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Silicon–oxygen bond

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Asilicon–oxygen bond(Si−Obond) is achemical bondbetweensiliconandoxygenatoms that can be found in manyinorganicandorganic compounds.[1]In a silicon–oxygen bond,electronsareshared unequallybetween the twoatoms,with oxygen taking the larger share due to its greaterelectronegativity.Thispolarisationmeans Si–O bonds show characteristics of bothcovalentandionic bonds.[2]Compounds containing silicon–oxygen bonds include materials of major geological and industrial significance such assilica,silicate mineralsandsilicone polymerslikepolydimethylsiloxane.[1][3]

Bond polarity, length and strength

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On thePauling electronegativity scale,silicon has anelectronegativityof 1.90 and oxygen 3.44. The electronegativity difference between the elements is therefore 1.54. Because of this moderately large difference in electronegativities, theSi−Obond ispolarbut not fullyionic.Carbon has an electronegativity of 2.55 socarbon–oxygen bondshave an electronegativity difference of 0.89 and are less polar than silicon–oxygen bonds. Silicon–oxygen bonds are thereforecovalentandpolar,with apartial positive chargeon silicon and a partial negative charge on oxygen: Siδ+—Oδ−.[2]

Silicon–oxygensingle bondsare longer (1.6 vs 1.4Å) but stronger (452 vs. about 360kJ mol−1) thancarbon–oxygensingle bonds.[1]However, silicon–oxygendouble bondsare weaker than carbon–oxygen double bonds (590 vs. 715 kJ mol−1) due to a better overlap ofp orbitalsforming a strongerpi bondin the latter. This is an example of thedouble bond rule.For these reasons,carbon dioxideis a molecular gas containing two C=O double bonds per carbon atom whereassilicon dioxideis a polymeric solid containing four Si–O single bonds per silicon atom; molecular SiO2containing two Si=O double bonds would polymerise.[4]Other compounds containing Si=O double bonds are normally very reactive and unstable with respect topolymerisationoroligomerization.Silanonesoligomerise tosiloxanesunless they are stabilised,[5]for example by coordination to a metal centre,[6]coordination toLewis acids or bases,[7]or bysteric shielding.[8]

Comparison of C–O and Si–O bonds
Bond Carbon–oxygen Silicon–oxygen
E C Si
Pauling electronegativityof E 2.55 1.90
Pauling electronegativity difference between E and O 0.89 1.54
H3E–O–EH3Bond angle / ° 111[9] 142[10]
Typicalsp3E–O single bond length / Å 1.43[11] 1.63[12]
Typicalsp2E–O single bond length / Å 1.34[11]
Typicalsp2E=O double bond length / Å 1.21[11] 1.52[8][13]
TypicalspE=O double bond length / Å 1.16[14] 1.48[15][16]
Typical E–O single bond strength / kJ mol−1 ~360[1] 452[1]
Typical E=O double bond strength / kJ mol−1 715[4] 590[4]

Bond angles

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Disiloxanegroups, Si–O–Si, tend to have largerbond anglesthan their carbon counterparts, C–O–C. The Si–O–Si angle ranges from about 130–180°, whereas the C–O–C angle inethersis 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 arehyperconjugation(donation from an oxygen p orbital to an Si–R σ*sigmaantibonding molecular orbital,for example) and ionic effects (such aselectrostatic repulsionbetween the two neighbouring partially positive silicon atoms). Recent calculations suggest πbackbondingfrom an oxygen 2p orbital to a silicon3d orbitalmakes only a minor contribution to bonding as the Si 3d orbital is too high in energy.[2]

The Si–O–Si angle is 144° inα-quartz,155° inβ-quartz,147° inα-cristobaliteand (153±20)° invitreous silica.It is 180° incoesite(another polymorph of SiO2), in Ph3Si–O–SiPh3,[17]and in the [O3Si–O–SiO3]6−ion inthortveitite,Sc2Si2O7.It increases progressively from 133° to 180° in Ln2Si2O7as the size and coordination number of the lanthanide decreases from neodymium to lutetium. It is 150° inhemimorphiteand 134° inlithium metasilicateandsodium metasilicate.[1]

Coordination number

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In silicate minerals, silicon often forms single bonds to four oxygen atoms in atetrahedral molecular geometry,forming asilicon–oxygen tetrahedron.At high pressures, silicon can increase itscoordination numberto six, as instishovite.[1]

