Editing Inert-pair effect (section)

==Steric activity of the lone pair==
The chemical inertness of the s electrons in the lower oxidation state is not always related to steric inertness (where steric inertness means that the presence of the s-electron lone pair has little or no influence on the geometry of the molecule or crystal). A simple example of steric activity is [[tin(II) chloride|SnCl<sub>2</sub>]], which is bent in accordance with [[valence shell electron repulsion theory|VSEPR theory]]. Some examples where the lone pair appears to be inactive are [[bismuth(III) iodide]], BiI<sub>3</sub>, and the {{chem|BiI|6|3-}} anion. In both of these the central Bi atom is octahedrally coordinated with little or no distortion, in contravention to VSEPR theory.<ref>{{cite journal
| title =  Stereochemically active or inactive lone pair electrons in some six-coordinate, group 15 halides
| author = Ralph A. Wheeler and P. N. V. Pavan Kumar
| journal = Journal of the American Chemical Society
| year = 1992
| volume = 114
| issue = 12
| pages = 4776–4784
| doi = 10.1021/ja00038a049
}}</ref>
The steric activity of the lone pair has long been assumed to be due to the orbital having some p character, i.e. the orbital is not spherically symmetric.<ref name = "Greenwood"/> More recent theoretical work shows that this is not always necessarily the case. For example, the [[litharge]] structure of [[lead(II) oxide|PbO]] contrasts to the more symmetric and simpler rock-salt structure of [[lead(II) sulfide|PbS]], and this has been explained in terms of Pb<sup>II</sup>–anion interactions in PbO leading to an asymmetry in electron density. Similar interactions do not occur in PbS.<ref>{{cite journal
| title =  The origin of the stereochemically active Pb(II) lone pair: DFT calculations on PbO and PbS
|author=Walsh A. |author2=Watson G. W.
| journal = Journal of Solid State Chemistry
| year = 2005
| volume = 178
| issue = 5
| pages = 1422–1428
| doi = 10.1016/j.jssc.2005.01.030
| bibcode = 2005JSSCh.178.1422W }}</ref> Another example are some thallium(I) salts where the asymmetry has been ascribed to s electrons on Tl interacting with antibonding orbitals.<ref>{{cite journal
| title =  Lone Pair Effect in Thallium(I) Macrocyclic Compounds
|author=Mudring A. J. |author2=Rieger F.
| journal = Inorg. Chem.
| year = 2005
| volume = 44
| issue = 18
| pages = 6240–6243
| doi = 10.1021/ic050547k
| pmid = 16124801
}}</ref>