{"id":1183,"date":"2009-12-01T10:58:43","date_gmt":"2009-12-01T09:58:43","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1183"},"modified":"2009-12-01T11:59:08","modified_gmt":"2009-12-01T10:59:08","slug":"multi-centre-bonding-in-the-grignard-reagent","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1183","title":{"rendered":"Multi-centre bonding in the Grignard Reagent"},"content":{"rendered":"
\n

The Grignard reaction<\/a> is encountered early on in most chemistry courses, and most labs include the preparation of this reagent, typically by the following reaction:<\/p>\n

2PhBr + 2Mg \u2192 2PhMgBr\u00a0\u2194 MgBr2<\/sub> + Ph2<\/sub>Mg<\/p>\n

The reagent itself exists as part of an equilibrium, named after Schlenk<\/a>, in which a significant concentration of a dialkyl or diarylmagnesium species is formed. The topic of this blog entry is to analyse the structure and bonding in this latter species.<\/p>\n

First, the structure is shown below (for 2,6-diethylphenyl magnesium). This reveals a dimeric structure with a four membered ring core, comprising two \u00a0Mg atoms \u00a0connected by two bridging \u00a0aryl groups.<\/p>\n

\"The

The crystal structure of a di-aryl magnesium. Click to view 3D<\/p><\/div>The question to be addressed here is the nature of the aryl groups. Put simply, it seems as if their bridging role means that one of the six carbons involved in the benzene ring has become sp3<\/sup> hybridized. This would in turn mean that the cyclic conjugation of the benzene ring is interrupted, and a species akin to the Wheland intermediate is formed in which the aromaticity of two of the benzene rings is no longer sustained. This situation could be depicted thus;<\/p>\n

\"A

A Simple bonding representation in Ph2Mg dimer<\/p><\/div>\n

Is this really the best way of depicting the bonding in this species? A more subtle analysis of the bonding can be achieved using a technique known as ELF (involving analysis of the electron localization function). This reveals bonds as so-called synaptic basins, which come in two varieties; disynaptic basins corresponding to two-centre bonds, and trisynaptic basins which reveal three-centre bonds (there is also a monosynaptic basin which corresponds to electron lone pairs). Such an ELF analysis (based on a B3LYP\/6-311G(d,p) computed wavefunction for Ph2<\/sub>Mg dimer) is shown below;<\/p>\n

\"ELF

ELF analysis of the bonding in Ph2Mg dimer. Click for 3D model<\/p><\/div>The small purple dots represent synaptic basins. Several of these are circled. The \u00a0ones circled in orange are conventional disynaptic forms, and the basins can be integrated to to 2.48 electrons each. The red basin however is clearly revealed as a trisynaptic form (covering both metal centres and the carbon) and integrating to \u00a02.7 electrons. The \u00a0three basins surrounding each Mg atom integrate to 7.91 electrons, which reveal the metal to have a conventional octet of electrons in its valence shell. The bonding in the central region could therefore be described as comprising two three-centre-three-electron<\/strong><\/em> bonds. The key aspect of this is that the two bridging phenyl groups do not break their aromaticity, ie all four<\/strong> phenyl\/aryl groups largely retain their aromaticity! Thus the disynaptic basins for \u00a0the normal <\/em>non-bridging\u00a0phenyl group and \u00a0circled in green integrates to 2.6 electrons and the blue to 2.8 (an ideal aromatic bond would of course integrate to 3.0 electrons), whereas the equivalent basins for the bridging phenyl (brown and purple, 2.5 and \u00a02.8) are virtually the same.<\/p>\n

It is interesting how a veritable mainstay of most taught chemistry courses, the Grignard reagent, \u00a0can have such subtle aspects of the bonding surrounding both the metal atom and the aromatic groups, and how rarely this bonding is actually dissected in most text books.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

The Grignard reaction is encountered early on in most chemistry courses, and most labs include the preparation of this reagent, typically by the following reaction: 2PhBr + 2Mg \u2192 2PhMgBr\u00a0\u2194 MgBr2 + Ph2Mg The reagent itself exists as part of an equilibrium, named after Schlenk, in which a significant concentration of a dialkyl or diarylmagnesium […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[7,4],"tags":[2649,2648,157,159,158,160],"ppma_author":[2661],"class_list":["post-1183","post","type-post","status-publish","format-standard","hentry","category-hypervalency","category-interesting-chemistry","tag-hypervalency","tag-interesting-chemistry","tag-metal","tag-metal-atom","tag-metal-centres","tag-mg-atom"],"yoast_head":"\nMulti-centre bonding in the Grignard Reagent - 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