View source for Chemistry:Silicon–oxygen bond

{{short description|Covalent chemical bond between silicon and oxygen atoms}}
A '''silicon–oxygen bond''' ('''{{chem2|Si\sO}} bond''') is a [[Chemistry:Chemical bond|chemical bond]] between [[Chemistry:Silicon|silicon]] and [[Chemistry:Oxygen|oxygen]] atoms that can be found in many [[Chemistry:Inorganic compound|inorganic]] and [[Chemistry:Organic compound|organic compound]]s.<ref name="G&E">{{Greenwood&Earnshaw2nd|pages=342–366}}</ref> In a silicon–oxygen bond, [[Physics:Electron|electron]]s are shared unequally between the two [[Physics:Atom|atom]]s, with oxygen taking the larger share due to its greater [[Chemistry:Electronegativity|electronegativity]]. This [[Chemistry:Chemical polarity#Polarity of bonds|polarisation]] means Si–O bonds show characteristics of both [[Chemistry:Covalent bond|covalent]] and ionic bonds.<ref name="Dankert&vonHänisch" /> Compounds containing silicon–oxygen bonds include materials of major geological and industrial significance such as [[Chemistry:Silica|silica]], [[Chemistry:Silicate minerals|silicate minerals]] and [[Chemistry:Silicone|silicone polymers]] like [[Chemistry:Polydimethylsiloxane|polydimethylsiloxane]].<ref name="G&E" /><ref name="Housecroft">{{Housecroft3rd|pages=413-424}}</ref>

== Bond polarity, length and strength ==
On the [[Chemistry:Electronegativities of the elements (data page)|Pauling electronegativity scale]], silicon has an [[Chemistry:Electronegativity|electronegativity]] of 1.90 and oxygen 3.44. The electronegativity difference between the elements is therefore 1.54. Because of this moderately large difference in electronegativities, the {{chem2|Si\sO}} bond is [[Chemistry:Chemical polarity|polar]] but not fully [[Chemistry:Ionic bonding|ionic]]. Carbon has an electronegativity of 2.55 so [[Chemistry:Carbon–oxygen bond|carbon–oxygen bonds]] have an electronegativity difference of 0.89 and are less polar than silicon–oxygen bonds. Silicon–oxygen bonds are therefore [[Chemistry:Covalent bond|covalent]] and [[Chemistry:Chemical polarity|polar]], with a [[Chemistry:Partial charge|partial positive charge]] on silicon and a partial negative charge on oxygen: Si<sup>δ+</sup>&mdash;O<sup>δ&minus;</sup>.<ref name="Dankert&vonHänisch">{{ cite journal | title = Siloxane Coordination Revisited: Si􏰉–O Bond Character, Reactivity and Magnificent Molecular Shapes | first1 = Fabian | last1 = Dankert | first2 = Carsten | last2 = von Hänisch | journal = Eur. J. Inorg. Chem. | year = 2021 | volume = 2021 | issue = 29 | pages = 2907–2927 | doi = 10.1002/ejic.202100275 | s2cid = 239645449 }}</ref>

Silicon–oxygen [[Chemistry:Single bond|single bond]]s are longer (1.6 vs 1.4 [[Angstrom|Å]]) but stronger (452 vs. about 360 [[Joule per mole|kJ mol<sup>−1</sup>]]) than [[Chemistry:Carbon–oxygen bond|carbon–oxygen]] single bonds.<ref name="G&E" /> However, silicon–oxygen [[Chemistry:Double bond|double bond]]s are weaker than carbon–oxygen double bonds (590 vs. 715 kJ mol<sup>−1</sup>) due to a better overlap of p orbitals forming a stronger [[Chemistry:Pi bond|pi bond]] in the latter. This is an example of the [[Chemistry:Double bond rule|double bond rule]]. For these reasons, [[Chemistry:Carbon dioxide|carbon dioxide]] is a molecular gas containing two C=O double bonds per carbon atom whereas [[Chemistry:Silicon dioxide|silicon dioxide]] is a polymeric solid containing four Si–O single bonds per silicon atom; molecular SiO<sub>2</sub> containing two Si=O double bonds would polymerise.<ref name="Norman">{{ cite book | title = Periodicity and the s- and p-Block Elements | author = N. C. Norman | publisher = Oxford University Press | year = 1997 | isbn = 978-0-19-855961-0 | pages = 50–52, 65–67 }}</ref> Other compounds containing Si=O double bonds are normally very reactive and unstable with respect to polymerisation or oligomerization. Silanones oligomerise to [[Chemistry:Siloxane|siloxane]]s unless they are stabilised,<ref>{{ cite journal | title = Chemical Tricks To Stabilize Silanones and Their Heavier Homologues with EO Bonds (E=Si–Pb): From Elusive Species to Isolable Building Blocks | first1 = Y. | last1 = Xiong | first2 = S. | last2 = Yao | first3 = M. | last3 = Driess | journal = Angew. Chem. Int. Ed. | year = 2013 | volume = 52 | issue = 16 | pages = 4302–4311 | doi = 10.1002/anie.201209766 | pmid = 23450830 }}</ref> for example by coordination to a metal centre,<ref>{{ cite journal | title = A Stable Silanone with a Three-Coordinate Silicon Atom: A Century-Long Wait is Over | first1 = S. S. | last1 = Sen | journal = Angew. Chem. Int. Ed. | year = 2014 | volume = 53 | issue = 34 | pages = 8820–8822 | doi = 10.1002/anie.201404793 | pmid = 24990653 }}</ref> coordination to [[Chemistry:Lewis acids and bases|Lewis acids or bases]],<ref>{{ cite journal | first1 = T. | last1 = Sun | first2 = J. | last2 = Li | first3 = H. | last3 = Wang | title = Recent Advances in the Chemistry of Heavier Group 14 Analogues of Carbonyls | journal = [[Chemistry:Chemistry: An Asian Journal|Chem. Asian J.]] | year = 2022 | pages = e202200611 | doi = 10.1002/asia.202200611 | pmid = 35883252 | s2cid = 251104394 }}</ref> or by steric shielding.<ref name="KobayashiIshidaIwamoto">{{ cite journal | title = An Isolable Silicon Analogue of a Ketone that Contains an Unperturbed Si=O Double Bond | first1 = Ryo | last1 = Kobayashi | first2 = Shintaro | last2 = Ishida | first3 = Takeaki | last3 = Iwamoto | journal = Angew. Chem. Int. Ed. | volume = 58 | issue = 28 | year = 2019 | pages = 9425–9428 | doi = 10.1002/anie.201905198 | pmid = 31095845 | s2cid = 157056381 | doi-access = free }}</ref>

