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 hydrogen difluoride ion. [Greenwood&Earnshaw p. 897.] [Weinhold, F.; Landis, C. "Valency and bonding", Cambridge, 2005; pp. 275-306.] It is also known as the Pimentel-Rundle three-center model after the work published by George C. Pimentel in 1951, [Pimentel, G. C. The Bonding of Trihalide and Bifluoride Ions by the Molecular Orbital Method. "J. Chem. Phys." 1951, "19", 446-448. doi|10.1063/1.1748245] which built on concepts developed earlier by Robert E. Rundle for electron-deficient bonding. [Rundle, R. E. Electron Deficient Compounds. II. Relative Energies of "Half-Bonds". "J. Chem. Phys" 1949, "17", 671-675.doi|10.1063/1.1747367]

The model considers bonding of three colinear atoms. For example in XeF2, the linear F-Xe-F subunit is described by a set of three molecular orbitals (MOs) derived from colinear p-orbitals on each atom. The Xe-F bonds result from the combination of a filled p orbital in the central atom (Xe) with two half-filled p orbitals on the axial atoms (F), resulting in a filled bonding orbital, a filled non-bonding orbital, and an empty antibonding orbital. The two lower energy MO's are doubly occupied. The HOMO is localized on the two terminal atoms. This localization of charge is accommodated by the fact that the terminal ligands are highly electronegative in hypervalent molecules. The molecules PF5 and SF4 are described, according to this model, as having one 3-center-4-electron bond as well as three and two other more conventionally described bonds, respectively. In SF6 and in the xenon fluorides, all bonds are described with the 3-center-4-electron model.

The bonding in XeF2 can also be shown qualitatively using resonant Lewis structures as shown below:

In this representation, the octet rule is not broken, the bond orders are 1/2, and there is increased electron density in the fluorine atoms. These results are consistent with the molecular orbital picture discussed above. Older models for explaining hypervalency invoked d orbitals. As of 2008, these models still appear in some beginning level college texts; however, quantum chemical calculations suggest that d-orbital participation is negligible due to the large energy difference between the relevant p (filled) and d (empty) orbitals. Furthermore, a distinction should be made between "d orbitals" in the valence bond sense and "d functions" that are included in the QM calculation as polarization functions. [E. Magnusson. Hypercoordinate molecules of second-row elements: d functions or d orbitals? "J. Am. Chem. Soc." 1990, "112", 7940-7951.] The 3-center-4-electron bonding model has the advantage of dispensing with the need for d orbitals, which has led to its acceptance. [Ramsden, C. A. Non-bonding molecular orbitals and the chemistry of non-classical organic molecules. [http://www.rsc.org/ejarchive/CS/1994/CS9942300111.pdf "Chem. Soc. Rev." 1994, 111-118.] ]

References


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Three-center two-electron bond — A three center two electron bond is an electron deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non bonding, and one anti bonding. The two… …   Wikipedia

  • Four-center two-electron bond — A four center two electron bond is a type of chemical bond in which four atoms share two electrons in bonding which is unusual because in ordinary chemical bonds two atoms share two electrons (2c 2e bonding). This type of bonding is postulated in …   Wikipedia

  • Three-center bond — In chemistry, there are two types of three center bonds: *Three center two electron bond, found in electron deficient compounds such as boranes *Three center four electron bond, found in hypervalent compounds such as the noble gas compounds …   Wikipedia

  • Electron — For other uses, see Electron (disambiguation). Electron Experiments with a Crookes tube first demonstrated the particle nature of electrons. In this illustration, the profile of the cross shaped target is projected against the tube face at right… …   Wikipedia

  • Chemical bond — A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and… …   Wikipedia

  • Covalent bond — Covalent redirects here. For other uses, see Covalent (disambiguation). A covalent bond forming H2 (right) where two h …   Wikipedia

  • Hydrogen bond — A hydrogen bond results from a dipole dipole force between an electronegative atom and a hydrogen atom bonded to nitrogen, oxygen or fluorine (thus the name hydrogen bond , which must not be confused with a covalent bond to hydrogen). The energy… …   Wikipedia

  • Metal ligand multiple bond — In Chemistry, ametal ligand multiple bond describes the interaction of certain ligands with a metal with a bond order greater than one. [ Metal Ligand Multiple Bonds: The Chemistry of Transition Metal Complexes Containing Oxo, Nitrido, Imido,… …   Wikipedia

  • Metal–ligand multiple bond — In Chemistry, a metal–ligand multiple bond describes the interaction of certain ligands with a metal with a bond order greater than one.[1] Coordination complexes featuring multiply bonded ligands are of both scholarly and practical interest.… …   Wikipedia

  • Nitrogen-vacancy center — The nitrogen vacancy center (N V center) is one of numerous point defects in diamond. Its most explored and useful property is photoluminescence, which can be easily detected from an individual N V center. Electron spins at N V centers, localized …   Wikipedia