{"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":"2009-12-30T22:45:07","modified_gmt":"2009-12-30T21:45:07","slug":"menage-a-dois-non-classical-sc-bonds","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/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,"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}},"categories":[5,6],"tags":[32,58,1525,198,1526],"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"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-lJ","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1347","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1347"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1347\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1347"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1347"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1347"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}