Shapes of trigonal bipyramidal molecules with lone pairs

Trigonal bipyramidal (tbp) molecules can also be found with lone pairs. This creates a bit of a dilemma since there are two differential positions available in these molecules; that is the axial and the equatorial positions. So if a tbp molecule has a lone pair of electrons will they go into the axial or the equatorial position? Now remember that a lone pair of electrons requires more space than a bonding pair of electrons; so where in a trigonal bipyramidal molecule will the lone pair of electrons have the most space? In the axial or equatorial positions?

In the axial position the lone pair of electrons will experience more repulsion since the three bonding pairs of electrons in the equatorial position are only 90⁰ away whereas in the equatorial position there are two bonding pairs at 90⁰ and the other equatorial bonding pairs are much further away. This means that in a trigonal bipyramidal molecule if there are lone pairs of electrons present they will occupy the equatorial positions.

Example - what shape is sulfur tetrafluoride (SF₄)

Using the VSEPR model we have used before we get:

1. Sulfur is the central atom and it is in group 6 of the periodic table. It has means it has 6 valency electrons it its outer shell.
2. Four fluorine atoms are bonded to the central sulfur atom and each fluorine atom will contribute 1 electron to the covalent bond with the central sulfur atom. So we have 4 electrons in total from the fluorine atoms.
3. The total number of electrons in the valency shells is therefore 10 electrons and dividing by 2 since each covalent bond involves the sharing of a pair of electrons gives 5 electron pairs; so the shape of the SF₄ molecule will be based on a trigonal bipyramidal shape. However the trigonal bipyramidal structure has 5 atoms bonded to the central atom but in SF₄ there are only 4 fluorine atoms around the central sulfur atom. This means that there is one lone pair or non-bonding of electrons in this molecule and it will occupy the equatorial position where there is more space for it.

Example 2- What shaped is chlorine trifluoride (ClF₃)?

1. Chlorine is the central atom and it is found in group7 of the periodic table; so it has 7 valency electrons.
2. Three fluorine atoms are bonded to the central atom chlorine atom with each fluorine atom contributing 1 electron to each covalent bond to the chlorine atom. So we have 3 electrons in total.
3. The total number of electrons in the valency shells is therefore 10 electrons; dividing by two gives 5 electron pairs; so the shape of the ClF₃ molecule will be based on a trigonal bipyramidal shape. There will be 2 lone pairs of electrons (4 electrons in total) and 3 bonding pairs of electrons (6 electrons in total); one pair for each of the fluorine chlorine bonds. The shape of this molecule is shown below; without the lone pairs of electrons present the molecule is simply described as T-shaped for obvious reasons!

However the presence of the two lone pairs of electrons in this molecule will distort the T-shaped molecule. The additional space requirements of the lone-pairs will mean that the 180⁰ angle formed by the F.....Cl.....F atoms will be compressed down to 175⁰, this is outlined in the image below:

Example 3- What shaped is xenon difluoride (XeF₂)?

The noble gases are generally very unreactive; however the larger noble gas such as xenon will react with fluorine to form xenon difluoride (XeF₂). What shape will this molecule have? Well as before simply use the VSEPR rules we have used so far to work out its shape:

1. Xenon is the central atom and it is in group8 of the periodic table. It has 8 valency electrons.
2. Two fluorine atoms are bonded to the central xenon atom and each fluorine atom will contribute 1 electron to the covalent bond with the central xenon atom. So we have 2 electrons in total from the fluorine atoms.
3. The total number of electrons in the valency shells is therefore 10 electrons; dividing by 2 gives 5 electron pairs; so the shape of the XeF₂ molecule will be based on a trigonal bipyramidal shape. There will be 3 lone pairs of electrons (6 electrons in total) and 2 bonding pairs of electrons; one pair for each of the xenon fluorine bonds.
   The 3 lone pairs will occupy the equatorial positions with bond angles of 120⁰ and the two fluorine atoms will in both axial positions. The overall shape of the molecule ignoring the lone pairs of electrons is simply linear. The molecule is perfectly straight!

Key Points

• To find the shape of a molecule it is necessary to check to see if it contains any lone or non-bonding pairs of electrons
• Lone pairs of electrons take up more space than bonding pairs of electrons. This can have a dramatic effect on the expected bond angles in a molecule.
• In deciding on the final shape of a molecule the lone pairs are NOT taken into account. Only the bonding pairs of electrons connected to other atoms are considered when taking into account the final shape of a molecule

Check your understanding - Questions on shapes and lone pairs