Hexamethyl tungsten
Encyclopedia
Hexamethyltungsten is the chemical compound
W
(CH3)6. Classified as an organometallic compound
, hexamethyltungsten is an air-sensitive, red, crystalline solid at room temperature; however, it is extremely volatile and sublimes at −30 °C. Owing to its six methyl groups it is extremely soluble in petroleum
, aromatic hydrocarbons, ethers, carbon disulfide
, and carbon tetrachloride
.
and Shortland, who described its preparation by the reaction of methyllithium with tungsten hexachloride
in diethyl ether
. The synthesis was motivated in part by previous work which indicated that tetrahedral methyl transition metal
compounds are thermally unstable, in the hopes that an octahedral
methyl compound would prove to be more robust. In 1976, Wilkinson and Galyer disclosed an improved synthesis using trimethylaluminium
in conjunction with trimethylamine
, instead of methyllithium. The stoichiometry of the improved synthesis is as follows:
Alternatively, the alkylation can employ dimethylzinc:
. The trigonal prismatic geometry is unusual in that the vast majority of six-coordinate organometallic compounds
adopt octahedral molecular geometry
. In the initial report, the IR spectroscopy results were interpreted in terms of an octahedral structure. In 1978, a study using photoelectron spectroscopy
appeared to confirm the initial assignment of an Oh structure.
The octahedral assignment remained for nearly 20 years until 1989 when Girolami and Morse showed that was trigonal prismatic as indicated by X-ray crystallography
. They predicted that other d0 ML6 species such as , , and W(CH3)6 would also prove to be trigonal prismatic. This report prompted other investigations into the structure of W(CH3)6 Using gas-phase electron diffraction
, Volden et al. confirmed that W(CH3)6 is indeed trigonal prismatic structure with either D3h or C3v symmetry. In 1996, Seppelt et al. reported that W(CH3)6 had a strongly distorted trigonal prismatic coordination geometry
based on single-crystal X-ray diffraction, which they later confirmed in 1998.
As shown in the top figure at right, the ideal or D3h trigonal prism in which all six carbons are equivalent is distorted to the C3v structure observed by Seppelt et al. by opening up one triplet of methyl groups (upper triangle) to wider C-W-C angles (94-97°) with slightly shorter C-W bond lengths, while closing the other triplet (lower triangle) to 75-78° with longer bond lengths.
Deviation from octahedral geometry can be ascribed to an effect known as a second-order Jahn-Teller distortion
. In 1995, before the work of Seppelt and Pfennig, Landis and coworkers had already predicted a distorted trigonal prismatic structure based on valence bond theory
and VALBOND
calculations.
The history of the structure of W(CH3)6 illustrates an inherent difficulty in interpreting spectral data for new compounds: initial data may not provide reason to believe the structure deviates from a presumed geometry based on significant historical precedence, but there is always the possibility that the initial assignment will prove to be incorrect. Prior to 1989, there was no reason to suspect that ML6 compounds were anything but octahedral
, yet new evidence and improved characterization methods suggested that perhaps there were exceptions to the rule, as evidenced by the case of W(CH3)6. These discoveries helped to spawn re-evaluation of the theoretical considerations for ML6 geometries.
Other 6-coordinate complexes with distorted trigonal prismatic structures include [MoMe6], , and . All are d0 complexes. Some 6-coordinate complexes with regular trigonal prismatic structures (D3h symmetry) include [ReMe6] (d1), (d0), and the aforementioned (d0).
, hexamethyltungsten decompose
s, releasing methane
and trace amounts of ethane
. The black residue is purported to contain polymethylene and tungsten, but the decomposition of W(CH3)6 to form tungsten metal is highly unlikely. The following equation is the approximate stoichiometry
proposed by Wilkinson and Shortland:
Like many organometallic complexes, WMe6 is destroyed by oxygen
. Similarly, acids give methane and unidentified tungsten derivatives, while halogens cause release give the methyl halide and leave the tungsten halide.
A patent application was submitted in 1991 suggesting the use of W(CH3)6 in the manufacture of semiconductor
devices for chemical vapor deposition
of tungsten
thin films; however, to date it has not been used for this purpose. Rather, tungsten hexafluoride and hydrogen
are used instead.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
W
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
(CH3)6. Classified as an organometallic compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
, hexamethyltungsten is an air-sensitive, red, crystalline solid at room temperature; however, it is extremely volatile and sublimes at −30 °C. Owing to its six methyl groups it is extremely soluble in petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
, aromatic hydrocarbons, ethers, carbon disulfide
Carbon disulfide
Carbon disulfide is a colorless volatile liquid with the formula CS2. The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non-polar solvent...
, and carbon tetrachloride
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
.
Synthesis
Hexamethyltungsten was first reported in 1973 by WilkinsonGeoffrey Wilkinson
Sir Geoffrey Wilkinson FRS was a Nobel laureate English chemist who pioneered inorganic chemistry and homogeneous transition metal catalysis.-Biography:...
and Shortland, who described its preparation by the reaction of methyllithium with tungsten hexachloride
Tungsten hexachloride
Tungsten hexachloride is the chemical compound of tungsten and chlorine with the formula WCl6. This dark violet blue species exists as a volatile solid under standard conditions. It is an important starting reagent in the preparation of tungsten compounds. WCl6 is a rare example of a...
in diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
. The synthesis was motivated in part by previous work which indicated that tetrahedral methyl transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
compounds are thermally unstable, in the hopes that an octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
methyl compound would prove to be more robust. In 1976, Wilkinson and Galyer disclosed an improved synthesis using trimethylaluminium
Trimethylaluminium
Trimethylaluminium is the chemical compound with the formula Al26, abbreviated as Al2Me6, 2 or the abbreviation TMA. This pyrophoric, colorless liquid is an industrially important organoaluminium compound...
in conjunction with trimethylamine
Trimethylamine
Trimethylamine is an organic compound with the formula N3. This colorless, hygroscopic, and flammable tertiary amine has a strong "fishy" odor in low concentrations and an ammonia-like odor at higher concentrations...
