Square pyramidal

In molecular geometry, square based pyramidal geometry describes the shape of certain compounds with the formula ML₅ where L is a ligand. If the ligand atoms were connected, the resulting shape would be that of a pyramid with a square base. The point group symmetry involved is of type C_4v. The geometry is common for certain main group compounds that have a stereochemically active lone pair, as described by VSEPR theory. Certain compounds crystallize in both the trigonal bipyramidal and the square pyramidal structures, notably [Ni(CN)₅]³⁻.

As a trigonal bipyramidal molecule undergoes Berry pseudorotation, it proceeds via an intermediary stage with the square planar geometry. Thus even though the geometry is rarely seen as the ground state, it is accessed by a low energy distortion from a trigonal bipyramid.

Pseudorotation also occurs in square pyramidal molecules. Molecules with this geometry, as opposed to trigonal bipyramidal, exhibit heavier vibration. The mechanism used is similar to the Berry mechanism.

Some molecular compounds that adopt square pyramidal geometry are XeOF₄, and various halogen pentafluorides (XF₅, where X = Cl, Br, I). Complexes of vanadium(IV), such as [VO(acac)₂] are square pyramidal (acac = acetylacetonate, the deprotonated anion of acetylacetone (2,4-pentanedione)).

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