Trigonal bipyramid molecular geometry
Encyclopedia
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.
. 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.
ion is also based upon a trigonal bipyramid, but with the equatorial positions filled with lone pair
s of electrons. In phosphorus
compounds with mixed substituent
s apicophilicity
is observed.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
a trigonal bipyramid formation is a molecular geometry
Molecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...
with one atom at the center and 5 more atoms at the corners of a triangular dipyramid
Triangular dipyramid
In geometry, the triangular bipyramid is the first in the infinite set of face-transitive bipyramids. It is the dual of the triangular prism with 6 isosceles triangle faces....
. This is one of the few cases where bond angles surrounding an atom are not identical (see also pentagonal dipyramid
Pentagonal dipyramid
In geometry, the pentagonal bipyramid is third of the infinite set of face-transitive bipyramids.Each bipyramid is the dual of a uniform prism.If the faces are equilateral triangles, it is a deltahedron and a Johnson solid...
), which is simply because there is no geometrical arrangement which can result in five equally sized bond angles in three dimensions.
Behavior
Isomers with a trigonal bipyramidal geometry are able to interconvert through a process known as Berry pseudorotationBerry mechanism
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 for two of the equatorial ones. It is the most widely accepted mechanism for 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.
Examples
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 triiodideTriiodide
In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...
ion is also based upon a trigonal bipyramid, but with the equatorial positions filled with lone pair
Lone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
s of electrons. In phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
compounds with mixed substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...
s apicophilicity
Apicophilicity
Apicophilicity is the phenomenon in which electronegative substituents of trigonal bipyramidal pentacoordinate compounds prefer to occupy apical positions ....
is observed.