Editing VSEPR theory (section)

==Exceptions==
There are groups of compounds where VSEPR fails to predict the correct geometry.

===Some AX<sub>2</sub>E<sub>0</sub> molecules===

The shapes of heavier Group 14 element alkyne analogues (RM≡MR, where M = Si, Ge, Sn or Pb) have been computed to be bent.<ref>{{cite journal| title = Silicon, germanium, tin and lead analogues of acetylenes| first = Philip P.| last = Power| journal = [[ChemComm|Chem. Commun.]]| date = September 2003| issue = 17| pages = 2091–2101| doi = 10.1039/B212224C| pmid = 13678155}}</ref><ref>{{cite journal| title = Triple bonds between heavier Group 14 elements. A theoretical approach| first1 = Shigeru| last1 = Nagase| first2 = Kaoru| last2 = Kobayashi| first3 = Nozomi| last3 = Takagi| journal = [[Journal of Organometallic Chemistry|J. Organomet. Chem.]]| date = 6 October 2000| volume = 11 | issue = 1–2| pages = 264–271| doi = 10.1016/S0022-328X(00)00489-7}}</ref><ref>{{cite journal| title = A Stable Compound Containing a Silicon–Silicon Triple Bond| first1 = Akira| last1 = Sekiguchi| first2 = Rei| last2 = Kinjō| first3 = Masaaki| last3 = Ichinohe| journal = [[Science (journal)|Science]]| date = September 2004| volume = 305| issue = 5691| pages = 1755–1757| doi = 10.1126/science.1102209| pmid = 15375262| bibcode = 2004Sci...305.1755S| s2cid = 24416825| url = http://people.ok.ubc.ca/wsmcneil/339/Sci2004.pdf}}{{dead link|date=December 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

===Some AX<sub>2</sub>E<sub>2</sub> molecules===
One example of the AX<sub>2</sub>E<sub>2</sub> geometry is molecular [[lithium oxide]], Li<sub>2</sub>O, a linear rather than bent structure, which is ascribed to its bonds being essentially ionic and the strong lithium-lithium repulsion that results.<ref>{{cite journal | last1 = Bellert | first1 = D. | last2 = Breckenridge | first2 = W. H. | year = 2001 | title = A spectroscopic determination of the bond length of the LiOLi molecule: Strong ionic bonding | journal = [[J. Chem. Phys.]] | volume = 114 | issue = 7| page = 2871 | doi = 10.1063/1.1349424 | bibcode = 2001JChPh.114.2871B }}</ref> Another example is O(SiH<sub>3</sub>)<sub>2</sub> with an Si–O–Si angle of 144.1°, which compares to the angles in Cl<sub>2</sub>O (110.9°), (CH<sub>3</sub>)<sub>2</sub>O (111.7°), and N(CH<sub>3</sub>)<sub>3</sub> (110.9°).<ref name="Gillespie&Robinson"/> Gillespie and Robinson rationalize the Si–O–Si bond angle based on the observed ability of a ligand's lone pair to most greatly repel other electron pairs when the ligand electronegativity is greater than or equal to that of the central atom.<ref name = "Gillespie&Robinson">{{cite journal | last1 = Gillespie | first1 = R. J. | last2 = Robinson | first2 = E. A. | year = 2005 | title = Models of molecular geometry | journal = [[Chem. Soc. Rev.]] | volume = 34 | issue = 5| pages = 396–407 | doi = 10.1039/b405359c | pmid = 15852152 }}</ref> In O(SiH<sub>3</sub>)<sub>2</sub>, the central atom is more electronegative, and the lone pairs are less localized and more weakly repulsive.  The larger Si–O–Si bond angle results from this and strong ligand-ligand repulsion by the relatively large -SiH<sub>3</sub> ligand.<ref name="Gillespie&Robinson"/>  Burford et al. showed through X-ray diffraction studies that Cl<sub>3</sub>Al–O–PCl<sub>3</sub> has a linear Al–O–P bond angle and is therefore a non-VSEPR molecule.<ref>{{cite journal |last1=Burford |first1=Neil |last2=Phillips |first2=Andrew |last3=Schurko |first3=Robert |last4=Wasylishen |first4=Roderick |last5=Richardson |first5=John |title=Isolation and comprehensive solid state characterization of Cl<sub>3</sub>Al–O–PCl<sub>3</sub> |journal=Chemical Communications |date=1997 |volume=1997 |issue=24 |pages=2363–2364 |doi=10.1039/A705781D |url=https://doi.org/10.1039/A705781D |access-date=3 April 2024|url-access=subscription }}</ref>

