Trigonal bipyramid molecular geometry
In chemistry a trigonal bipyramid is a molecular geometry with one atom at the center and 5 more at the corners of a triangular dipyramid. This is one of the only 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 this geometry are able to interconvert through a process called Berry pseudorotation.
Phosphorus pentachloride is a molecule with a trigonal bipyramidal geometry. Phosphorus shares a plane with three chloride atoms at 120 degrees angles of each other (equatorial positions) and two more above and below at 90 degrees of the plane (apical positions). The triiodide ion is also 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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