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Silicon monoxide is the chemical compound with the formula SiO where silicon is present in the oxidation state +2. In the vapour phase, it is a diatomic molecule. It has been detected in stellar objects and has been described as the most common
oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of th ...
of silicon in the universe.Peter Jutzi and Ulrich Schubert (2003) ''Silicon chemistry: from the atom to extended systems''. Wiley-VCH .


Solid form

When SiO gas is cooled rapidly, it condenses to form a brown/black polymeric
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) o ...
y material, (SiO)''n'', which is available commercially and used to deposit films of SiO. Glassy (SiO)''n'' is air and moisture sensitive.


Oxidation

Its surface readily oxidizes in air at
room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
, giving an SiO2 surface layer that protects the material from further oxidation. However, (SiO)''n'' irreversibly
disproportionates In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...
into SiO2 and Si in a few hours between 400 °C and 800 °C and very rapidly between 1,000 °C and 1,440 °C, although the reaction does not go to completion.


Production

The first precise report on the formation of SiO was in 1887J. W. Mellor "A Comprehensive Treatise on Inorganic and Theoretical Chemistry" Vol VI, Longmans, Green and Co. (1947) p. 235. by the chemist Charles F. Maybery (1850–1927) at the
Case School of Applied Science The Case School of Engineering is the engineering school at Case Western Reserve University in Cleveland, Ohio, USA. It traces its roots to the 1880 founding of the Case School of Applied Science. It became the Case Institute of Technology in 1947 ...
in
Cleveland Cleveland ( ), officially the City of Cleveland, is a city in the U.S. state of Ohio and the county seat of Cuyahoga County. Located in the northeastern part of the state, it is situated along the southern shore of Lake Erie, across the U ...
. Maybery claimed that SiO formed as an amorphous greenish-yellow substance with a vitreous luster when silica was reduced with charcoal in the absence of metals in an electric furnace. The substance was always found at the interface between the charcoal and silica particles. By investigating some of the chemical properties of the substance, its specific gravity, and a combustion analysis, Maybery deduced that the substance must be SiO. The equation representing the partial chemical reduction of SiO2 with C can be represented as: : + ⇌ Complete reduction of SiO2 with twice the amount of carbon yields elemental silicon and twice the amount of carbon monoxide. In 1890, the German chemist
Clemens Winkler Clemens Alexander Winkler (December 26, 1838 – October 8, 1904) was a German chemist who discovered the element germanium in 1886, solidifying Dmitri Mendeleev's theory of periodicity. Life Winkler was born in 1838 in Freiberg, Kingdom ...
(the discoverer of germanium) was the first to attempt to synthesize SiO by heating silicon dioxide with silicon in a combustion furnace. : + ⇌ However, Winkler was not able to produce the monoxide since the temperature of the mixture was only around 1000 °C. The experiment was repeated in 1905 by Henry Noel Potter (1869–1942), a Westinghouse engineer. Using an electric furnace, Potter was able to attain a temperature of 1700 °C and observe the generation of SiO. Potter also investigated the properties and applications of the solid form of SiO.


Gaseous form

Because of the volatility of SiO, silica can be removed from ores or minerals by heating them with silicon to produce gaseous SiO in this manner. However, due to the difficulties associated with accurately measuring its vapor pressure, and because of the dependency on the specifics of the experimental design, various values have been reported in the literature for the vapor pressure of SiO (g). For the pSiO above molten silicon in a quartz (SiO2) crucible at the melting point of silicon, one study yielded a value of 0.002 atm. For the direct vaporization of pure, amorphous SiO solid, 0.001 atm has been reported. For a coating system, at the phase boundary between SiO2 and a silicide, 0.01 atm was reported. Silica itself, or refractories containing SiO2, can be reduced with H2 or CO at high temperatures, e.g.: :(g) ⇌ As the SiO product volatilizes off (is removed), the equilibrium shifts to the right, resulting in the continued consumption of SiO2. Based on the dependence of the rate of silica weight loss on the gas flow rate normal to the interface, the rate of this reduction appears to be controlled by convective diffusion or mass transfer from the reacting surface.


Gaseous (molecular) form

Silicon monoxide molecules have been trapped in an argon matrix cooled by helium. In these conditions, the SiO bond length is between 148.9 pm and 151 pm.Inorganic Chemistry, Holleman-Wiberg, Academic Press (2001) p. 858. This bond length is similar to the length of Si=O double bonds (148 pm) in the matrix-isolated linear molecule (O=Si=O), indicative of the absence of a triple bond as in
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simp ...
. However, the SiO triple bond has a calculated bond length of 150 pm and a bond energy of 794 kJ/mol, which are also very close to those reported for SiO. The SiO double bond structure is, notably, an exception to Lewis'
octet rule The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The ...
for molecules composed of the light main group elements, whereas the SiO triple bond satisfies this rule. That anomaly not withstanding, the observation that monomeric SiO is short-lived and that (SiO)'n'
oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
s with 'n' = 2,3,4,5 are known, all having closed ring structures in which the silicon atoms are connected through bridging oxygen atoms (i.e. each oxygen atom is singly bonded to two silicon atoms; no Si-Si bonds), suggests the Si=O double bond structure, with a hypovalent silicon atom, is likely for the monomer. Condensing molecular SiO in argon matrix together with
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 ...
,
chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine ...
or
carbonyl sulfide Carbonyl sulfide is the chemical compound with the linear formula OCS. It is a colorless flammable gas with an unpleasant odor. It is a linear molecule consisting of a carbonyl group double bonded to a sulfur atom. Carbonyl sulfide can be conside ...
(COS), followed by irradiation with light, produces the planar molecules (with Si-O distance 148 pm) and (Si-O 149 pm), and the linear molecule (Si-O 149 pm, Si-S 190 pm). Matrix-isolated molecular SiO reacts with oxygen atoms generated by microwave discharge to produce molecular which has a linear structure. When metal atoms (such as Na, Al, Pd, Ag, and Au) are co-deposited with SiO, triatomic molecules are produced with linear (AlSiO and PdSiO), non-linear (AgSiO and AuSiO), and ring (NaSiO) structures.


Solid (polymeric) form

Potter reported SiO solid as yellowish-brown in color and as being an electrical and thermal insulator. The solid burns in oxygen and decomposes water with the liberation of hydrogen. It dissolves in warm alkali hydroxides and in hydrofluoric acid. Even though Potter reported the heat of combustion of SiO to be 200 to 800 calories higher than that of an equilibrium mixture of Si and SiO2 (which could, arguably, be used as evidence that SiO is a unique chemical compound), some studies characterized commercially available solid silicon monoxide materials as an inhomogeneous mixture of amorphous SiO2 and amorphous Si with some chemical bonding at the interface of the Si and SiO2 phases. Recent spectroscopic studies in a correlation with Potter's report suggest that commercially available solid silicon monoxide materials can not be considered as an inhomogeneous mixture of amorphous SiO2 and amorphous Si.


Interstellar occurrence

Interstellar SiO was first reported in 1971 after detection in the giant molecular cloud Sgr B2. SiO is used as a molecular tracer of shocked gas in protostellar outflows.


References

{{Authority control Inorganic silicon compounds Oxides Inorganic polymers