===Some AX<sub>6</sub>E<sub>1</sub> and AX<sub>8</sub>E<sub>1</sub> molecules===
[[File:Xenon-hexafluoride-3D-SF.png|200px|thumb|[[Xenon hexafluoride]], which has a distorted octahedral geometry]]
Some AX<sub>6</sub>E<sub>1</sub> molecules, e.g. [[xenon hexafluoride]] (XeF<sub>6</sub>) and the Te(IV) and Bi(III) anions, {{chem|TeCl|6|2-}}, {{chem|TeBr|6|2-}}, {{chem|BiCl|6|3-}}, {{chem|BiBr|6|3-}} and {{chem|BiI|6|3-}}, are octahedral, rather than pentagonal pyramids, and the lone pair does not affect the geometry to the degree predicted by VSEPR.<ref>{{cite book|last=Wells |first=A. F. |date=1984 |title=Structural Inorganic Chemistry |edition=5th |publisher=Oxford Science Publications |isbn=978-0-19-855370-0}}</ref> Similarly, the octafluoroxenate ion ({{chem|XeF|8|2-}}) in [[nitrosonium octafluoroxenate(VI)]]<ref name=Housecroft/>{{rp|498}}<ref name="Peterson1971">{{Cite journal | first3 = A.| first2 = H. | first4 = M.| title = Antiprismatic Coordination about Xenon: the Structure of Nitrosonium Octafluoroxenate(VI)| first1 = W.| last2 = Holloway| last3 = Coyle| volume = 173| journal = [[Science (journal)|Science]]| issue = 4003| pages = 1238–1239| issn = 0036-8075| doi = 10.1126/science.173.4003.1238| last4 = Williams| pmid = 17775218| last1 = Peterson| date = Sep 1971|bibcode = 1971Sci...173.1238P | s2cid = 22384146 }}</ref><ref>{{cite book| title = Molecular origami: precision scale models from paper| first1 = Robert M.| last1 = Hanson| publisher = University Science Books| year = 1995| isbn = 978-0-935702-30-9}}</ref> is a square antiprism with minimal distortion, despite having a lone pair. One rationalization is that steric crowding of the ligands allows little or no room for the non-bonding lone pair;<ref name = "Gillespie&Robinson"/> another rationalization is the [[inert-pair effect]].<ref name=Housecroft/>{{rp|214}}

===Square planar ML<sub>4</sub> complexes===
The Kepert model predicts that ML<sub>4</sub> transition metal molecules are tetrahedral in shape, and it cannot explain the formation of square planar complexes.<ref name=Housecroft/>{{rp|542}} The majority of such complexes exhibit a d<sup>8</sup> configuration as for the [[potassium tetrachloroplatinate|tetrachloroplatinate]] ({{chem|PtCl|4|2-}}) ion. The explanation of the shape of square planar complexes involves electronic effects and requires the use of [[crystal field theory]].<ref name=Housecroft/>{{rp|562–4}}

