Octahedral molecular geometry

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Octahedral molecular geometry
Octahedral-3D-balls.png
Examples SF6, Mo(CO)6
Point group Oh
Coordination number 6
Bond angle(s)90°
μ (Polarity) 0

In chemistry, octahedral molecular geometry describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The octahedron has eight faces, hence the prefix octa . The octahedron is one of the Platonic solids, 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 Oh. Examples of octahedral compounds are sulfur hexafluoride SF6 and molybdenum hexacarbonyl 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, [Co(NH3)6]3+, which is not octahedral in the mathematical sense due to the orientation of the N−H bonds, is referred to as octahedral. [1]

Contents

The concept of octahedral coordination geometry was developed by Alfred Werner to explain the stoichiometries and isomerism in coordination compounds. 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.

Structure of sulfur hexafluoride, an example of a molecule with the octahedral coordination geometry. Sulfur-hexafluoride-3D-vdW.png
Structure of sulfur hexafluoride, an example of a molecule with the octahedral coordination geometry.

Isomerism in octahedral complexes

When two or more types of ligands (La, Lb, ...) 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 MLa
4
Lb
2
, two isomers exist. These isomers of MLa
4
Lb
2
are cis, if the Lb ligands are mutually adjacent, and trans, if the Lb groups are situated 180° to each other. It was the analysis of such complexes that led Alfred Werner to the 1913 Nobel Prize–winning postulation of octahedral complexes.

Facial and meridional isomers

For MLa
3
Lb
3
, 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.

Chirality

More complicated complexes, with several different kinds of ligands or with bidentate ligands can also be chiral, with pairs of isomers which are non-superimposable mirror images or enantiomers of each other.

Other

For MLa
2
Lb
2
Lc
2
, a total of six isomers are possible. [2]

  1. One isomer in which all three pairs of identical ligands are trans
  2. Three distinct isomers in which one pair of identical ligands (Laor Lbor Lc) is trans while the other two are cis.
  3. Two enantiomeric chiral isomers in which all three pairs of identical ligands are cis.

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:

FormulaNumber of isomersNumber of enantiomeric pairs
ML610
MLa
5
Lb
10
MLa
4
Lb
2
20
MLa
4
LbLc
20
MLa
3
Lb
3
20
MLa
3
Lb
2
Lc
30
MLa
3
LbLcLd
51
MLa
2
Lb
2
Lc
2
61
MLa
2
Lb
2
LcLd
82
MLa
2
LbLcLdLe
156
MLaLbLcLdLeLf3015

Thus, all 15 diastereomers of MLaLbLcLdLeLf are chiral, whereas for MLa
2
LbLcLdLe, six diastereomers are chiral and three are not (the ones where La are trans). One can see that octahedral coordination allows much greater complexity than the tetrahedron that dominates organic chemistry. The tetrahedron MLaLbLcLd 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. This reduces the symmetry of the molecule from Oh to D4h and is known as a tetragonal distortion.

Distorted octahedral geometry

Some molecules, such as XeF6 or IF
6
, have a lone pair that distorts the symmetry of the molecule from Oh to C3v. [3] [4] 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). [5] These both represent a divergence from the geometry predicted by VSEPR, which for AX6E1 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 ligands. Two motifs for fusing octahedra are common: edge-sharing and face-sharing. Edge- and face-shared bioctahedra have the formulas [M2L8(μ-L)]2 and M2L6(μ-L)3, respectively. Polymeric versions of the same linking pattern give the stoichiometries [ML2(μ-L)2] and [M(μ-L)3], respectively.

The sharing of an edge or a face of an octahedron gives a structure called bioctahedral. Many metal pentahalide and pentaalkoxide compounds exist in solution and the solid with bioctahedral structures. One example is niobium pentachloride. Metal tetrahalides often exist as polymers with edge-sharing octahedra. Zirconium tetrachloride is an example. [6] Compounds with face-sharing octahedral chains include MoBr3, RuBr3, and TlBr3.

