Trigonal Bipyramid
If you are following the structures in order from
Chemical Structures, you should be a pro with the
interactive java applets by now.
The trigonal bipyramid structure is characterized by five regions of electron density
around the central atom. A region of electron density is either a bonding pair
of electrons or a lone pair (non-bonding pair) of electrons. There are four molecular
geometries that we will discuss which are distinguished by the number of bonding pairs
and lone pairs of electrons. All four molecular structures have the same trigonal bipyramid
electronic geometry. All the regions of electron density occupy sp3d
hybrid orbitals.
The trigonal bipyramid structures are discussed in Chapter 10 of your textbook.
By this time, I am assuming that you are comfortable with the terms,
"molecular geometry" and "electronic geometry." If you do not understand these terms,
go back to your textbook to review their definitions.
We will be introduced to the new terms, "equatorial" and "axial" in this web page.
AB5 Molecular Structure
In the 3D applet shown below, the central atom is white, five other atoms are red,
and the five bonds are shown in blue. All five regions of electron density
are bonding electron pairs. The molecular geometry is trigonal bipyramid and the
electronic geometry is trigonal bipyramid.
Place the cursor over the molecule, press and hold the left button down, and move the mouse.
You should be able to spin the molecule in any direction, and examine the orientation of
the atoms. (If the image looks weird, like it is only partially shown or it looks like a double image,
click the refresh button of your browser. It is at the top in the toolbar.)
As you rotate the molecule with your mouse, convince yourself that the five
red atoms are not geometrically equivalent
(ignore possible chemical differences).
As you rotate the molecule, try to identify a group of three red atoms
where each red atom is indistinguishable from each other. The three red atoms that you identify and the
white central atom should all lie in one geometric plane. That group should also
look like the trigonal planar structure. The
three red atoms in this group occupy the equatorial positions on the central atom.
Continue to rotate the molecule and convince yourself that the remaining two red
atoms and the central white atom lie in a geometric line. The two red atoms and the
central white atom should look like the linear structure.
These two red atoms occupy the axial positions on the central white atom.
In a sense, the trigonal bipyramid molecular structure is a combination of the
linear and trigonal planar
molecular structures.
PCl5 is an example of a molecule that has the trigonal bipyramid molecular and electronic
structure.
How many of the Cl- ions are in equatorial positions on the phosphorus atom?
How many of the Cl- ions are in axial positions on the phosphorus atom?
Does the phosphorous atom obey the octet rule? Why or why not?
AB4U Molecular Structure
In this structure, there are four red atoms and one lone (non-bonding) electron pair around
the central white atom. There are still five regions of electron density
around the central atom. Thus, the electronic geometry is trigonal bipyramid.
There are two possible positions, equatorial and axial, that the lone pair of electrons
could occupy. Only one position is normally observed in real molecules.
The lone electron pair will go to the position around the central atom that maximizes the
distance from and minimizes the interactions with the other electron density regions.
The lone electron pair will normally occupy one of the equatorial positions on
the central atom. The result is the molecular geometry called seesaw.
Would it matter which equatorial position that the
lone electron pair occupied?
SF4 is an example of a molecule that has the seesaw molecular structure and
trigonal bipyramid electronic structure.
Could the lone electron pair occupy any of the other
equatorial positions on the central sulfur atom and SF4 still have the
same molecule structure?
Convince yourself that the remaining red atoms are not all geometrically equivalent.
How many of the remaining red atoms are in equatorial positions
and how many are in axial positions?
The molecular geometry is called "seesaw." Rotate the molecule until you
recognize the seesaw.
The 3D applet was written in such a away that when you rotate
the molecule to where the seesaw would be on a playground, you would be viewing
the seesaw from the end.
Which parts of a seesaw, the legs or the arms, would correspond to the
equatorial and axial positions?
AB3U2 Molecular Structure
Now we have three red atoms and two lone (non-bonding) electron pairs around the central white atom.
Since there are still five total regions of electron density around the central atom,
the electronic geometry is still trigonal bipyramid.
There are again two different positions, equatorial and axial, that the two lone
electron pairs can occupy. Both lone electron pairs occupy equatorial positions in
real molecules. The molecular geometry is T-shaped.
Rotate the molecule until you recognize the "T" in the T-shaped molecular geometry.
As with the 3D applet for the seesaw molecular geometry, you won't be able to
stand the "T" upright and facing towards you.
Convince yourself that the remaining red atoms are not all geometrically equivalent.
How many of the remaining red atoms are in equatorial positions
and how many are in axial positions?
AB2U3 Molecular Structure
Now we have two red atoms and three lone (non-bonding) electron pairs around the central white atom.
The electronic geometry is trigonal bipyramid.
All three lone electron pairs are in equatorial positions around the central atom.
The molecular geometry is linear.
How many of the remaining red atoms are in equatorial positions
and how many are in axial positions?
Are the remaining two red atoms geometrically equivalent to each?
How does the electronic structure of this molecule compare BeCl2
(linear molecular structure)?
URL: http://www.rretc.com/chemistry/trigonalbipyramid.html
Copyright (c) 2006 with Michael Keyes (michael.keyes@rretc.com)
Feel free to use for single-user, educational purposes only. All other purposes, please email.