===Complexes with strong d-contribution===
[[File:Hexamethyl-tungsten-3D-balls.png|200px|thumb|[[Hexamethyltungsten]], a transition metal complex whose geometry is different from main-group coordination]]
Some transition metal complexes with low d electron count have unusual geometries, which can be ascribed to d subshell bonding interaction.<ref name="d0kaupp">{{cite journal | title = "Non-VSEPR" Structures and Bonding in d<sup>0</sup> Systems | first = Martin | last = Kaupp | journal = [[Angewandte Chemie|Angew. Chem. Int. Ed. Engl.]] | year = 2001 | volume = 40 | issue = 1 | pages = 3534–3565 | doi =  10.1002/1521-3773(20011001)40:19<3534::AID-ANIE3534>3.0.CO;2-#| pmid = 11592184 | url = http://www.chimdocet-inorganica.it/SITO_ESERCIZI/Complementi/COMP1/VSEPREccezioni.pdf}}</ref> [[Ronald Gillespie|Gillespie]] found that this interaction produces bonding pairs that also occupy the respective [[antipodal point]]s (ligand opposed) of the sphere.<ref>{{cite journal | title = An Electron Localization Function Study of the Geometry of d<sup>0</sup> Molecules of the Period 4 Metals Ca to Mn | first1 = Ronald J. | last1 = Gillespie | first2 =  Stéphane | last2 =  Noury | first3 = Julien | last3 = Pilmé | first4 =  Bernard | last4 =  Silvi | journal = [[Inorganic Chemistry (journal)|Inorg. Chem.]] | year = 2004 | volume = 43 | issue = 10 | pages = 3248–3256 | doi =  10.1021/ic0354015| pmid = 15132634 }}</ref><ref name="Fiftyyears"/> This phenomenon is an electronic effect resulting from the bilobed shape of the underlying sd<sup>x</sup> [[hybridization (chemistry)|hybrid orbitals]].<ref>{{cite journal | last1 = Landis | first1 = C. R. | last2 = Cleveland | first2 = T. | last3 = Firman | first3 = T. K. | year = 1995 | title = Making sense of the shapes of simple metal hydrides | journal = [[J. Am. Chem. Soc.]] | volume = 117 | issue = 6| pages = 1859–1860 | doi=10.1021/ja00111a036| bibcode = 1995JAChS.117.1859L }}</ref><ref>{{cite journal | last1 = Landis | first1 = C. R. | last2 = Cleveland | first2 = T. | last3 = Firman | first3 = T. K. | year = 1996 | title = Structure of W(CH<sub>3</sub>)<sub>6</sub> | journal = [[Science (journal)|Science]] | volume = 272 | issue = 5259| pages = 179–183 |doi=10.1126/science.272.5259.179f| doi-access = free }}</ref> The repulsion of these bonding pairs leads to a different set of shapes.

{| class="wikitable" style="margin:1em auto;"
|- 
! Molecule type
! Shape
! Geometry
! Examples
|- 
! ML<sub>2</sub>
| [[Bent molecular geometry|Bent]]
| [[File:Bent-3D-balls.png|100px]]
| [[Titanium dioxide|TiO<sub>2</sub>]]<ref name="d0kaupp"/>
|- 
! ML<sub>3</sub>
| [[Trigonal pyramidal molecular geometry|Trigonal pyramidal]]
| [[File:Pyramidal-3D-balls.png|100px]]
| [[Chromium trioxide|CrO<sub>3</sub>]]<ref name=cdoi>{{Cite journal | doi = 10.1021/ja077984d| title = Probing the Electronic and Structural Properties of Chromium Oxide Clusters {{chem|(CrO|3|)|''n''|-}} and (CrO<sub>3</sub>)<sub>''n''</sub> (''n''&nbsp;= 1–5): Photoelectron Spectroscopy and Density Functional Calculations| journal = Journal of the American Chemical Society| volume = 130| issue = 15| pages = 5167–77| year = 2008| last1 = Zhai | first1 = H. J. | last2 = Li | first2 = S. | last3 = Dixon | first3 = D. A. | last4 = Wang | first4 = L. S. |pmid = 18327905}}</ref>
|-
! ML<sub>4</sub>
| [[Tetrahedral molecular geometry|Tetrahedral]]
| [[File:Tetrahedral-3D-balls.png|100px]]
| [[titanium tetrachloride|TiCl<sub>4</sub>]]<ref name=Housecroft/>{{rp|598–599}}
|- 
! ML<sub>5</sub>
| [[Square pyramidal molecular geometry|Square pyramidal]]
| [[File:Square-pyramidal-3D-balls.png|100px]]
| [[Pentamethyltantalum|Ta(CH<sub>3</sub>)<sub>5</sub>]]<ref>{{cite journal |journal= Coord. Chem. Rev. |volume= 197 |year= 2000 |pages= 141–168 |title= Atomic orbitals, symmetry, and coordination polyhedra |first= R. Bruce |last= King | doi = 10.1016/s0010-8545(99)00226-x }}</ref>
|- 
! ML<sub>6</sub>
| ''C<sub>3v</sub>'' [[Trigonal prismatic molecular geometry|Trigonal prismatic]]
| [[File:Prismatic_TrigonalP.png|100px]]
| [[Hexamethyltungsten|W(CH<sub>3</sub>)<sub>6</sub>]]<ref>{{cite journal|last1=Haalan |first1=A. |last2=Hammel |first2=A.|last3=Rydpal |first3=K. |last4=Volden |first4=H. V.|journal=[[J. Am. Chem. Soc.]]|year=1990|volume=112|pages= 4547–4549|title=The coordination geometry of gaseous hexamethyltungsten is not octahedral|doi=10.1021/ja00167a065|issue=11|bibcode=1990JAChS.112.4547H }}</ref>
|}

