In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or
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 elect ...
s symmetrically arranged around a central atom, defining the vertices of an
octahedron. The octahedron has eight faces, hence the
prefix
A prefix is an affix which is placed before the stem of a word. Adding it to the beginning of one word changes it into another word. For example, when the prefix ''un-'' is added to the word ''happy'', it creates the word ''unhappy''. Particul ...
''
octa''. The octahedron is one of the
Platonic solid
In geometry, a Platonic solid is a convex, regular polyhedron in three-dimensional Euclidean space. Being a regular polyhedron means that the faces are congruent (identical in shape and size) regular polygons (all angles congruent and all e ...
s, although octahedral molecules typically have an atom in their centre and no bonds between the ligand atoms. A perfect octahedron belongs to the
point group O
h. Examples of octahedral compounds are
sulfur hexafluoride
Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non- flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attach ...
SF
6 and
molybdenum hexacarbonyl
Molybdenum hexacarbonyl (also called molybdenum carbonyl) is the chemical compound with the formula Mo(CO)6. This colorless solid, like its chromium and tungsten analogues, is noteworthy as a volatile, air-stable derivative of a metal in its zero ...
Mo(CO)
6. The term "octahedral" is used somewhat loosely by chemists, focusing on the geometry of the bonds to the central atom and not considering differences among the ligands themselves. For example,
, which is not octahedral in the mathematical sense due to the orientation of the bonds, is referred to as octahedral.
The concept of octahedral coordination geometry was developed by
Alfred Werner
Alfred Werner (12 December 1866 – 15 November 1919) was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration ...
to explain the stoichiometries and isomerism in
coordination compound
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Man ...
s. His insight allowed chemists to rationalize the number of isomers of coordination compounds. Octahedral transition-metal complexes containing amines and simple anions are often referred to as
Werner-type complexes.
Isomerism in octahedral complexes
When two or more types of ligands (L
a, L
b, ...) are coordinated to an octahedral metal centre (M), the complex can exist as isomers. The naming system for these isomers depends upon the number and arrangement of different ligands.
''cis'' and ''trans''
For MLL, two isomers exist. These isomers of MLL are ''cis'', if the L
b ligands are mutually adjacent, and ''trans'', if the L
b groups are situated 180° to each other. It was the analysis of such complexes that led
Alfred Werner
Alfred Werner (12 December 1866 – 15 November 1919) was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration ...
to the 1913 Nobel Prize–winning postulation of octahedral complexes.
Image:Cis-dichlorotetraamminecobalt(III).png,
Image:Trans-dichlorotetraamminecobalt(III).png,
Facial and meridional isomers
For MLL, two isomers are possible - a facial isomer (''fac'') in which each set of three identical ligands occupies one face of the octahedron surrounding the metal atom, so that any two of these three ligands are mutually cis, and a meridional isomer (''mer'') in which each set of three identical ligands occupies a plane passing through the metal atom.
Image:Fac-trichlorotriamminecobalt(III).png,
Image:Mer-trichlorotriamminecobalt(III).png,
Δ vs Λ isomers
Complexes with three bidentate ligands or two cis
bidentate ligands can exist as
enantiomer
In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical anti ...
ic pairs. Examples are shown below.
Image:Delta-tris(oxalato)ferrate(III)-3D-balls.png,
Image:Lambda-tris(oxalato)ferrate(III)-3D-balls.png,
Image:Delta-cis-dichlorobis(ethylenediamine)cobalt(III).png,
Image:Lambda-cis-dichlorobis(ethylenediamine)cobalt(III).png,
Other
For MLLL, a total of five geometric isomers and six stereoisomers are possible.
# One isomer in which all three pairs of identical ligands are ''trans''
# Three isomers in which one pair of identical ligands (L
a ''or'' L
b ''or'' L
c) is ''trans'' while the other two pairs of ligands are mutually ''cis''.
# Two enantiomeric pair in which all three pairs of identical ligands are ''cis''. These are equivalent to the Δ vs Λ isomers mentioned above.