, instead of methyllithium. The stoichiometry of the improved synthesis is as follows:
- WCl6 + 6 Al(CH3)3 → W(CH3)6 + 6 Al(CH3)2Cl
Alternatively, the alkylation can employ dimethylzinc:
- WX6 + 3 Zn(CH3)2 → W(CH3)6 + 3 ZnX2 (X = F, Cl)
Molecular geometry
W(CH3)6 adopts a distorted trigonal prismatic geometry with C3v symmetryMolecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
. The trigonal prismatic geometry is unusual in that the vast majority of six-coordinate organometallic compounds
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
adopt octahedral molecular geometry
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
. In the initial report, the IR spectroscopy results were interpreted in terms of an octahedral structure. In 1978, a study using photoelectron spectroscopy
Photoemission spectroscopy
Photoemission spectroscopy , also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in a substance...
appeared to confirm the initial assignment of an Oh structure.
The octahedral assignment remained for nearly 20 years until 1989 when Girolami and Morse showed that was trigonal prismatic as indicated by X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
. They predicted that other d0 ML6 species such as , , and W(CH3)6 would also prove to be trigonal prismatic. This report prompted other investigations into the structure of W(CH3)6 Using gas-phase electron diffraction
Electron diffraction
Electron diffraction refers to the wave nature of electrons. However, from a technical or practical point of view, it may be regarded as a technique used to study matter by firing electrons at a sample and observing the resulting interference pattern...
, Volden et al. confirmed that W(CH3)6 is indeed trigonal prismatic structure with either D3h or C3v symmetry. In 1996, Seppelt et al. reported that W(CH3)6 had a strongly distorted trigonal prismatic coordination geometry
Coordination geometry
The term coordination geometry is used in a number of related fields of chemistry and solid state chemistry/physics.-Molecules:The coordination geometry of an atom is the geometrical pattern formed by atoms around the central atom....
based on single-crystal X-ray diffraction, which they later confirmed in 1998.
As shown in the top figure at right, the ideal or D3h trigonal prism in which all six carbons are equivalent is distorted to the C3v structure observed by Seppelt et al. by opening up one triplet of methyl groups (upper triangle) to wider C-W-C angles (94-97°) with slightly shorter C-W bond lengths, while closing the other triplet (lower triangle) to 75-78° with longer bond lengths.
Deviation from octahedral geometry can be ascribed to an effect known as a second-order Jahn-Teller distortion
Jahn-Teller effect
The Jahn–Teller effect, sometimes also known as Jahn–Teller distortion, or the Jahn–Teller theorem, describes the geometrical distortion of non-linear molecules under certain situations. This electronic effect is named after Hermann Arthur Jahn and Edward Teller, who proved, using group theory,...
. In 1995, before the work of Seppelt and Pfennig, Landis and coworkers had already predicted a distorted trigonal prismatic structure based on valence bond theory
Valence bond theory
In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds...
and VALBOND
VALBOND
In molecular mechanics, VALBOND is a method for computing the angle bending energy that is based on valence bond theory. It is based on orbital strength functions, which are maximized when the hybrid orbitals on the atom are orthogonal...
calculations.
The history of the structure of W(CH3)6 illustrates an inherent difficulty in interpreting spectral data for new compounds: initial data may not provide reason to believe the structure deviates from a presumed geometry based on significant historical precedence, but there is always the possibility that the initial assignment will prove to be incorrect. Prior to 1989, there was no reason to suspect that ML6 compounds were anything but octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
, yet new evidence and improved characterization methods suggested that perhaps there were exceptions to the rule, as evidenced by the case of W(CH3)6. These discoveries helped to spawn re-evaluation of the theoretical considerations for ML6 geometries.
Other 6-coordinate complexes with distorted trigonal prismatic structures include [MoMe6], , and . All are d0 complexes. Some 6-coordinate complexes with regular trigonal prismatic structures (D3h symmetry) include [ReMe6] (d1), (d0), and the aforementioned (d0).
Reactivity and potential uses
At room temperatureRoom temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
, hexamethyltungsten decompose
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...
s, releasing methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
and trace amounts of ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
. The black residue is purported to contain polymethylene and tungsten, but the decomposition of W(CH3)6 to form tungsten metal is highly unlikely. The following equation is the approximate stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
proposed by Wilkinson and Shortland:
- → 3 + + W
Like many organometallic complexes, WMe6 is destroyed by oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Similarly, acids give methane and unidentified tungsten derivatives, while halogens cause release give the methyl halide and leave the tungsten halide.
A patent application was submitted in 1991 suggesting the use of W(CH3)6 in the manufacture of semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
devices for chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
of tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
thin films; however, to date it has not been used for this purpose. Rather, tungsten hexafluoride and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
are used instead.