ax = axial eq = equatorial
In Chemistry, the trigonal bipyramidal molecular geometry describes the arrangement of six atoms, five of which are located around the remaining atom, forming a triangular bipyramid. It is one of the few cases in which the bond angles that surround an atom are not identical ( pentagonal bipyramid), which is simply because there is no possible geometric arrangement with five equal linking angles in three dimensions.
BehaviorIsomers with trigonal bipyramidal geometry are capable of interconverting through a process known as Berry pseudo-rotation. The pseudo-rotation is conceptually similar to the movement of a conformational diastereoisomer, even though full revolutions are not achieved. In the process of pseudo-rotation, two equatorial ligands (both with a smaller bond distance than the third) "move" towards the molecular axis, while the axial ligands "move" simultaneously towards the equator, creating a constant cyclic movement. The phenomenon of pseudorotation is especially noticeable in simple molecules such as phosphorus pentafluoride. Examples
Phosphorus pentachloride (PCl5) is the clearest example of trigonal bipyramidal structure. The phosphorus atom shares the plane with three "equatorial" chlorine atoms with bond angles of 120 °. The two "axial" chlorine atoms are located above and below the horizontal plane of the phosphorus.
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