In
molecular geometry
Molecular geometry is the three-dimensional space, three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometric ...
, square pyramidal geometry describes the shape of certain
compounds with the formula where L is a
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...
. If the ligand atoms were connected, the resulting shape would be that of a
pyramid with a square base. The
point group symmetry
In group theory, the symmetry group of a geometric object is the group of all transformations under which the object is invariant, endowed with the group operation of composition. Such a transformation is an invertible mapping of the amb ...
involved is of type C
4v. The geometry is common for certain
main group compounds that have a
stereochemically-active
lone pair
In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC '' Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. L ...
, as described by
VSEPR theory
Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm t ...
. Certain compounds
crystallize
Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely depo ...
in both the trigonal bipyramidal and the square pyramidal structures, notably .
As a transition state in Berry pseudorotation
As a
trigonal bipyramidal
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 bipyramid. This is one geometry for which the bond angles surrounding the central atom are not ident ...
molecule undergoes
Berry pseudorotation
The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands (see Figure at right) for two of the equatorial ones. It is the most widely ac ...
, it proceeds via an intermediary stage with the square pyramidal 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.
Examples
Some molecular compounds that adopt square pyramidal geometry are XeOF
4, and various
halogen pentafluorides (XF
5, where X = Cl, Br, I).
Complexes of
vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pass ...
(IV), such as
vanadyl acetylacetonate
Vanadyl acetylacetonate is the chemical compound with the formula VO(acac)2, where acac– is the conjugate base of acetylacetone. It is a blue-green solid that dissolves in polar organic solvents. The coordination complex consists of the vanadyl ...
,
O(acac)2 are square pyramidal (acac = acetylacetonate, the deprotonated anion of
acetylacetone
Acetylacetone is an organic compound with the chemical formula . It is a colorless liquid, classified as a 1,3- diketone. It exists in equilibrium with a tautomer . These tautomers interconvert so rapidly under most conditions that they are tr ...
(2,4-pentanedione)).
See also
*
AXE method
Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm t ...
*
Square pyramid
In geometry, a square pyramid is a pyramid having a square base. If the apex is perpendicularly above the center of the square, it is a right square pyramid, and has symmetry. If all edge lengths are equal, it is an equilateral square pyramid ...
*
Hypervalent molecule
In chemistry, a hypervalent molecule (the phenomenon is sometimes colloquially known as expanded octet) is a molecule that contains one or more main group elements apparently bearing more than eight electrons in their valence shells. Phosphorus ...
*
Molecular geometry
Molecular geometry is the three-dimensional space, three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometric ...
References
External links
Chem, Chemistry, Structures, and 3D Molecules Indiana University Molecular Structure Center
{{MolecularGeometry
Molecular geometry