{"id":17805,"date":"2017-03-23T15:18:14","date_gmt":"2017-03-23T15:18:14","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=17805"},"modified":"2017-03-26T08:00:54","modified_gmt":"2017-03-26T07:00:54","slug":"silylium-cations","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805","title":{"rendered":"Silyl cations?"},"content":{"rendered":"
\n

It is not only the non-classical norbornyl cation<\/a> that has proved controversial in the past. A colleague mentioned at lunch (thanks Paul!) that tri-coordinate group 14 cations such as R3<\/sub>Si+<\/sup> have also had an interesting history.[1]<\/a><\/span> Here I take a brief look at some of these systems.<\/p>\n

\"\"<\/p>\n

Their initial characterisations, as with the carbon analogues, was by 29<\/sup>Si NMR. The first (of around 25) crystal structures appeared in 1994 (below) and they continue to fascinate to this day. I decided to focus on searching the Cambridge structure database (CSD), using the search query shown below (NM = non-metal). For a planar system the three angles subtended at the Si would of course total to\u00a0360\u00b0.<\/p>\n

\"\"<\/p>\n

The first such structure, published in 1994[2]<\/a><\/span> is shown in 2D representation below<\/p>\n

\"\"<\/p>\n

However, the three angles subtended at the Si are 113, 115 and 114\u00b0. Could it be that these types of cation are not planar but pyramidal\u00a0(a \u03c9B97XD\/Def2-TZVPP\u00a0calculation of SiH3<\/sub>+<\/sup> certainly gives it<\/strong> as planar). Below is a plot of the three angles:<\/p>\n

\"\"<\/p>\n

Ringed in red are two\u00a0systems where all three angles are ~120\u00b0 (the ones with red dots). The blue circle contains examples where all three angles are <110\u00b0. So I took a closer look at the first of these published[2]<\/a><\/span> and known by the code HAGCIB10 (angles of 113, 115 and 114\u00b0). The Si appears to be connected to a toluene present in the crystals via<\/em> an Si-C bond (blue arrow). If correct, that would account for the angles around Si being <120\u00b0 and indeed closer to tetrahedral, but it would also mean that the species was actually an arenium cation, otherwise known as a “Wheland intermediate”. That extra bond means that it is not a tri-coordinate silicon, but a four-coordinate silicon and that perhaps the indexing in the CSD needs correcting (as was done here<\/a>).\"\"<\/p>\n

Looking further, quite a few of the 25 examples contain so-called “N-heterocyclic carbene” ligands, as below (DOI for 3D model:\u00a010.5517\/CC12FWM0<\/a>[3]<\/a><\/span>).<\/i><\/p>\n

\"\"<\/p>\n

Again one might question the location of the formal +ve charge. Perhaps it might instead reside on the nitrogen as per below, in which case we again do not have a true tri-coordinate silicon cation for systems with such ligands.<\/p>\n

\"\"<\/a>This cannot be the whole story, although I would note that Si=C bonds can contain pyramidalised Si. The bonding clearly needs more investigation!\u00a0<\/p>\n

Very probably each of the 25 examples\u00a0identified by this search as a silylium or silyl cation has its own story to tell. But in unravelling these stories, one should always perhaps take the 2D representations shown in both the CSD and the original publications with a pinch of salt until other possibly better representations such as the one above are\u00a0excluded.<\/p>\n