The number of possible isomers can reach 30 for an octahedral complex with six different ligands (in contrast, only two stereoisomers are possible for a tetrahedral complex with four different ligands). The following table lists all possible combinations for monodentate ligands:
Thus, all 15 diastereomers of ML
aL
bL
cL
dL
eL
f are chiral, whereas for MLL
bL
cL
dL
e, six diastereomers are chiral and three are not (the ones where L
a are ''trans''). One can see that octahedral coordination allows much greater
complexity
Complexity characterises the behaviour of a system or model whose components interact in multiple ways and follow local rules, leading to nonlinearity, randomness, collective dynamics, hierarchy, and emergence.
The term is generally used to ch ...
than the tetrahedron that dominates
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
. The tetrahedron ML
aL
bL
cL
d exists as a single enantiomeric pair. To generate two diastereomers in an organic compound, at least two carbon centers are required.
Deviations from ideal symmetry
Jahn–Teller effect
The term can also refer to octahedral influenced by the
Jahn–Teller effect, which is a common phenomenon encountered in
coordination chemistry
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Man ...
. This reduces the symmetry of the molecule from O
h to D
4h and is known as a tetragonal distortion.
Distorted octahedral geometry
Some molecules, such as
XeF6 or , have a lone pair that distorts the
symmetry
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
of the molecule from O
h to C
3v. The specific geometry is known as a
''monocapped'' octahedron, since it is derived from the octahedron by placing the lone pair over the centre of one triangular face of the octahedron as a "cap" (and shifting the positions of the other six atoms to accommodate it). These both represent a divergence from the geometry predicted by VSEPR, which for AX
6E
1 predicts a
pentagonal pyramidal shape.
Bioctahedral structures
Pairs of octahedra can be fused in a way that preserves the octahedral coordination geometry by replacing terminal ligands with
bridging ligand
In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually ...
s. Two motifs for fusing octahedra are common: edge-sharing and face-sharing. Edge- and face-shared bioctahedra have the formulas
2L8(μ-L)">2L8(μ-L)sub>2 and M
2L
6(μ-L)
3, respectively. Polymeric versions of the same linking pattern give the stoichiometries
2(μ-L)2">L2(μ-L)2sub>∞ and
3">(μ-L)3sub>∞, respectively.
The sharing of an edge or a face of an octahedron gives a structure called bioctahedral. Many metal penta
halide
In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a flu ...
and penta
alkoxide compounds exist in solution and the solid with bioctahedral structures. One example is
niobium pentachloride
Niobium(V) chloride, also known as niobium pentachloride, is a yellow crystalline solid. It hydrolyzes in air, and samples are often contaminated with small amounts of NbOCl3. It is often used as a precursor to other compounds of niobium. NbCl ...
. Metal tetrahalides often exist as
polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s with edge-sharing octahedra.
Zirconium tetrachloride
Zirconium(IV) chloride, also known as zirconium tetrachloride, () is an inorganic compound frequently used as a precursor to other compounds of zirconium. This white high-melting solid hydrolyzes rapidly in humid air.
Structure
Unlike molecular T ...
is an example.
Compounds with face-sharing octahedral chains include MoBr
3, RuBr
3, and TlBr
3.
Image:Niobium-pentachloride-from-xtal-3D-balls.png, Ball-and-stick model of niobium pentachloride
Niobium(V) chloride, also known as niobium pentachloride, is a yellow crystalline solid. It hydrolyzes in air, and samples are often contaminated with small amounts of NbOCl3. It is often used as a precursor to other compounds of niobium. NbCl ...
, a bioctahedral coordination compound.
Image:Zirconium-tetrachloride-3D-balls-A.png, Ball-and-stick model of zirconium tetrachloride
Zirconium(IV) chloride, also known as zirconium tetrachloride, () is an inorganic compound frequently used as a precursor to other compounds of zirconium. This white high-melting solid hydrolyzes rapidly in humid air.
Structure
Unlike molecular T ...
, an inorganic polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
based on edge-sharing octahedra.
Image:MoBr3sideview.jpg, Ball-and-stick model of molybdenum(III) bromide
Molybdenum(III) bromide is the inorganic compound with the formula MoBr3. It is a black solid that is insoluble in most solvents but dissolves in donor solvents such as pyridine.
Preparation
Molybdenum(III) bromide is produced by the reaction o ...
, an inorganic polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
based on face-sharing octahedra.
Image:TiI3end-on.jpg, View almost down the chain of titanium(III) iodide
Titanium(III) iodide is an inorganic compound with the formula TiI3. It is a dark violet solid that is insoluble in solvents, except upon decomposition.
