{"id":1347,"date":"2009-12-30T22:45:07","date_gmt":"2009-12-30T21:45:07","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1347"},"modified":"2010-11-02T10:04:52","modified_gmt":"2010-11-02T09:04:52","slug":"menage-a-dois-non-classical-sc-bonds","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1347","title":{"rendered":"M\u00e9nage \u00e0 deux: Non-classical SC bonds."},"content":{"rendered":"
\n

A previous post<\/a> posed the question; during the transformation of one molecule to another, what is the maximum number of electron pairs that can simultaneously move either to or from any one atom-pair bond as part of the reaction? A rather artificial example (atom-swapping between three nitrosonium cations) was used to illustrate the concept, in which three<\/strong> electron pairs would all<\/strong> move from a triple bond to a region not previously containing any electrons to form new triple bonds and destroy the old. Here is a slightly more realistic example of the phenomenon, illustrated by the (narcisistic) reaction below of a bis(sulfur trifluoride) carbene. Close relatives of this molecule are actually known, with either one SF3<\/sub> of the units replaced by a CF3<\/sub> group or a SF5<\/sub> replacing the SF3<\/sub> (DOI: 10.1021\/ja00290a038<\/a> ).<\/p>\n

\"\"<\/a>

F3SCSF3 and the nature of its S-C bonds<\/p><\/div>\n

The two C-S bonds in this molecule are not the same (and similarly for the CF3<\/sub> analogue), one being long (single), the other short (assumed triple), and the angle subtended at the central carbon is around 150\u00b0 (B3LYP\/cc-pVTZ calculation, DOI: 10042\/to-3643<\/a>). The transition state for interconverting one form to the other would presumably correspond to the concerted movement of two<\/strong> pairs of electrons from one CS region to the other as shown above, not so much a M\u00e9nage \u00e0 trois, as a M\u00e9nage \u00e0 deux! The transition state itself (DOI: 10042\/to-3644<\/a>) has C2<\/sub> symmetry, with a calculated free energy barrier of 31 kcal\/mol and \u03bd 284i<\/em> cm-1<\/sup> for the bond shifting process.<\/p>\n

\"Transition

Transition state for bond equalisation. Click for animation<\/p><\/div>\n

The molecule above does have a further point of interest; one of the sulfur atoms (the triply bonded one) is approximately tetrahedral in coordination, whilst the other has a “T-shape”. An inorganic chemist would describe one sulfur as tetravalent (oxidation state IV), the other as hexavalent (oxidation state VI) and the equilibrium between them a dismutation of the two oxidation states. Does this have any reality? The ELF method has been mentioned a number of times in these posts, and it is applied here to seek an answer. The ELF basin centroids are shown below.<\/p>\n

\"The

ELF basins for F3SCSF3. Click for 3D<\/p><\/div>\n

The integrations are as follows: 14<\/strong> = 2.24 (a single C-S bond), 30<\/strong>=1.66 (an incipient carbene forming, as implied above), 13+15+16<\/strong> = 4.34 (a reasonably persuasive triple bond, comprising, unusually, three separated basins). The fluorines 2<\/strong>, 3<\/strong> and 6<\/strong> all exhibit bonding basins to the S (respectively 2.17, 2.17 and 2.09), but fluorines 1<\/strong>,5<\/strong> and 4<\/strong> do not! Sulfur 8<\/strong> additionally has a lone pair, 29<\/strong>=2.31, but sulfur 9<\/strong> does not. One aspect of this analysis is the nature of the triple bond between S9<\/strong>-C7<\/strong>. Because the three basins are separate, does that mean that the bond cannot rotate about its axis?<\/p>\n

\"\"<\/a>

AIM Analysis of F3SCSF3<\/p><\/div>\n

An alternative AIM analysis is shown above. Now, the CS triple bond is reduced to a single bond critical point (BCP), labelled 10<\/strong>. AIM allows a property known as bond ellipticity to be computed at that BCP. Typically, single and triple bonds have ellipticities close to zero, whilst double bonds have a value of around 0.4 to 0.5. That for point 10<\/strong> is 0.18, which seems to support the ELF analysis above. Pretty unsual bonding it would have to be agreed!<\/p>\n

\"\"<\/a>

ELF centroids for transition state for dismutation.<\/p><\/div>\n

But what of the original question posed at the start in the diagram; do two<\/strong> pairs of electrons move away together from one triple bond to form another. A further ELF analysis at the transition state for this process reveals that in effect the two pairs do different things. One localizes onto the carbon, to form a proper carbene, the other becomes a sulfur lone pair. So the valence dismutation involves three pairs of electrons, not two as shown at the start, with each pair doing its own thing.<\/p>\n

