{"id":19133,"date":"2017-11-28T21:25:42","date_gmt":"2017-11-28T21:25:42","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=19133"},"modified":"2017-11-28T21:25:42","modified_gmt":"2017-11-28T21:25:42","slug":"octet-expansion-and-hypervalence-in-dimethylidyne-%ce%bb6-sulfane","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=19133","title":{"rendered":"Octet expansion and hypervalence in dimethylidyne-\u03bb6-sulfane."},"content":{"rendered":"
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I started this story by looking at octet expansion and hypervalence in non-polar hypercoordinate species such as S(-CH3<\/sub>)6<\/sub>, then moved on to S(=CH2<\/sub>)3<\/sub>. Finally now its the turn of S(\u2261CH)2<\/sub>.\u2021<\/sup><\/p>\n

\"\"<\/a><\/p>\n

As the triple bonds imply, this seems to represent twelve shared valence electrons surround the sulfur, six from S itself and three from each carbon. The octet is clearly expanded from eight to twelve. But is all as it seems?<\/p>\n

The linear form reveals the following localized orbitals. Six NBOs are localized to the S-C regions, of which four are bonding, two \u03c3 and two\u00a0\u03c0. The remaining four electrons are in two non-bonding lone pairs, with a mild anti-bonding S-C component. So the bond order comes out as ~four, not six! This corresponds to the story told in the earlier blogs that the electrons in excess of the octet tend to occupy either non or antibonding orbitals.<\/p>\n

\"\"<\/p>\n

In fact the full NBO analysis gives a value of\u00a04.0920 for the S bond index and little Rydberg character; S: [core]3S(1.02)3p(3.61)3d(0.13).<\/p>\n

Next, the ELF analysis, based not on orbitals but the derived electron densities. Each S-C region shows an ELF circular attractor integrating to 5.44e (or 10.88e for the S valence region). So the ELF reflects not only the density arising from bonding orbitals, but the non-bonding ones as well!\u00a0<\/p>\n

\"\"<\/p>\n

Take a look at the ELF basin for the two hydrogen atoms; at 2.42e each this shell is ALSO expanded from the normal 2! Apart from the normal C-H localised NBO orbital, one can also see small C-H bonding contributions from the four NBOs labelled B above as well. So ELF analysis of the shared electrons in this species seems to show octet expansion for S and similar shell expansion for H. But we now know that simply taking the ELF basin population and dividing by two to get the bond or valence index can be misleading. The ELF analysis includes non or even anti-bonding density contributions and so it cannot be used to infer hyperbonding (hypervalence).<\/p>\n

I must now confess to withholding some vital information from you. The linear HC\u2261S\u2261CH molecule is not a minimum, having four computed negative force constants, the normal mode of one of which is animated \u00a0below.\u00a0<\/p>\n

\"\"<\/p>\n

The true minimum has C2<\/sub> symmetry as follows and it corresponds to that mysterious structure shown at the top and hitherto not mentioned. This form is 14.6 kcal\/mol lower in free energy than the linear variety.\u00a0\"\"<\/p>\n

The ELF analysis confirms this species as bis(carbene), with two “lone pairs” on S. All the octet expansion has vanished; of the ~six electrons hitherto located in each C-S region, four have morphed into lone pairs, leaving only ~two in the S-C regions. The sulfur is now allocated 7.44e, a \u00a0“normal” octet.<\/p>\n

\"\"<\/p>\n

At this point, I remind that the great G. N. Lewis himself, the original coiner of the eight electron valence rule, pondered whether acetylene might have a related bis(carbene) form<\/a>. It is nice to come up with an example of this more than 100 years after his original suggestion.<\/p>\n

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\u2021<\/sup>FAIR Data DOI for the collection: 10.14469\/hpc\/3333<\/a><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

I started this story by looking at octet expansion and hypervalence in non-polar hypercoordinate species such as S(-CH3)6, then moved on to S(=CH2)3. Finally now its the turn of S(\u2261CH)2.\u2021 As the triple bonds imply, this seems to represent twelve shared valence electrons surround the sulfur, six from S itself and three from each carbon. […]<\/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":[558,5],"tags":[2337,2347,510,164,2364,2371,2375,2327],"class_list":["post-19133","post","type-post","status-publish","format-standard","hentry","category-historical","category-hypervalency","tag-1-decyne","tag-ch2","tag-chemical-bonding","tag-free-energy","tag-g-n-lewis","tag-lewis-structure","tag-music","tag-octet"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-4YB","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/19133","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=19133"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/19133\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=19133"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=19133"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=19133"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}