Preparation and structure
Titanium(III) iodide can be prepared by reaction of titanium wit ...
highlighting the eclipsing of the halide ligands in such face-sharing octahedra.
Trigonal prismatic geometry
For compounds with the formula MX
6, the chief alternative to octahedral geometry is a trigonal prismatic geometry, which has
symmetry
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
D
3h. In this geometry, the six ligands are also equivalent. There are also distorted trigonal prisms, with C
3v symmetry; a prominent example is . The interconversion of ''Δ''- and ''Λ''-complexes, which is usually slow, is proposed to proceed via a trigonal prismatic intermediate, a process called the "
Bailar twist". An alternative pathway for the
racemization of these same complexes is the
Ray–Dutt twist
The Ray–Dutt twist is a mechanism proposed for the racemization of octahedral complexes containing three bidentate chelate rings. Such complexes typically adopt an octahedral molecular geometry in their ground states, in which case they possess ...
.
Splitting of d-orbital energies
For a free ion, e.g. gaseous Ni
2+ or Mo
0, the energy of the
d-orbital
In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any sp ...
s are equal in energy; that is, they are "degenerate". In an octahedral complex, this degeneracy is lifted. The energy of the d
''z''2 and d
''x''2−''y''2, the so-called e
g set, which are aimed directly at the ligands are destabilized. On the other hand, the energy of the d
''xz'', d
''xy'', and d
''yz'' orbitals, the so-called t
2g set, are stabilized. The labels t
2g and e
g refer to
irreducible representations, which describe the symmetry properties of these orbitals. The energy gap separating these two sets is the basis of
crystal field theory Crystal field theory (CFT) describes the breaking of degeneracies of electron orbital states, usually ''d'' or ''f'' orbitals, due to a static electric field produced by a surrounding charge distribution (anion neighbors). This theory has been used ...
and the more comprehensive
ligand field theory. The loss of degeneracy upon the formation of an octahedral complex from a free ion is called
crystal field splitting Crystal field theory (CFT) describes the breaking of degeneracies of electron orbital states, usually ''d'' or ''f'' orbitals, due to a static electric field produced by a surrounding charge distribution (anion neighbors). This theory has been used ...
or
ligand field splitting. The energy gap is labeled ''Δ''
o, which varies according to the number and nature of the ligands. If the symmetry of the complex is lower than octahedral, the e
g and t
2g levels can split further. For example, the t
2g and e
g sets split further in ''trans''-MLL.
Ligand strength has the following order for these electron donors:
:weak:
iodine
Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
<
bromine
Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
<
fluorine
Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
<
acetate <
oxalate <
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
<
pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...
<
cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
:strong
So called "weak field ligands" give rise to small ''Δ''
o and
absorb light at longer
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s.
Reactions
Given that a virtually uncountable variety of octahedral complexes exist, it is not surprising that a wide variety of reactions have been described. These reactions can be classified as follows:
* Ligand substitution reactions (via a variety of mechanisms)
* Ligand addition reactions, including among many, protonation
*
Redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
reactions (where electrons are gained or lost)
* Rearrangements where the relative stereochemistry of the ligand changes within the
coordination sphere.
Many reactions of octahedral transition metal complexes occur in water. When an
anionic ligand replaces a coordinated water molecule the reaction is called an
anation. The reverse reaction, water replacing an anionic ligand, is called
aquation Aquation is the chemical reaction involving "incorporation of one or more integral molecules of water" with or without displacement of other atoms or groups. The term is typically employed to refer to reactions of metal complexes where an anion is ...
. For example, the slowly yields in water, especially in the presence of acid or base. Addition of concentrated HCl converts the aquo complex back to the chloride, via an anation process.
See also
*
Octahedral cluster Octahedral clusters are inorganic or organometallic cluster compounds composed of six metals in an octahedral array.Eric J. Welch and Jeffrey R. Long ''Atomlike Building Units of Adjustable Character: Solid-State and Solution Routes to Manipulating ...
s
*
AXE method
*
Molecular geometry
References
External links
Example of octahedral geometry at 3dCHEM.com
Indiana University Molecular Structure Center
{{MolecularGeometry
Molecular geometry
Coordination chemistry