See also

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References

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  1. ^abcdefgGreenwood, Norman N.;Earnshaw, Alan (1997).Chemistry of the Elements(2nd ed.).Butterworth-Heinemann.pp. 342–366.ISBN978-0-08-037941-8.
  2. ^abcDankert, Fabian; von Hänisch, Carsten (2021). "Siloxane Coordination Revisited: Si􏰉–O Bond Character, Reactivity and Magnificent Molecular Shapes".Eur. J. Inorg. Chem.2021(29): 2907–2927.doi:10.1002/ejic.202100275.S2CID239645449.
  3. ^Housecroft, C. E.; Sharpe, A. G. (2008).Inorganic Chemistry(3rd ed.). Prentice Hall. pp. 413–424.ISBN978-0-13-175553-6.
  4. ^abcN. C. Norman (1997).Periodicity and the s- and p-Block Elements.Oxford University Press. pp. 50–52, 65–67.ISBN978-0-19-855961-0.
  5. ^Xiong, Y.; Yao, S.; Driess, M. (2013). "Chemical Tricks To Stabilize Silanones and Their Heavier Homologues with EO Bonds (E=Si–Pb): From Elusive Species to Isolable Building Blocks".Angew. Chem. Int. Ed.52(16): 4302–4311.doi:10.1002/anie.201209766.PMID23450830.
  6. ^Sen, S. S. (2014). "A Stable Silanone with a Three-Coordinate Silicon Atom: A Century-Long Wait is Over".Angew. Chem. Int. Ed.53(34): 8820–8822.doi:10.1002/anie.201404793.PMID24990653.
  7. ^Sun, T.; Li, J.; Wang, H. (2022). "Recent Advances in the Chemistry of Heavier Group 14 Analogues of Carbonyls".Chem. Asian J.17(18): e202200611.doi:10.1002/asia.202200611.PMID35883252.S2CID251104394.
  8. ^abKobayashi, Ryo; Ishida, Shintaro; Iwamoto, Takeaki (2019)."An Isolable Silicon Analogue of a Ketone that Contains an Unperturbed Si=O Double Bond".Angew. Chem. Int. Ed.58(28): 9425–9428.doi:10.1002/anie.201905198.PMID31095845.S2CID157056381.
  9. ^Vojinović, Krunoslav; Losehand, Udo; Mitzel, Nobert W. (2004). "Dichlorosilane–dimethyl ether aggregation: a new motif in halosilane adduct formation".Dalton Trans.(16): 2578–2581.doi:10.1039/B405684A.PMID15303175.
  10. ^Barrow, M. J.; Ebsworth, E. A. V.; Harding, M. M. (1979). "The crystal and molecular structures of disiloxane (at 108 K) and hexamethyldisiloxane (at 148 K)".Acta Crystallogr. B.35(9): 2093–2099.doi:10.1107/S0567740879008529.
  11. ^abcSmith, Michael B.; March, Jerry (2007).March's Advanced Organic Chemistry(6th ed.). John Wiley & Sons. pp. 24–25.ISBN978-0-471-72091-1.
  12. ^Kaftory, Menahem; Kapon, Moshe; Botoshansky, Mark (1998)."The Structural Chemistry of Organosilicon Compounds".In Rappoport, Zvi; Apeloig, Yitzhak (eds.).The Chemistry of Organic Silicon Compounds, Volume 2.PATAI'S Chemistry of Functional Groups. John Wiley & Sons, Ltd.doi:10.1002/0470857250.ISBN9780471967576.
  13. ^Bogey, Marcel; Delcroix, Bruno; Jean-Claude Guillemin, Adam Walters (1996). "Experimentally Determined Structure of H2SiO by Rotational Spectroscopy and Isotopic Substitution ".J. Mol. Spectrosc.175(2): 421–428.Bibcode:1996JMoSp.175..421B.doi:10.1006/jmsp.1996.0048.
  14. ^Greenwood, Norman N.;Earnshaw, Alan (1997).Chemistry of the Elements(2nd ed.).Butterworth-Heinemann.pp. 292, 304–314.ISBN978-0-08-037941-8.
  15. ^Schnöckel, Hansgeorg (1978). "IR Spectroscopic Detection of Molecular SiO2".Angew. Chem. Int. Ed.17(8): 616–617.doi:10.1002/anie.197806161.
  16. ^Jutzi, Peter; Schubert, Ulrich (2003).Silicon Chemistry: From the Atom to Extended Systems.Wiley-VCH. pp. 27–28.ISBN9783527306473.
  17. ^Glidewell, C.; Liles, D. C. (1978). "The crystal and molecular structure of oxobis[triphenylsilicon(IV)]".Acta Crystallogr. B.34:124–128.doi:10.1107/S0567740878002435.S2CID98347658.
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