Square pyramidal molecular geometry
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In
molecular geometry Molecular geometry is the 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 geometrical parameters that determ ...
, 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 electr ...
. 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 ambient ...
involved is of type C4v. The geometry is common for certain
main group In chemistry and atomic physics, the main group is the group of elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine as arra ...
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. Lone ...
, 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 theo ...
. 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 deposi ...
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 identi ...
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 XeOF4, and various
halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
pentafluorides (XF5, 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 ( pas ...
(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 g ...
, 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 tre ...
(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 theo ...
*
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 p ...
*
Molecular geometry Molecular geometry is the 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 geometrical parameters that determ ...


References


External links


Chem, Chemistry, Structures, and 3D Molecules

Indiana University Molecular Structure Center


{{MolecularGeometry Molecular geometry