{| class="wikitable" style="text-align:center;"
|+Comparison of C–O and Si–O bonds
|-
! Bond
| '''Carbon–oxygen'''
| '''Silicon–oxygen'''
|-
! E
| C
| Si
|-
! Pauling electronegativity of E
| 2.55 
| 1.90
|-
! Pauling electronegativity difference between E and O
| 0.89
| 1.54
|-
! H<sub>3</sub>E–O–EH<sub>3</sub> Bond angle / °
| 111<ref>{{ cite journal | first1 = Krunoslav | last1 = Vojinović | first2 = Udo | last2 = Losehand | first3 = Nobert W. | last3 = Mitzel | title = Dichlorosilane–dimethyl ether aggregation: a new motif in halosilane adduct formation | journal = Dalton Trans. | year = 2004 | issue = 16 | pages = 2578–2581  | doi = 10.1039/B405684A | pmid = 15303175 }}</ref>
| 142<ref>{{ cite journal | first1 = M. J. | last1 = Barrow | first2 = E. A. V. | last2 = Ebsworth | first3 = M. M. | last3 = Harding | title = The crystal and molecular structures of disiloxane (at 108 K) and hexamethyldisiloxane (at 148 K) | journal = Acta Crystallogr. B | year = 1979 | volume = 35 | issue = 9 | pages = 2093–2099 | doi = 10.1107/S0567740879008529 }}</ref>
|-
! Typical ''sp''<sup>3</sup> E–O single bond length / Å
| 1.43<ref name="March">{{ cite book | first1 = Michael B. | last1 = Smith | first2 = Jerry | last2 = March | title = March's Advanced Organic Chemistry | publisher = John Wiley & Sons | year = 2007 | edition=6th | isbn =  978-0-471-72091-1 | pages = 24–25 }}</ref>
| 1.63<ref>{{ cite book | first1 = Menahem | last1 = Kaftory |first2 = Moshe | last2 = Kapon | first3 = Mark | last3 = Botoshansky | chapter = The Structural Chemistry of Organosilicon Compounds | chapter-url = https://doi.org/10.1002/0470857250.ch5 | year = 1998 | editor-first1 = Zvi | editor-last1 = Rappoport | editor-first2 = Yitzhak | editor-last2 = Apeloig | title = The Chemistry of Organic Silicon Compounds, Volume 2 | publisher = John Wiley & Sons, Ltd | isbn = 9780471967576 | doi = 10.1002/0470857250 | series = PATAI'S Chemistry of Functional Groups }}</ref>
|-
! Typical ''sp''<sup>2</sup> E–O single bond length / Å
| 1.34<ref name="March" />
| 
|-
! Typical ''sp''<sup>2</sup> E=O double bond length / Å
| 1.21<ref name="March" />
| 1.52<ref name="KobayashiIshidaIwamoto" /><ref name="H2SiO_mw">{{ cite journal | first1 = Marcel | last1 = Bogey | first2 = Bruno | last2 = Delcroix | first3 = Adam Walters | last3 = Jean-Claude Guillemin | title = Experimentally Determined Structure of H<sub>2</sub>SiO by Rotational Spectroscopy and Isotopic Substitution | journal = [[Physics:Journal of Molecular Spectroscopy|J. Mol. Spectrosc.]] | volume = 175 | issue = 2 | year = 1996 | pages = 421–428 | doi = 10.1006/jmsp.1996.0048 | bibcode = 1996JMoSp.175..421B }}</ref>
|-
! Typical ''sp'' E=O double bond length / Å
| 1.16<ref name="G&E_carbon_oxygen">{{Greenwood&Earnshaw2nd|pages=292, 304–314}}</ref>
| 1.48<ref>{{ cite journal | first1 = Hansgeorg | last1 = Schnöckel | title = IR Spectroscopic Detection of Molecular SiO<sub>2</sub> | journal = Angew. Chem. Int. Ed. | year = 1978 | volume = 17 | issue = 8 | pages = 616–617 | doi = 10.1002/anie.197806161 }}</ref><ref name="Jutzi">{{ cite book | title = Silicon Chemistry: From the Atom to Extended Systems | first1 = Peter | last1 = Jutzi | first2 = Ulrich | last2 = Schubert | publisher = Wiley-VCH | year = 2003 | isbn = 9783527306473 | pages = 27–28 }}</ref>
|-
! Typical E–O single bond strength / kJ mol<sup>−1</sup>
| ~360<ref name="G&E" />
| 452<ref name="G&E" />
|-
! Typical E=O double bond strength / kJ mol<sup>−1</sup>
| 715<ref name="Norman" />
| 590<ref name="Norman" />
|-
|}

