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VSEPR (Read 5567 times)
Gerrit-Jan Linker
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Posts: 75
21.03.11 at 13:02:41
Valence shell electron pair repulsion (VSEPR) model. By Gillespie and Nyholm  
The method can be presented in a simpel emperical way and in a analysis on the basis of electron densities.
  • Fundamental basis is the Pauli principle and not electrostatics.
  • Fundamental assumption: electron pairs keep as far apart as possible
    • Empircal presentation

      • Draw the Lewis structure
      • Electron pairs, both bonding and non bonding, behave as they will repel eachother. Therefore they keep apart as far as possible.
      • Deviations from optimal angles with lone pairs: electron pair repulsion increases in magnitude of the order bond pair: bond pair - bond pair < bond pair - lone pair < lone pair - lone pair

    • Presentation on the basis of electron densities
      • Consider a valence shell with 8 electrons, 4 with spin up and 4 with spin down.
      • Applying Pauli's principle:  
        • same spin electrons keep as far apart as possible.
        • Most probable relative location of spin up electrons is along the vertices of an tetrahedron.
        • The same is the case for spin down electrons. It forms its own tetrahedron, independent of the one for spin up electrons.
        • Tetrahedron of spin up electrons may have any orientation with respect to the the tetrahedron of spin down electrons.
        • In other words, both spin up and spin down electrons 'live in their own independent worlds'
        • In a free atom this leads to a spherical electron density.

      • In a molecule the attraction of the ligand atoms cause the spin up and down tetrahedra to come into approximate coincidence.
      • That increases the probability that a pair of bonding electrons may be found in the bonding region.
      • When the tetrahedra align, it increases the probability that a pair of electrons can be found in the long pair directions.
      • Electron pair domains are introduced: regions where there is an enhanced probability of finding the electron pair.

        • An electron pair domain has a certain shape in which the electron pair is most probable to be found.
        • A lone pair domain is generally larger than a bonding domain: it is more spread out along the nucleus.
        • A bonding domain takes up less space compared to a lone pair domain because it is stretched out toward the ligand.
        • Differences in the relative size and shape of the lone pair and bonding pair domains provide a rationale for the relative sizes of the bond angles.
          • Bond angles decrease with increasing electronegativity of the ligands.
          • Bond angles decrease with decreasing electronegativity of the central atom A.

    Chemistry Education: Research and Practise in Europe, Gillespie, 2001, 2, 2, p73-90 ; Gillespie, Chemical Bonding and Molecular Geometry - From Lewis to Electron Densities, 2001)
    Wikipedia (
    VSEPR is a model in chemistry used to model the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion.

    The premise of VSEPR is that the valence electron pairs surrounding an atom mutually repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecular geometry. The number of electron pairs surrounding an atom, both bonding and nonbonding, is called its steric number.

    I doubt whether VSEPR can be used to PREDICT the geometry of molecules. Perhaps the shapes of lone pairs are not such that are usually depicted when VSEPR theory is used. Then, whether the repulsion argument can uphold is uncertain. To my knowledge a lone pair as a p or sp function has not been found. E.g. when MO theory is used to model H2O can we 'see' the lone pairs of Oxigen? It is claimed that H2O is nearly precisely spherical with even the protons entering the Oxigen electron cloud.  
    Theorem: VSEPR has no predictive power (and neither has sp2/sp3 hybridisation)
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    « Last Edit: 27.12.11 at 14:12:57 by Gerrit-Jan Linker »  

    Gerrit-Jan Linker
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    Gerrit-Jan Linker
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    VSEPR: difficult molecules
    Reply #1 - 07.01.12 at 10:38:09
    VSEPR: difficult molecules
    CaF2, 145; SrF2, 120; BaF2, 108; SrCl2, 130; BaCl2, 115; BaBr2, 115; BaI2, 105
    ref (not confirmed): Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419.
    It has been proposed by Gillespie that this is caused by interaction of the ligands with the electron core of the metal atom, polarising it so that the inner shell is not spherically symmetric, thus influencing the molecular geometry.
    ref: Models of molecular geometry, Gillespie R. J., Robinson E.A. Chem. Soc. Rev., 2005, 34, 396407
    ref: Core Distortions and Geometries of the Difluorides and Dihydrides of Ca, Sr, and Ba Bytheway I, Gillespie R.J, Tang T.H., Bader R.F. Inorganic Chemistry, 34,9, 24072414, 1995
    O(SiH3)2 144.1, Cl2O 110.9, (CH3)2O 111.7 and N(CH3)3 110.9
    TeCl62−, TeBr62−, BiCl63−, BiBr63− and BiI63− are regular octahedra and the lone pair does not affect the geometry
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    « Last Edit: 07.01.12 at 10:43:12 by Gerrit-Jan Linker »  

    Gerrit-Jan Linker
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