Editing Lone pair (section)

==Unusual lone pairs==
A stereochemically active lone pair is also expected for divalent [[lead]] and [[tin]] ions due to their formal electronic configuration of n''s''<sup>2</sup>. In the solid state this results in the distorted metal coordination observed in the [[tetragonal crystal system|tetragonal]] [[litharge]] structure adopted by both PbO and SnO.
The formation of these heavy metal n''s''<sup>2</sup> lone pairs which was previously attributed to intra-atomic [[Orbital hybridisation|hybridization]] of the metal s and p states<ref>''Stereochemistry of Ionic Solids'' J.D.Dunitz and L.E.Orgel, Advan. Inorg. and Radiochem. '''1960''', 2, 1–60</ref> has recently been shown to have a strong anion dependence.<ref>{{cite journal |doi=10.1103/PhysRevLett.96.157403 |volume=96 |title=Electronic Origins of Structural Distortions in Post-Transition Metal Oxides: Experimental and Theoretical Evidence for a Revision of the Lone Pair Model |year=2006 |journal=Physical Review Letters |last1=Payne |first1=D. J. |issue=15 |page=157403 |pmid=16712195 |bibcode=2006PhRvL..96o7403P |url=https://ora.ox.ac.uk/objects/uuid:da90e4c7-566c-4a37-bab0-cd00b17043ff}}</ref> This dependence on the electronic states of the anion can explain why some divalent lead and tin materials such as PbS and SnTe show no stereochemical evidence of the lone pair and adopt the symmetric rocksalt crystal structure.<ref>{{cite journal |doi=10.1016/j.jssc.2005.01.030 |volume=178 |title=The origin of the stereochemically active Pb(II) lone pair: DFT calculations on PbO and PbS |year=2005 |journal=Journal of Solid State Chemistry |pages=1422–1428 |last1=Walsh |first1=Aron |issue=5 |bibcode=2005JSSCh.178.1422W}}</ref><ref>{{cite journal |doi=10.1021/jp051822r |volume=109 |title=Influence of the Anion on Lone Pair Formation in Sn(II) Monochalcogenides: A DFT Study |year=2005 |journal=The Journal of Physical Chemistry B |pages=18868–18875 |last1=Walsh |first1=Aron |issue=40 |pmid=16853428}}</ref>

In molecular systems the lone pair can also result in a distortion in the coordination of ligands around the metal ion. The lone-pair effect of lead can be observed in supramolecular complexes of [[lead(II) nitrate]], and in 2007 a study linked the lone pair to [[lead poisoning]].<ref>{{cite journal |last1=Gourlaouen |first1=Christophe |last2=Parisel |first2=Olivier |title=Is an Electronic Shield at the Molecular Origin of Lead Poisoning? A Computational Modeling Experiment |journal=Angewandte Chemie International Edition |date=15 January 2007 |volume=46 |issue=4 |pages=553–556 |doi=10.1002/anie.200603037 |pmid=17152108}}</ref> Lead ions can replace the native metal ions in several key enzymes, such as zinc cations in the [[ALAD]] enzyme, which is also known as [[porphobilinogen synthase]], and is important in the synthesis of [[heme]], a key component of the oxygen-carrying molecule [[hemoglobin]]. This inhibition of heme synthesis appears to be the molecular basis of lead poisoning (also called "saturnism" or "plumbism").<ref>{{cite journal |last1=Jaffe |first1=E. K. |last2=Martins |first2=J. |last3=Li |first3=J. |last4=Kervinen |first4=J. |last5=Dunbrack |first5=R. L. |display-authors=2 |title=The Molecular Mechanism of Lead Inhibition of Human Porphobilinogen Synthase |journal=Journal of Biological Chemistry |date=13 October 2000 |volume=276 |issue=2 |pages=1531–1537 |doi=10.1074/jbc.M007663200 |pmid=11032836 |doi-access=free}}</ref><ref>{{cite journal |last1=Scinicariello |first1=Franco |last2=Murray |first2=H. Edward |last3=Moffett |first3=Daphne B. |last4=Abadin |first4=Henry G. |last5=Sexton |first5=Mary J. |last6=Fowler |first6=Bruce A.|title=Lead and δ-Aminolevulinic Acid Dehydratase Polymorphism: Where Does It Lead? A Meta-Analysis |journal=Environmental Health Perspectives |date=15 September 2006 |volume=115 |issue=1 |pages=35–41 |doi=10.1289/ehp.9448 |pmid=17366816 |pmc=1797830}}</ref><ref>{{cite web |last1=Chhabra |first1=Namrata |title=Effect of Lead poisoning on heme biosynthetic pathway |url=http://usmle.biochemistryformedics.com/effect-of-lead-poisoning-on-heme-biosynthetic-pathway/ |website=Clinical Cases: Biochemistry For Medics |access-date=30 October 2016 |url-status=dead |archive-url=https://web.archive.org/web/20160403160650/http://usmle.biochemistryformedics.com/effect-of-lead-poisoning-on-heme-biosynthetic-pathway/ |archive-date=3 April 2016 |date=November 15, 2015}}</ref>

