Berry mechanism

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Trigonal bipyramidal molecular shape
ax = axial ligands (on unique axis)
eq = equatorial ligand (in plane perpendicular to unique axis)

The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to

square pyramidal geometry.[1] The Berry mechanism is named after R. Stephen Berry, who first described this mechanism in 1960.[2][3]

Berry mechanism in trigonal bipyramidal structure

The process of pseudorotation occurs when the two axial ligands close like a pair of scissors pushing their way in between two of the equatorial groups which scissor out to accommodate them. Both the axial and equatorial constituents move at the same rate of increasing the angle between the other axial or equatorial constituent.

PF5.[3]

Iron-pentacarbonyl-Berry-mechanism
Iron-pentacarbonyl-Berry-mechanism

This rapid exchange of axial and equatorial ligands renders complexes with this geometry unresolvable (unlike carbon atoms with four distinct substituents), except at low temperatures or when one or more of the ligands is bi- or poly-dentate.

Berry mechanism in square pyramidal structure

The Berry mechanism in square pyramidal molecules (such as IF5) is somewhat like the inverse of the mechanism in bipyramidal molecules. Starting at the "transition phase" of bipyramidal pseudorotation, one pair of fluorines scissors back and forth with a third fluorine, causing the molecule to vibrate. Unlike with pseudorotation in bipyramidal molecules, the atoms and ligands which are not actively vibrating in the "scissor" motion are still participating in the process of pseudorotation; they make general adjustment based on the movement of the actively vibrating atoms and ligands. However, this geometry requires a significant amount of energy to occur of about 26.7 kcal/mol.[3]

See also

References

  1. doi:10.1351/goldbook.P04934
    .
  2. ^ RS Berry, 1960, "Correlation of rates of intramolecular tunneling processes, with application to some Group V compounds," J. Chem. Phys. 32:933-938, DOI 10.1063/1.1730820; see [1] or [2], accessed 28 May 2014
  3. ^
    KK Hii & HS Rzepa, 2005, "Mechanisms that interchange axial and equatorial atoms in fluxional processes: Illustration of the Berry pseudorotation, the turnstile and the lever mechanisms via animation of transition state normal vibrational modes", J. Chem. Educ. (online), 2005; see [3] Archived 2019-10-19 at the Wayback Machine
    , accessed 28 May 2014
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