AX4E

AX4E is a designation used in VSEPR theory to describe a molecule in which a central atom A is bonded to four atoms X (four bonding pairs) and has one lone pair of electrons (E). This combination results in five electron domains around A.

The electron-domain geometry for AX4E is trigonal bipyramidal, but the molecular geometry is seesaw. In this arrangement, the lone pair occupies one equatorial position to minimize repulsion with the bonding pairs. As a consequence, the molecule loses the full symmetry of a trigonal bipyramid and typically adopts a C2v-symmetric shape known as a seesaw.

In terms of bond angles and lengths, the two axial X atoms lie opposite each other along the vertical axis, with an approximate 180-degree separation. The remaining two X atoms occupy equatorial positions and are arranged around the central atom with an angle of about 102 degrees between them due to the influence of the lone pair. Axial–equatorial X–A–X angles are generally near 90 degrees but are slightly distorted by lone-pair repulsion. Axial bonds tend to be longer than equatorial bonds because they experience greater lone-pair–bond-pair repulsion.

Common examples of AX4E molecules include sulfur tetrafluoride (SF4) and certain ions such as IF4−, where the central atom has four bonding pairs and one lone pair. AX4E structures illustrate how a lone pair can direct deviations from idealized geometries in five-coordinate centers.

In summary, AX4E denotes a five-electron-domain system with four bonds and one lone pair, yielding a seesaw molecular geometry derived from a trigonal bipyramidal electron arrangement.