References<\/h2>\n
  1. J.B. Lambert, Y. Zhao, H. Wu, W.C. Tse, and B. Kuhlmann, \"The Allyl Leaving Group Approach to Tricoordinate Silyl, Germyl, and Stannyl Cations\", Journal of the American Chemical Society<\/i>, vol. 121, pp. 5001-5008, 1999. https:\/\/doi.org\/10.1021\/ja990389u<\/a>\n\n<\/li>\n
  2. J.B. Lambert, S. Zhang, and S.M. Ciro, \"Silyl Cations in the Solid and in Solution\", Organometallics<\/i>, vol. 13, pp. 2430-2443, 1994. https:\/\/doi.org\/10.1021\/om00018a041<\/a>\n\n<\/li>\n
  3. T. Agou, N. Hayakawa, T. Sasamori, T. Matsuo, D. Hashizume, and N. Tokitoh, \"Reactions of Diaryldibromodisilenes with N\u2010Heterocyclic Carbenes: Formation of Formal Bis\u2010NHC Adducts of Silyliumylidene Cations\", Chemistry \u2013 A European Journal<\/i>, vol. 20, pp. 9246-9249, 2014. https:\/\/doi.org\/10.1002\/chem.201403083<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    It is not only the non-classical norbornyl cation that has proved controversial in the past. A colleague mentioned at lunch (thanks Paul!) that tri-coordinate group 14 cations such as R3Si+ have also had an interesting history. Here I take a brief look at some of these systems. Their initial characterisations, as with the carbon analogues, […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":true,"_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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[1745],"tags":[2051,2083,557,1395,157,1442,1849,734,2084],"ppma_author":[2661],"class_list":["post-17805","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","tag-2-norbornyl-cation","tag-carbocations","tag-chemical-bonding","tag-chemistry","tag-metal","tag-physical-organic-chemistry","tag-reactive-intermediates","tag-search-query","tag-tri-coordinate"],"yoast_head":"\nSilyl cations? - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Silyl cations? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"It is not only the non-classical norbornyl cation that has proved controversial in the past. A colleague mentioned at lunch (thanks Paul!) that tri-coordinate group 14 cations such as R3Si+ have also had an interesting history. Here I take a brief look at some of these systems. Their initial characterisations, as with the carbon analogues, […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2017-03-23T15:18:14+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2017-03-26T07:00:54+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/164-1024x748.jpg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Silyl cations? - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805","og_locale":"en_GB","og_type":"article","og_title":"Silyl cations? - Henry Rzepa's Blog","og_description":"It is not only the non-classical norbornyl cation that has proved controversial in the past. A colleague mentioned at lunch (thanks Paul!) that tri-coordinate group 14 cations such as R3Si+ have also had an interesting history. Here I take a brief look at some of these systems. Their initial characterisations, as with the carbon analogues, […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805","og_site_name":"Henry Rzepa's Blog","article_published_time":"2017-03-23T15:18:14+00:00","article_modified_time":"2017-03-26T07:00:54+00:00","og_image":[{"url":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/164-1024x748.jpg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"2 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Silyl cations?","datePublished":"2017-03-23T15:18:14+00:00","dateModified":"2017-03-26T07:00:54+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805"},"wordCount":462,"commentCount":8,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/164-1024x748.jpg","keywords":["2-Norbornyl cation","Carbocations","chemical bonding","Chemistry","metal","Physical organic chemistry","Reactive intermediates","search query","tri-coordinate"],"articleSection":["crystal_structure_mining"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805","name":"Silyl cations? - Henry Rzepa's Blog","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#website"},"primaryImageOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805#primaryimage"},"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/164-1024x748.jpg","datePublished":"2017-03-23T15:18:14+00:00","dateModified":"2017-03-26T07:00:54+00:00","author":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"breadcrumb":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805#breadcrumb"},"inLanguage":"en-GB","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805"]}]},{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805#primaryimage","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/164.jpg","contentUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/164.jpg","width":1204,"height":880},{"@type":"BreadcrumbList","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog"},{"@type":"ListItem","position":2,"name":"Silyl cations?"}]},{"@type":"WebSite","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#website","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/","name":"Henry Rzepa's Blog","description":"Chemistry with a twist","potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-GB"},{"@type":"Person","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281","name":"Henry Rzepa","image":{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g370be3a7397865e4fd161aefeb0a5a85","url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","contentUrl":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","caption":"Henry Rzepa"},"description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London.","sameAs":["https:\/\/orcid.org\/0000-0002-8635-8390"],"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?author=1"}]}},"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/pDef7-4Db","jetpack-related-posts":[{"id":17633,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17633","url_meta":{"origin":17805,"position":0},"title":"George Olah and the norbornyl cation.","author":"Henry Rzepa","date":"March 10, 2017","format":false,"excerpt":"George Olah passed away on March 8th. He was part of the generation of scientists in the post-war 1950s who had access to chemical instrumentation that truly revolutionised chemistry. In particular he showed how the then newly available NMR spectroscopy illuminated structures of cations in solvents such \"Magic acid\". The\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":17692,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17692","url_meta":{"origin":17805,"position":1},"title":"What is the (calculated) structure of a norbornyl cation anion-pair in water?","author":"Henry Rzepa","date":"April 1, 2017","format":false,"excerpt":"In a comment appended to an earlier post, I mused about the magnitude of the force constant relating to the interconversion between a classical and a non-classical structure for the norbornyl cation. Most calculations indicate the force constant for an \"isolated\" symmetrical cation\u00a0is +ve, which means it is a true\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/b3lypd3bj.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17662,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17662","url_meta":{"origin":17805,"position":2},"title":"Expanding on the curious connection between the norbornyl cation and small-ring aromatics.","author":"Henry Rzepa","date":"March 12, 2017","format":false,"excerpt":"This is another of those posts that has morphed from an earlier one noting\u00a0the death of the great chemist George Olah. The discussion about the norbornyl cation concentrated on whether this species existed in a single minimum symmetric energy well (the non-classical Winstein\/Olah proposal) or a double minimum well connected\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":17702,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17702","url_meta":{"origin":17805,"position":3},"title":"How does methane invert (its configuration)?","author":"Henry Rzepa","date":"March 16, 2017","format":false,"excerpt":"This is a spin-off from the table I constructed here for further chemical examples of the classical\/non-classical norbornyl cation conundrum. One possible entry would include the transition state for inversion of methane via a square planar geometry as compared with e.g. NiH4 for which the square planar motif is its\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":14423,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14423","url_meta":{"origin":17805,"position":4},"title":"Intermolecular atom-atom bonds in crystals? The O…O case.","author":"Henry Rzepa","date":"July 25, 2015","format":false,"excerpt":"I recently followed this bloggers trail; link1 \u2192 link2 to arrive at this delightful short commentary on atom-atom bonds in crystals by Jack Dunitz. Here he discusses that age-old question (to chemists), what is a bond? Even almost 100 years after Gilbert Lewis' famous analysis, we continue to ponder this\u2026","rel":"","context":"In "Chemical IT"","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"OO-query","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/07\/OO-query.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10448,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10448","url_meta":{"origin":17805,"position":5},"title":"The \u03c0-complex theory of metal-alkene compounds.","author":"Henry Rzepa","date":"May 13, 2013","format":false,"excerpt":"The period 1951--1954 was a golden one for structural chemistry; proteins, DNA, Ferrocene (1952) and the one I discuss here, a bonding model for Zeise's salt (3). In\u00a0\"A review of \u03c0 Complex Theory\",\u00a0 Bull. Soc. Chim. Fr., 1951, 1 8 , C79 (it is not online) M. J. S.\u00a0Dewar\u00a0sets out\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17805","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=17805"}],"version-history":[{"count":10,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17805\/revisions"}],"predecessor-version":[{"id":17817,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17805\/revisions\/17817"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=17805"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=17805"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=17805"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=17805"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}