The gas phase structures of the triatomic halides of the heavier members of [[alkaline earth metal|group 2]], (i.e., calcium, strontium and barium halides, MX<sub>2</sub>), are not linear as predicted but are bent, (approximate X–M–X angles: [[calcium fluoride|CaF<sub>2</sub>]], 145°; [[strontium fluoride|SrF<sub>2</sub>]], 120°; [[barium fluoride|BaF<sub>2</sub>]], 108°; [[strontium chloride|SrCl<sub>2</sub>]], 130°; [[barium chloride|BaCl<sub>2</sub>]], 115°; [[barium bromide|BaBr<sub>2</sub>]], 115°; [[barium iodide|BaI<sub>2</sub>]], 105°).<ref name = "Greenwood">{{Greenwood&Earnshaw}}</ref> It has been proposed by [[Ronald Gillespie|Gillespie]] that this is also caused by bonding interaction of the ligands with the d subshell of the metal atom, thus influencing the molecular geometry.<ref name = "Gillespie&Robinson"/><ref>{{cite journal | doi = 10.1063/1.459748 | title = Ab initio model potential study of the equilibrium geometry of alkaline earth dihalides: MX<sub>2</sub> (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I) | year = 1991 | author = Seijo, Luis | journal = [[J. Chem. Phys.]] | volume = 94 | pages = 3762 | last2 = Barandiarán | first2 = Zoila | last3 = Huzinaga | first3 = Sigeru | issue = 5| bibcode = 1991JChPh..94.3762S | url = https://repositorio.uam.es/bitstream/10486/7315/1/41581_jchemphysseijo_91_jcp_94_3762.pdf | hdl = 10486/7315 | hdl-access = free }}</ref>

===Superheavy elements===
[[Relativistic quantum chemistry|Relativistic effects]] on the electron orbitals of [[superheavy element]]s is predicted to influence the molecular geometry of some compounds. For instance, the 6d<sub>5/2</sub> electrons in [[nihonium]] play an unexpectedly strong role in bonding, so NhF<sub>3</sub> should assume a T-shaped geometry, instead of a trigonal planar geometry like its lighter congener BF<sub>3</sub>.<ref>{{cite journal |last1=Seth |first1=Michael |last2=Schwerdtfeger |first2=Peter |first3=Knut |last3=Fægri |date=1999 |title=The chemistry of superheavy elements. III. Theoretical studies on element 113 compounds |journal=Journal of Chemical Physics |volume=111 |issue=14 |pages=6422–6433 |doi=10.1063/1.480168 |bibcode=1999JChPh.111.6422S|s2cid=41854842 |doi-access=free |hdl=2292/5178 |hdl-access=free }}</ref> In contrast, the extra stability of the 7p<sub>1/2</sub> electrons in [[tennessine]] are predicted to make TsF<sub>3</sub> trigonal planar, unlike the T-shaped geometry observed for IF<sub>3</sub> and predicted for [[Astatine|At]]F<sub>3</sub>;<ref>{{Cite journal |last1=Bae |first1=Ch. |last2=Han |first2=Y.-K. |last3=Lee |first3=Yo. S. |doi=10.1021/jp026531m |title=Spin−Orbit and Relativistic Effects on Structures and Stabilities of Group 17 Fluorides EF<sub>3</sub> (E = I, At, and Element 117): Relativity Induced Stability for the ''D<sub>3h</sub>'' Structure of (117)F<sub>3</sub> |journal=The Journal of Physical Chemistry A |volume=107 |issue=6 |pages=852–858 |date=18 January 2003 |bibcode=2003JPCA..107..852B}}</ref> similarly, [[Oganesson|Og]]F<sub>4</sub> should have a tetrahedral geometry, while XeF<sub>4</sub> has a square planar geometry and [[Radon|Rn]]F<sub>4</sub> is predicted to have the same.<ref name=fluoride>{{cite journal|journal=Journal of Physical Chemistry A|volume=103|issue=8|pages=1104–1108|date=1999|title=Structures of RgFn (Rg = Xe, Rn, and Element 118. n = 2, 4.) Calculated by Two-component Spin-Orbit Methods. A Spin-Orbit Induced Isomer of (118)F<sub>4</sub>|first1=Young-Kyu|last1=Han|first2=Yoon Sup|last2=Lee|doi=10.1021/jp983665k|bibcode=1999JPCA..103.1104H}}</ref>