Trigonal prismatic geometry

For compounds with the formula MX6, the chief alternative to octahedral geometry is a trigonal prismatic geometry, which has symmetry D3h. In this geometry, the six ligands are also equivalent. There are also distorted trigonal prisms, with C3v symmetry; a prominent example is W(CH3)6 . 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.

Splitting of the energy of the d-orbitals in octahedral complexes

For a free ion, e.g. gaseous Ni2+ or Mo0, the energy of the d-orbitals are equal in energy; that is, they are "degenerate". In an octahedral complex, this degeneracy is lifted. The energy of the dz2 and dx2y2, the so-called eg set, which are aimed directly at the ligands are destabilized. On the other hand, the energy of the dxz, dxy, and dyz orbitals, the so-called t2g set, are stabilized. The labels t2g and eg 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 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 or ligand field splitting. The energy gap is labeled Δo, which varies according to the number and nature of the ligands. If the symmetry of the complex is lower than octahedral, the eg and t2g levels can split further. For example, the t2g and eg sets split further in trans-MLa
4
Lb
2
.

Ligand strength has the following order for these electron donors:

weak: iodine < bromine < fluorine < acetate < oxalate < water < pyridine < cyanide :strong

So called "weak field ligands" give rise to small Δo and absorb light at longer wavelengths.

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:

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. For example, the [CoCl(NH3)5]2+ slowly yields [Co(NH3)5(H2O)]3+ 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

Related Research Articles

Coordination complex Molecule or ion containing ligands datively bonded to a central metallic atom

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. Many metal-containing compounds, especially those that include transition metals, are coordination complexes.

<i>Cis</i>–<i>trans</i> isomerism pairs of molecules with the same formula but in different orientations

Cistrans isomerism, also known as geometric isomerism or configurational isomerism, is a term used in organic chemistry. The prefixes "cis" and "trans" are from Latin: "this side of" and "the other side of", respectively. In the context of chemistry, cis indicates that the functional groups (substituents) are on the same side of some plane, while trans conveys that they are on opposing (transverse) sides. Cis-trans isomers are stereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are in different orientations in three-dimensional space. Cis-trans notation does not always correspond to EZ isomerism, which is an absolute stereochemical description. In general, cis-trans stereoisomers contain double bonds that do not rotate, or they may contain ring structures, where the rotation of bonds is restricted or prevented. Cis and trans isomers occur both in organic molecules and in inorganic coordination complexes. Cis and trans descriptors are not used for cases of conformational isomerism where the two geometric forms easily interconvert, such as most open-chain single-bonded structures; instead, the terms "syn" and "anti" are used.

Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, as there is much overlap in the subdiscipline of organometallic chemistry. It has applications in every aspect of the chemical industry, including catalysis, materials science, pigments, surfactants, coatings, medications, fuels, and agriculture.

Ligand Chelating or binding agent

In coordination chemistry, a ligand is an ion or molecule that binds to a central atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs often through Lewis Bases. The nature of metal–ligand bonding can range from covalent to ionic. Furthermore, the metal–ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic "ligands".

Stereocenter particular instance of a stereogenic element that is geometrically a point

In a molecule, a stereocenter is a particular instance of a stereogenic element that is geometrically a point. A stereocenter or stereogenic center is any point in a molecule, though not necessarily an atom, bearing different substituents, such that interchanging any two substituents leads to a stereoisomer. The term stereocenter was introduced in 1984 by Kurt Mislow and Jay Siegel. A chirality center is a stereocenter consisting of an atom holding a set of ligands in a spatial arrangement which is not superimposable on its mirror image. The concept of a chirality center generalizes the concept of an asymmetric carbon atom such that an interchanging of any two groups gives rise to an enantiomer. In organic chemistry, a chirality center usually refers to a carbon, phosphorus, or sulfur atom, though it is also possible for other atoms to be chirality centers, especially in areas of organometallic and inorganic chemistry.