\"\"<\/a>

Six-electron model for valence isomerism in F3SCSF3<\/p><\/div>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

A previous post posed the question; during the transformation of one molecule to another, what is the maximum number of electron pairs that can simultaneously move either to or from any one atom-pair bond as part of the reaction? A rather artificial example (atom-swapping between three nitrosonium cations) was used to illustrate the concept, in […]<\/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":[152,169,2649,168,2648],"ppma_author":[2661],"class_list":["post-1347","post","type-post","status-publish","format-standard","hentry","category-hypervalency","category-interesting-chemistry","tag-animation","tag-calculated-free-energy-barrier","tag-hypervalency","tag-inorganic-chemist","tag-interesting-chemistry"],"yoast_head":"\nM\u00e9nage \u00e0 deux: Non-classical SC bonds. - 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=1347\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"M\u00e9nage \u00e0 deux: Non-classical SC bonds. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"A previous post posed the question; during the transformation of one molecule to another, what is the maximum number of electron pairs that can simultaneously move either to or from any one atom-pair bond as part of the reaction? A rather artificial example (atom-swapping between three nitrosonium cations) was used to illustrate the concept, in […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1347\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2009-12-30T21:45:07+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2010-11-02T09:04:52+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/12\/SF3.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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"M\u00e9nage \u00e0 deux: Non-classical SC bonds. - 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=1347","og_locale":"en_GB","og_type":"article","og_title":"M\u00e9nage \u00e0 deux: Non-classical SC bonds. - Henry Rzepa's Blog","og_description":"A previous post posed the question; during the transformation of one molecule to another, what is the maximum number of electron pairs that can simultaneously move either to or from any one atom-pair bond as part of the reaction? 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By replacing the HO group by one with greater \u03c3-electron withdrawing propensity, the stereo-electronic effect between\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"The H2BSCH molecule. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/12\/CSBH2.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4479,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4479","url_meta":{"origin":1347,"position":1},"title":"Hexavalent carbon revisited (and undecavalent boron thrown in).","author":"Henry Rzepa","date":"June 26, 2011","format":false,"excerpt":"A little while ago, I speculated (blogs are good for that sort of thing) about hexavalent carbon, and noted how one often needs to make (retrospectively) obvious connections between two different areas of chemistry. That post has attracted a number of comments in the two years its been up, along\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/06\/CB11.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10937,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10937","url_meta":{"origin":1347,"position":2},"title":"VSEPR Theory: A closer look at chlorine trifluoride, ClF3.","author":"Henry Rzepa","date":"July 27, 2013","format":false,"excerpt":"Valence shell electron pair repulsion theory is a simple way of rationalising the shapes of many compounds in which a main group element is surrounded by ligands. ClF3 is a good illustration of this theory. The standard application of VSEPR theory to this molecule is as follows: Central atom: chlorine\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"VSEPR","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/07\/VSEPR.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1183,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1183","url_meta":{"origin":1347,"position":3},"title":"Multi-centre bonding in the Grignard Reagent","author":"Henry Rzepa","date":"December 1, 2009","format":false,"excerpt":"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\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"The crystal structure of a di-aryl magnesium. Click to view 3D","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/12\/udaqiz.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1210,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1210","url_meta":{"origin":1347,"position":4},"title":"The nature of the C\u2261S triple bond","author":"Henry Rzepa","date":"December 1, 2009","format":false,"excerpt":"Steve Bachrach has just blogged on a recent article (DOI: 10.1002\/anie.200903969) claiming the isolation of a compound with a C\u2261S triple bond; Steve notes that Schreiner and co claim a \u201cstructure with a rather strong CS double bond or a weak triple bond\u201d. With this size of molecule, the proverbial\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"A compound with a CS triple bond","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/12\/CSCC.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":15552,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15552","url_meta":{"origin":1347,"position":5},"title":"VSEPR Theory: A closer look at trifluorothionitrile, NSF3.","author":"Henry Rzepa","date":"January 16, 2016","format":false,"excerpt":"The post on applying VSEPR (\"valence shell electron pair repulsion\") theory to the geometry of ClF3\u00a0has proved perennially popular. So here is a follow-up on another little molecue,\u00a0F3SN. As the name implies, it is often represented with an\u00a0S\u2261N bond. Here I take a look at the conventional analysis. This is\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"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\/1347","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=1347"}],"version-history":[{"count":0,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1347\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1347"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1347"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1347"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=1347"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}