{"id":11538,"date":"2013-11-05T17:40:43","date_gmt":"2013-11-05T17:40:43","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11538"},"modified":"2013-11-05T17:40:43","modified_gmt":"2013-11-05T17:40:43","slug":"multiple-personalities-of-magnesium","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2013\/11\/05\/multiple-personalities-of-magnesium\/","title":{"rendered":"Multiple personalities of  Magnesium."},"content":{"rendered":"<p>The following is a short question in a problem sheet associated with introductory organic chemistry.<\/p>\n<ul>\n<li><strong>Q<\/strong>: &#8220;<em>Show curly arrows for the formation of the product of the following reaction, together with a Lewis representation of that product: Et<sub>2<\/sub>O + MgBr<sub>2<\/sub><\/em>&#8220;.<\/li>\n<li><strong>A<\/strong>:\u00a0<em>Et<sub>2<\/sub>O<sup>+<\/sup>-Mg<sup>&#8211;<\/sup>Br<sub>2<\/sub><\/em> (a product by the way that is known as magnesium bromide ethyl etherate, and which is commercially available as a solution).<\/li>\n<\/ul>\n<p>First a few tutor-like comments. The Mg is tri-coordinate in this simple representation, and if we assume that the bonds are covalent, has six electrons in the Mg valence shell. In modern notation, the Mg has a formal charge of -1 and the oxygen +1. The Mg thus does not have a filled 3s\/3p valence shell, which would be eight.\u00a0But few (students or tutors) go on to apply a reality check.\u00a0So here is one.<\/p>\n<p>The reality check involves a search for a crystal structure, which is really trivial to set up. And what we find are the following.<\/p>\n<ol>\n<li>The first hit with exactly this stoichiometry has the CCDC code\u00a0<strong>TOQKIT<\/strong> and a polymeric structure as below. Each Mg is coordinated by four (bridged) bromines and one oxygen, giving <em>trigonal bipyramidal<\/em> penta-coordination. The valence electron count at Mg is now eight, but distributed around five bonds, not four. Since we no longer have formal Lewis two-electron covalent bonds, it is difficult to assign a Lewis-like charge to the atoms.\u00a0<br \/>\n<figure id=\"attachment_11541\" aria-describedby=\"caption-attachment-11541\" style=\"width: 307px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\" wp-image-11541 \" onclick=\"jmolInitialize('..\/Jmol\/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2013\/11\/TOQKIT.mol;');\" alt=\"Click for  3D\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/11\/TOQKIT.jpg\" width=\"307\" height=\"206\" \/><figcaption id=\"caption-attachment-11541\" class=\"wp-caption-text\">Click for 3D<\/figcaption><\/figure>\n<\/li>\n<li>The next hit actually corresponds to the stoichiometry 2R<sub>2<\/sub>O + MgBr<sub>2<\/sub> (R=thf). This again is polymeric, but differs from the first structure in having <em>octahedral<\/em> Mg (six coordination).<br \/>\n<figure id=\"attachment_11544\" aria-describedby=\"caption-attachment-11544\" style=\"width: 293px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\" wp-image-11544  \" onclick=\"jmolInitialize('..\/Jmol\/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2013\/11\/MGBEET.mol2;');\" alt=\"Click for  3D.\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/11\/MGBEET.jpg\" width=\"293\" height=\"290\" \/><figcaption id=\"caption-attachment-11544\" class=\"wp-caption-text\">Click for 3D.<\/figcaption><\/figure>\n<\/li>\n<li>OK, even more ether: 4R<sub>2<\/sub>O + MgBr<sub>2<\/sub>. Finally, non polymeric but again with six-coordinate <em>octahedral<\/em> Mg. The Mg again has a filled valence octet, and again the bonds are not two-electron ones, hence no charges are attempted.\u00a0So just a change in the stoichiometry can result in fascinating changes to the resulting structure.<br \/>\n<figure id=\"attachment_11545\" aria-describedby=\"caption-attachment-11545\" style=\"width: 307px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\" wp-image-11545  \" onclick=\"jmolInitialize('..\/Jmol\/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2013\/11\/ZZZVBQ.mol2;');\" alt=\"Click for  3D\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/11\/ZZZ.jpg\" width=\"307\" height=\"302\" \/><figcaption id=\"caption-attachment-11545\" class=\"wp-caption-text\">Click for 3D<\/figcaption><\/figure>\n<\/li>\n<li>Finally, a variation; benzyl magnesium bromide (a Grignard reagent) shows<em> tetrahedral<\/em> coordination.<\/li>\n<\/ol>\n<figure id=\"attachment_11557\" aria-describedby=\"caption-attachment-11557\" style=\"width: 302px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\" wp-image-11557  \" onclick=\"jmolInitialize('..\/Jmol\/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2013\/11\/XECTOQ.mol2;');\" alt=\"Click for 3D\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/11\/XECTOQ.jpg\" width=\"302\" height=\"233\" \/><figcaption id=\"caption-attachment-11557\" class=\"wp-caption-text\">Click for 3D<\/figcaption><\/figure>\n<p>Students (and tutors) who get as far as this are amply rewarded I hope!<\/p>\n","protected":false},"excerpt":{"rendered":"<p>The following is a short question in a problem sheet associated with introductory organic chemistry. Q: &#8220;Show curly arrows for the formation of the product of the following reaction, together with a Lewis representation of that product: Et2O + MgBr2&#8220;. A:\u00a0Et2O+-Mg&#8211;Br2 (a product by the way that is known as magnesium bromide ethyl etherate, and [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11],"tags":[1119,1784,1879,1880,2400],"class_list":["post-11538","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-grignard","tag-octahedral","tag-pence","tag-pentagonalbipyramid","tag-tetrahedral"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/11538","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=11538"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/11538\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=11538"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=11538"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=11538"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}