In chemistry a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular dipyramid. This is one of the few cases where bond angles surrounding an atom are not identical (see also pentagonal dipyramid), which is simply because there is no geometrical arrangement which can result in five equally sized bond angles in three dimensions.
Isomers with a trigonal bipyramidal geometry are able to interconvert through a process known as Berry pseudorotation. Pseudorotation is similar in concept to the movement of a conformational diastereomer, though no full revolutions are completed. In the process of pseudorotation, two equatorial ligands (both of which have a shorter bond length than the third) "shift" toward the molecule's axis, while the axial ligands simultaneously "shift" toward the equator, creating a constant cyclical movement. Pseudorotation is particularly notable in simple molecules such as PF5.
Phosphorus pentachloride is a molecule with a trigonal bipyramidal geometry. The phosphorus atom shares a plane with three chlorine atoms which are at 120 degrees angles to each other (equatorial positions), with two more chlorine atoms above and below the plane (apical or axial positions). The triiodide ion is also based upon a trigonal bipyramid, but with the equatorial positions filled with lone pairs of electrons. In phosphorus compounds with mixed substituents apicophilicity is observed.
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