== Bond angles ==
[[Chemistry:Disiloxane|Disiloxane]] groups, Si–O–Si, tend to have larger bond angles than their carbon counterparts, C–O–C. The Si–O–Si angle ranges from about 130–180°, whereas the C–O–C angle in [[Chemistry:Ether|ether]]s is typically 107–113°. Si–O–C groups are intermediate, tending to have bond angles smaller than Si–O–Si but larger than C–O–C. The main reasons are [[Chemistry:Hyperconjugation|hyperconjugation]] (donation from an oxygen p orbital to an Si–R σ* [[Chemistry:Sigma bond|sigma]] [[Chemistry:Antibonding molecular orbital|antibonding molecular orbital]], for example) and ionic effects (such as [[Physics:Electrostatics|electrostatic repulsion]] between the two neighbouring partially positive silicon atoms). Recent calculations suggest π backbonding from an oxygen 2p orbital to a silicon 3d orbital makes only a minor contribution to bonding as the Si 3d orbital is too high in energy.<ref name="Dankert&vonHänisch" />

The Si–O–Si angle is 144° in α-quartz, 155° in β-quartz, 147° in [[Chemistry:Cristobalite|α-cristobalite]] and (153±20)° in vitreous silica. It is 180° in [[Chemistry:Coesite|coesite]] (another polymorph of SiO<sub>2</sub>), in Ph<sub>3</sub>Si–O–SiPh<sub>3</sub>,<ref>{{ cite journal | first1 = C. | last1 = Glidewell | first2 = D. C. | last2 = Liles | title = The crystal and molecular structure of oxobis[triphenylsilicon(IV)] | journal = Acta Crystallogr. B | year = 1978 | volume = 34 | pages = 124–128 | doi = 10.1107/S0567740878002435 | s2cid = 98347658 | doi-access =  }}</ref> and in the [O<sub>3</sub>Si–O–SiO<sub>3</sub>]<sup>6−</sup> ion in [[Chemistry:Thortveitite|thortveitite]], Sc<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>. It increases progressively from 133° to 180° in Ln<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> as the size and coordination number of the lanthanide decreases from neodymium to lutetium. It is 150° in [[Chemistry:Hemimorphite|hemimorphite]] and 134° in [[Chemistry:Lithium metasilicate|lithium metasilicate]] and [[Chemistry:Sodium metasilicate|sodium metasilicate]].<ref name="G&E" />

== Coordination number ==
In silicate minerals, silicon often forms single bonds to four oxygen atoms in a [[Physics:Tetrahedral molecular geometry|tetrahedral molecular geometry]], forming a [[Chemistry:Silicon–oxygen tetrahedron|silicon–oxygen tetrahedron]]. At high pressures, silicon can increase its [[Physics:Coordination number|coordination number]] to six, as in [[Chemistry:Stishovite|stishovite]].<ref name="G&E" />

== See also ==
*[[Chemistry:Organosilicon compound|Organosilicon compound]]
*[[Chemistry:Carbon–hydrogen bond|Carbon–hydrogen bond]]
*[[Chemistry:Carbon–carbon bond|Carbon–carbon bond]]
*[[Chemistry:Carbon–fluorine bond|Carbon–fluorine bond]]
*[[Physics:Bonding in solids|Bonding in solids]]

==References==
{{Reflist}}

{{DEFAULTSORT:Silicon-oxygen bond}}
[[Category:Chemical bonding]]

{{Sourceattribution|Silicon–oxygen bond}}