Computational experiments reveal that although the [[coordination number]] does not change upon substitution in calcium-binding proteins, the introduction of lead distorts the way the ligands organize themselves to accommodate such an emerging lone pair: consequently, these proteins are perturbed. This lone-pair effect becomes dramatic for zinc-binding proteins, such as the above-mentioned porphobilinogen synthase, as the natural substrate cannot bind anymore – in those cases the protein is [[enzyme inhibitor|inhibited]].

In [[Group 14]] elements (the [[carbon group]]), lone pairs can manifest themselves by shortening or lengthening [[single bond]] ([[bond order]] 1) lengths,<ref>{{cite journal |last1=Richards |first1=Anne F. |last2=Brynda |first2=Marcin |last3=Power |first3=Philip P. |title=Effects of the alkali metal counter ions on the germanium–germanium double bond length in a heavier group 14 element ethenide salt |journal=Chem. Commun. |date=2004 |issue=14 |pages=1592–1593 |doi=10.1039/B401507J |pmid=15263933}}</ref> as well as in the effective order of [[triple bond]]s as well.<ref>{{cite journal |last1=Power |first1=Philip P. |title=π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements |journal=Chemical Reviews |date=December 1999 |volume=99 |issue=12 |pages=3463–3504 |doi=10.1021/cr9408989 |pmid=11849028}}</ref><ref name="LeeSekiguchi2011">{{cite book |author1=Vladimir Ya. Lee |author2=Akira Sekiguchi |title=Organometallic Compounds of Low-Coordinate Si, Ge, Sn, and Pb: From Phantom Species to Stable Compounds |url=https://books.google.com/books?id=kAS9u4If26wC&pg=PA23 |date=22 July 2011 |publisher=John Wiley & Sons |isbn=978-1-119-95626-6 |page=23}}</ref> The familiar [[alkyne]]s have a carbon-carbon triple bond ([[bond order]] 3) and a linear geometry of 180° bond angles (figure '''A''' in reference <ref name="SpikesDigermyne">{{cite journal |last1=Spikes |first1=Geoffrey H. |last2=Power |first2=Philip P. |title=Lewis base induced tuning of the Ge–Ge bond order in a "digermyne" |journal=Chem. Commun. |date=2007 |issue=1 |pages=85–87 |doi=10.1039/b612202g |pmid=17279269}}</ref>). <!-- I'm assuming this is what the original editor was referring to by "germanium to germanium [..] effective bond order 2" -->However, further down in the group ([[silicon]], [[germanium]], and [[tin]]), formal triple bonds have an effective bond order 2 with one lone pair (figure '''B'''<ref name="SpikesDigermyne"/>) and [[cis-trans isomerism|trans]]-bent geometries. In [[lead]], the effective bond order is reduced even further to a single bond, with two lone pairs for each lead atom (figure ''C''<ref name="SpikesDigermyne"/>). In the [[organogermanium compound]] (''Scheme 1'' in the reference), the effective bond order is also 1, with complexation of the [[Lewis acid|acidic]] [[isonitrile]] (or ''isocyanide'') C-N groups, based on interaction with germanium's empty 4p orbital.<ref name="SpikesDigermyne"/><ref>{{cite journal |last1=Power |first1=Philip P. |title=Silicon, germanium, tin, and lead analogues of acetylenes |journal=Chemical Communications |date=2003 |issue=17 |pages=2091–101 |doi=10.1039/B212224C |pmid=13678155}}</ref>

[[File:Digermina.png|center|600px|Lone pair trends in group 14 triple bonds]]