The seesaw molecular geometry features distinct bond angles, primarily around 90° and 120°, though these values are typically slightly distorted due to the presence of a lone pair of electrons on the central atom. This shape arises from a central atom bonded to four other atoms, with one lone pair of electrons (AX4E1 in VSEPR notation).
Understanding Seesaw Geometry
The seesaw shape is a derivative of the trigonal bipyramidal electron domain geometry, which ideally has five electron domains around the central atom. In a seesaw molecule, four of these domains are bonding pairs, and one is a lone pair.
For optimal stability and minimized electron-electron repulsion, the lone pair occupies an equatorial position in the trigonal bipyramidal arrangement. This placement is advantageous because it maximizes the bond angles of the lone pair with the other atoms, providing angles of 120° and 90° to the surrounding electron domains, compared to only 90° bond angles if the lone pair were placed in an axial position.
This arrangement results in the characteristic seesaw shape, which includes:
- Two axial atoms (positioned above and below the equatorial plane).
- Two equatorial atoms (located in the same plane as the central atom and the lone pair).
Ideal vs. Actual Bond Angles
While the parent trigonal bipyramidal geometry provides ideal angles of 90° (between axial and equatorial positions) and 120° (between equatorial positions), the lone pair exerts a stronger repulsive force than bonding pairs. This increased repulsion compresses the bond angles between the atoms, leading to deviations from the ideal values.
Here's a breakdown of the bond angles in a seesaw geometry:
| Angle Type | Ideal Angle (from Trigonal Bipyramidal) | Actual Angle (Approximate in Seesaw) | Description |
|---|---|---|---|
| Axial-Equatorial | 90° | < 90° | Angle between an axial atom and an equatorial atom, compressed by lone pair repulsion. |
| Equatorial-Equatorial | 120° | < 120° | Angle between the two equatorial atoms, compressed by lone pair repulsion. |
| Axial-Axial | 180° | ~170-175° | Angle between the two axial atoms, often slightly bent due to overall molecular strain. |
For example, in sulfur tetrafluoride (SF₄), a classic example of a seesaw molecule, the experimental bond angles are approximately:
- Axial S-F to Equatorial S-F: ~86.5° (less than 90°)
- Equatorial F-S-F: ~101.5° (less than 120°)
- Axial F-S-F: ~173° (less than 180°)
These precise values illustrate how lone pair repulsion significantly influences the final geometry and bond angles of the molecule.
Examples of Seesaw Molecules
Several compounds exhibit the seesaw molecular geometry, including:
- Sulfur Tetrafluoride (SF₄): The most common example, where sulfur is the central atom.
- Tellurium Tetrachloride (TeCl₄): Similar to SF₄, with tellurium as the central atom.
- Bromine Trifluoride Oxide (BrF₃O): Another example where bromine is the central atom.
Understanding these bond angles is crucial for predicting molecular polarity and reactivity, as the specific arrangement of atoms and lone pairs dictates the overall shape and properties of a molecule.