VSEPR theory Theoretical model used in chemistry

Valence shell electron pair repulsion theory, or 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 theory after its two main developers, Ronald Gillespie and Ronald Nyholm.

Alfred Werner

Alfred Werner 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 of transition metal complexes. Werner developed the basis for modern coordination chemistry. He was the first inorganic chemist to win the Nobel prize, and the only one prior to 1973.

Metal ammine complex

In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia (NH3) ligand. "Ammine" is spelled this way due to historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost all metal ions bind ammonia as a ligand, but the most prevalent examples of ammine complexes are for Cr(III), Co(III), Ni(II), Cu(II) as well as several platinum group metals.

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In stereochemistry, topicity is the stereochemical relationship between substituents and the structure to which they are attached. Depending on the relationship, such groups can be heterotopic, homotopic, enantiotopic, or diastereotopic.

Metal nitrosyl complex

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In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central ion/molecule/atom is called a ligand. This number is determined somewhat differently for molecules than for crystals.

Denticity

Denticity refers to the number of donor groups in a single ligand that bind to a central atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be monodentate. Ligands with more than one bonded atom are called polydentate or multidentate. The word denticity is derived from dentis, the Latin word for tooth. The ligand is thought of as biting the metal at one or more linkage points. The denticity of a ligand is described with the Greek letter κ ('kappa'). For example, κ6-EDTA describes an EDTA ligand that coordinates through 6 non-contiguous atoms.

Isomer Chemical compounds with the same molecular formula but different atomic arrangements

In chemistry, isomers are molecules or polyatomic ions with identical molecular formulas — that is, same number of atoms of each element — but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.

Capped octahedral molecular geometry

In chemistry, the capped octahedral molecular geometry describes the shape of compounds where seven atoms or groups of atoms or ligands are arranged around a central atom defining the vertices of a gyroelongated triangular pyramid. This shape has C3v symmetry and is one of the three common shapes for heptacoordinate transition metal complexes, along with the pentagonal bipyramid and the capped trigonal prism.

Niobium(V) ethoxide Chemical compound

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A descriptor is in chemical nomenclature a prefix placed before the systematic substance name, which describes the configuration or the stereochemistry of the molecule. Some listed descriptors are only of historical interest and should not be used in publications anymore as they do not correspond with the modern recommendations of the IUPAC. Stereodescriptors are often used in combination with locants to clearly identify a chemical structure unambiguously.

Transplatin Chemical compound

trans-Dichlorodiammineplatinum(II) is the trans isomer of the coordination complex with the formula trans-PtCl2(NH3)2, sometimes called transplatin. It is a yellow solid with low solubility in water but good solubility in DMF. The existence of two isomers of PtCl2(NH3)2 led Alfred Werner to propose square planar molecular geometry. It belongs to the molecular symmetry point group D2h.

References

  1. Von Zelewsky, A. (1995). Stereochemistry of Coordination Compounds . Chichester: John Wiley. ISBN   0-471-95599-X.
  2. Miessler, G. L.; Tarr, D. A. (1999). Inorganic Chemistry (2nd ed.). Prentice-Hall. p. 290. ISBN   0-13-841891-8.
  3. Crawford, T. Daniel; Springer, Kristen W.; Schaefer, Henry F. (1994). "A contribution to the understanding of the structure of xenon hexafluoride". J. Chem. Phys. 102 (8): 3307–3311. Bibcode:1995JChPh.102.3307C. doi:10.1063/1.468642.
  4. Mahjoub, Ali R.; Seppelt, Konrad (1991). "The Structure of IF
    6
    ". Angewandte Chemie International Edition . 30 (3): 323–324. doi:10.1002/anie.199103231.
  5. Winter, Mark (2015). "VSEPR and more than six electron pairs". University of Sheffield: Department of Chemistry. Retrieved 25 September 2018. the structure of XeF6 is based upon a distorted octahedron, probably towards a monocapped octahedron
  6. Wells, A.F. (1984). Structural Inorganic Chemistry. Oxford: Clarendon Press. ISBN   0-19-855370-6.