In a
Tetrahedral molecular geometry a central
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
is located at the center with four
substituentIn 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 that are located at the corners of a
tetrahedronIn geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
. The bond angles are cos
−1(
−1/3) ≈ 109.5° when all four substituents are the same, as in CH
4. This
molecular geometryMolecular 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...
is common throughout the first half of the periodic table. The perfectly symmetrical tetrahedron belongs to
point groupIn chemistry, a point group is a group of geometric symmetries leaving a point fixed.-Overview:Point groups can exist in a Euclidean space of any dimension. The discrete point groups in two dimensions, also called rosette groups, are used to describe the symmetries of an ornament...
T
d, but most tetrahedral molecules are not of such high symmetry.
In a
Tetrahedral molecular geometry a central
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
is located at the center with four
substituentIn 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 that are located at the corners of a
tetrahedronIn geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
. The bond angles are cos
−1(
−1/3) ≈ 109.5° when all four substituents are the same, as in CH
4. This
molecular geometryMolecular 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...
is common throughout the first half of the periodic table. The perfectly symmetrical tetrahedron belongs to
point groupIn chemistry, a point group is a group of geometric symmetries leaving a point fixed.-Overview:Point groups can exist in a Euclidean space of any dimension. The discrete point groups in two dimensions, also called rosette groups, are used to describe the symmetries of an ornament...
T
d, but most tetrahedral molecules are not of such high symmetry. Tetrahedral molecules can be
chiralThe term chiral is used to describe an object that is non-superposable on its mirror image. Achiral objects are objects that are identical to their mirror image....
.
Main group chemistry
Aside virtually all saturated organic compounds, most compounds of Si, Ge, and Sn are tetrahedral. Often tetrahedral molecules feature multiple bonding to the outer ligands, as in
xenon tetroxideXenon tetroxide is a chemical compound of xenon and oxygen with molecular formula XeO4, remarkable for being a relatively stable compound of a noble gas...
(XeO
4), the
perchloratePerchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are also used as an oxidizer in rocket fuel and explosives and can be found in airbags...
ion (ClO
4−), the
sulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
ion (SO
42−), the
phosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
ion (PO
43−]).
Thiazyl trifluorideThiazyl trifluoride, NSF3, is a stable, colourless gas and an important precursor to other sulfur-nitrogen-fluorine compounds.NSF3 can be synthesised by the fluorination of thiazyl fluoride, NSF, with silver fluoride, AgF2:NSF + 2AgF2 → NSF3 +...
, SNF
3 is tetrahedral, featuring a sulfur-to-nitrogen triple bond.
Ammonia can be classified as tetrahedral, if one considers the lone pair as a ligand as in the language of VSEPR theory. The H-N-H angles are 107°, being contracted from 109.4°, a difference attributed to the influence of the lone pair.
Transition metal chemistry
Again the geometry is widespread, particularly so for complexes where the metal has d
0 or d
10 configuration. Illustrative examples include
tetrakis(triphenylphosphine)palladium(0)Tetrakispalladium is the chemical compound Pd[P3]4, often abbreviated Pd4, or even PdP4...
,
nickel carbonylNickel carbonyl is a colorless organometallic complex that is a versatile reagent, first described in 1890 by Ludwig Mond. It was the first metal simple carbonyl complex to be reported...
, and
titanium tetrachlorideTitanium tetrachloride is the inorganic compound with the formula TiCl
4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl
4 is an unusual example of a metal halide that is highly volatile...
. Many complexes with incompletely filled d-shells are often tetrahedral, e.g. the tetrahalides of iron(II), cobalt(II), and nickel(II).
Water Structure
The most common arrangement of liquid water molecules is tetrahedral with two hydrgen atoms covalently attached to oxygen and two ( covalently attached to another water molecule ) attached by hydrogen bonds. Since the hydrogen bonds vary in length many of these water molecules are not symmetrical and form transient irregular tetrahedra between their four associated hydrogen atoms..
Exceptions and distortions
Inversion of tetrahedral occurs widely in organic and main group chemistry. The so-called
Walden inversionWalden inversion is the inversion of a chiral center in a molecule in a chemical reaction. Since a molecule can form two enantiomers around a chiral center, the Walden inversion converts the configuration of the molecule from one enantiomeric form to the other. For example, in a SN2...
illustrates the stereochemical consequences of inversion at carbon.
Nitrogen inversionIn chemistry, a nitrogen compound like ammonia in a trigonal pyramid geometry undergoes rapid nitrogen inversion whereby the molecule turns inside out. This interconversion is a room temperature process because the energy barrier is relatively small. Contrast this to phosphine which does not show...
in ammonia also entails transient formation of planar NH
3.
Inverted tetrahedral geometry
Geometrical constraints in a molecule can cause a severe distortion of idealized tetrahedral geometry. In compounds featuring "inverted carbon," for instance, the carbon is pyramidal..
Organic molecules displaying inverted carbon are
tetrahedraneTetrahedrane is a platonic hydrocarbon with chemical formula 44 and a tetrahedral structure. Extreme angle strain prevents this molecule from forming outside of man-made production.In 1978, Günther Maier prepared a stable tetrahedrane with four tert-butyl substituents...
s,
propellanePropellanes are a class of tricyclic organic compounds sharing a common carbon-carbon covalent bond. They are characterized by the presence of carbon with an inverted tetrahedral geometry, large steric strain and high reactivity and are for these reasons much studied in organic chemistry...
s or more generally paddlanes, and pyramidanes . Such molecules are typically
strainedIn a molecule, strain energy is released when the constituent atoms are allowed to rearrange themselves in a chemical reaction or a change of chemical conformation in a way that:* angle strain,* torsional strain,* ring strain and/or steric strain,...
, resulting in increased reactivity.
Planarization
A tetrahedron can also be distorted by increasing the angle between the two opposite bonds. In the extreme case, flattening results. For carbon this phenomenon can be observed in a class of compounds called the
fenestraneA fenestrane in organic chemistry is a type of chemical compound with a central quaternary carbon atom which serves as a common vertex for four fused carbocycles. They can be regarded as spiro compounds twice over. Because of their inherent strain and instability fenestranes are of theoretical...
s.
External links