2.9.1: Trigonal Bipyramidal Species

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Trigonal Bipyramidal Species are those that have a central p-block atom and are attached to 5 other atoms. It is classified as a EX5 molecule, where E stands for the central atom, and X stands for the atoms that are attached. It makes sense, then, to classify this molecule a 5-coordinate system. The VSEPR (valence shell electron pair repulsion) model is the what helps us identify how a molecule may or may not be trigonal bipyramidal.

Trigonal Bipyramidal Species

The central atom of the triognal bipyramidal species is bonded to 5 separate molecules. This means that the central atom must have an extended valence shell, in order to all for the 5 bonds to occur. The central atom is thus typically a p-block atom.

There are two different ways to classify the 5 X atoms, either as axial or equitorial. It is important to understand the difference between the two, so that we can reference them in future discussion.

The axial atoms are the ones above and below the central atom. These are the points of the 'pyramids' the molecule makes. (NOTE: Bipyramidal means two pyramids. If you look at a picture you should be able to see the pyramids relative to the top half and the bottom half of the central atom." The equatorial atoms are the ones that lie in the horizontal plane of the central atom. (Think of the equator relative to the Earth.)

Let us continue this discussion with the assumption that the EX5 is composed of just atoms (and not lone pairs). The bond angles between the equatorial atoms is 120 degrees. The bond angles between the axial and equitorial atoms is 90 degrees. The bond angles between the two axial atoms is 180 degrees. The angles formed by these atoms is the most stable conformation that can be maintained by the trigonal bipyramidal species.

Other Common Shapes for 5 Coordinate Molecules

As we have already discussed, when the central atom has 5 bonding pairs of electrons (those electrons being shared between two atoms), the shape of the molecule is trigonal bipyramidal. This section gives a little bit of information about how the shape of a molecule is affected when not all of the pairs of electrons are nonbonding.

Before that, however, it is important to discuss how bond angles are affected by lone pairs. According to VSEPR, electron-electron repulsion decreases in the following manner:

lone pair - lone pair > lone pair - bonding pair > bonding pair - bonding pair.

This means that presence of lone pairs will increase electron-electron repulsion, causing a change in the bond angles.

The first alternate shape for a 5 coordinate molecule, then, is where one of the bonding pairs of electrons is replaced with a lone pair. The best place for the lone pair to be is equatorial to the central atom. This allows for the least amount of electron-electron repulsion possible. This shape is called Seesaw.

The next possible shape is where there are two lone pairs on the central atom. Once again, the best place for these two molecules is equatorial to the central atom, for the least amount of electron repulsion to occur. This molecule is called T-shaped.

The last alternate shape for a 5 coordinate system is when there are three lone pairs. These are all equatorial to the central atom to establish the lowest energy conformation. This molecule is considered to be Linear.

Stereoisomers

Due to the fact that there are two possible types of X atoms (axial or equatorial) stereoisomers of trigonal bipyramidal species (with more than one type of A atom) are possible. If there are two different X atoms, then those atoms have the option of being cis or trans to each other. Meaning if one is axial, then the other can be equatorial (cis) or also axial (trans). Stereoisomers occur when two (or more) molecules with the same molecular formula have different molecular shapes.

References

Housecroft, Catherine E., and Alan G. Sharpe. Inorganic Chemistry. London: Pearson Education, 2008. Print.
Petrucci, Ralph H., et al. General Chemistry: Principles and Modern Applications. Upper Saddle River, NJ: Pearson Education, 2007. Print.

Outside Links

The Relationship Between the Number of Regions of Valence Electrons
and the Molecular Geometry Around an Atom. Tutor-Page.com. 05 Nov. 2010.
<www.tutor-pages.com/Chemistry..._Nonpolar.html>
Trigonal Bipyramidal Molecular Geometry. Wikipedia. 05 Nov. 2010. <http://en.Wikipedia.org/wiki/Trigona...cular_geometry>
Molecular Shape. Spark Notes. 05 Nov. 2010. <http://img.sparknotes.com/content/te...17002_0423.gif>

Problems

Explain why a trigonal bipyramidal molecule takes on the shape that it does.
What are the other possible shapes a 5-coordinate molecule can have? Give examples if possible.
What is a stereoisomer? Give an example using a trigonal bipyramidal molecule.