trifluoride is the Chemical compound with the formula ClF3 In chemistry triiodide (sometimes written tri-iodide can have several meanings Hypervalent molecules were first defined by Jeremy I. Musher in 1969 as molecules of the elements of group 15-18 in any oxidation state other than the lowest. In Chemistry, the oxidation state is an indicator of the degree of Oxidation of an Atom in a Chemical compound. [1]

Several specific classes of hypervalent molecules exist:

Contents

Bonding in hypervalent molecules

Since the hypervalent molecules do not seem to obey the octet rule, some models have been proposed to describe their bonding properties. The octet rule is a simple chemical Rule of thumb that states that Atoms tend to combine in such a way that they each have eight Electrons in Irving Langmuir in the 1920s held the view that the octet rule did prevail and that bonding was based on ionic interactions (e. Irving Langmuir ( January 31, 1881 in Brooklyn New York – August 16, 1957 in Woods Hole Massachusetts) was an g. SF42+F22−). His opponent in this era, Gilbert N. Lewis on the other hand believed in octet expansion. Gilbert Newton Lewis ( October 23, 1875 - March 23, 1946) was a famous American physical chemist known for the discovery [2]

Hypervalent bonding has also been believed to be described as sp3d and sp3d2 hybrid orbitals composed of s, p, and d-orbitals at higher energy levels. -->In Chemistry 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. Ab Initio Software Corporation was founded in the mid 1990's by the former CEO of Thinking Machines Corporation Sheryl Handler, and several other former employees

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, was proposed. Year 1951 ( MCMLI) was a Common year starting on Monday. Events of 1951 January The 3-center-4-electron bond is a model used to explain bonding in Hypervalent molecules such as Phosphorus pentafluoride, Sulfur hexafluoride, the In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule. 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 orbitals: an occupied bonding orbital, an occupied non-bonding orbital (HOMO), and an unoccupied anti-bonding orbital (LUMO). In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally HOMO and LUMO are Acronyms for highest occupied Molecular orbital and lowest unoccupied Molecular orbital, respectively HOMO and LUMO are Acronyms for highest occupied Molecular orbital and lowest unoccupied Molecular orbital, respectively This model in which the octet rule is preserved was also advocated by Musher. [2]

N-X-L notation

The N-X-L nomenclature introduced in 1980. [3] is often used to classify hypervalent compounds of main group elements, where:

Examples are

Criticism of the term

As a replacement for the term hypervalency, Paul von Ragué Schleyer in 1984 proposed hypercoordination because this term does not imply any mode of chemical bonding. Xenon difluoride is a powerful Fluorinating agent, with the chemical formula, is one of the most stable Xenon compounds. Phosphorus pentachloride is the Chemical compound with the formula PCl5 Sulfur hexafluoride is an Inorganic compound with the formula. Iodine heptafluoride, also known as iodine(VII fluoride or even iodine fluoride is the compound I[[fluoride F]]7 Paul von Ragué Schleyer (born February 27 1930 is an organic physical Chemist of substantial significance whose research has been cited with great frequency [2]

The concept of hypervalency has been criticized by Ronald Gillespie 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. Ronald J Gillespie, CM, ( August 21, 1924) a Chemistry professor at McMaster University, specializes in the field of Molecular "[4]

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. " Electronegativity " is the opposite of " Electropositivity," which describes an element's ability to donate electrons Consistent with this alternative view is the finding that hypercoordinated molecules based on fluorine ligands, for example PF5 do not have hydride counterparts e. Hydride is the name given to the negative Ion of Hydrogen, H− g. phosphorane PH5 which is an unstable molecule. A phosphorane is a Functional group in Chemistry with Pentavalent Phosphorus.

Even an ionic model holds up well in thermochemical calculations. In Thermodynamics and Physical chemistry, thermochemistry is the study of the Heat evolved or absorbed in Chemical reactions Thermochemistry 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. In Thermodynamics, the word exothermic "outside heating" describes a process or reaction that releases Energy usually in the form of Heat, but Phosphorus trifluoride (formula P[[Fluorine F]]3 is a colourless and odourless Gas. Fluorine, fluorum meaning "to flow" is the Chemical element with the symbol F and Atomic number 9 [5]

References

  1. ^ Musher, J. L. The Chemistry of Hypervalent Molecules Angew. Chem. Int. Ed. Engl. 1969, 8, 54-68. [1]
  2. ^ a b c The Origin of the Term "Hypervalent" Jensen, William B. J. Chem. Educ. 2006 83 1751. Link
  3. ^ Perkins,C. W. ; Martin, J. C. ; Arduengo, A. J. ; Lau, W. ; Alegria, A,; Kochi, J. K. ; An Electrically Neutral a-Sulfuranyl Radical from the Homolysis of a Perester with Neighboring Sulfenyl Sulfur: 9-S-3 species J. Am. Chem. Soc. 1980, 102, 7753-7759 doi:10.1021/ja00546a019
  4. ^ Gillespie, R. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. J. ; Silvi, B. The octet rule and hypervalence: two misunderstood concepts. Coord. Chem. Rev. 2002, 233-234, 53-62. [2]
  5. ^ Predicting the Stability of Hypervalent Molecules Mitchell, Tracy A. ; Finocchio, Debbie; Kua, Jeremy. J. Chem. Educ. 2007, 84, 629. Link
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