{"id":15552,"date":"2016-01-16T16:50:37","date_gmt":"2016-01-16T16:50:37","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=15552"},"modified":"2016-01-16T16:50:37","modified_gmt":"2016-01-16T16:50:37","slug":"vsepr-theory-a-closer-look-at-trifluorothionitrile-nsf3","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2016\/01\/16\/vsepr-theory-a-closer-look-at-trifluorothionitrile-nsf3\/","title":{"rendered":"VSEPR Theory: A closer look at trifluorothionitrile, NSF3."},"content":{"rendered":"

The post on applying<\/a> VSEPR ("valence shell electron pair repulsion") theory to the geometry of ClF3<\/sub> has proved perennially popular. So here is a follow-up on another little molecue, F3<\/sub>SN<\/a>. As the name implies, it is often represented with an S≡N bond. Here I take a look at the conventional analysis.<\/p>\n

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

This<\/a> is as follows:<\/p>\n

    \n
  1. \n Six valence electrons on the central S atom.\n <\/li>\n
  2. \n Three F atoms contribute one electron each.\n <\/li>\n
  3. \n One electron from the N σ-bond.\n <\/li>\n
  4. \n Donate two electrons from S to the two π-bonds.\n <\/li>\n
  5. \n Eight electrons left around central S, ≡ four valence shell electron pairs.\n <\/li>\n
  6. \n Hence a tetrahedral<\/strong> geometry.\n <\/li>\n
  7. \n The bond-bond repulsions however are not all equal. The SN bond repels the three SF bonds more than the S-F bonds repel each-other.\n <\/li>\n
  8. \n Hence the N-S-F angle is greater than the F-S-F angle, a distorted tetrahedron.\n <\/li>\n<\/ol>\n

    Now for a calculation[cite]10.14469\/ch\/191808[\/cite];  ωB97XD\/Def2-TZVP, where the wavefunction is analysed using ELF (electron localisation function), which is a useful way of locating the centroids of bonds and lone pairs (click on diagram below to see 3D model).<\/p>\n

    \"Trifluorosulfonitrile\"<\/p>\n

      \n
    • \n At the outset one notes that there are six<\/strong> ELF disynaptic basins surrounding the central S, integrating to a total of 7.05e. The sulfur is NOT<\/strong> hypervalent; it does not exceed the octet rule.\n <\/li>\n
    • \n These six "electron sub-pair<\/em>" basins are arranged octahedrally<\/strong> around the sulfur. The coordination is NOT tetrahedral, as implied above.\n <\/li>\n
    • \n The three S-N basins have slightly more electrons (1.25e) than the three S-F basins (1.10e), resulting in …\n <\/li>\n
    • \n the angle subtended at the S for the SN basins being 96° (a bit larger than octahedral) whilst the angle subtended at the S for the SF basins being smaller (89.9°). This matches point 7<\/strong> above, but is achieved in an entirely different manner.\n <\/li>\n
    • \n As a result, the N-S-F angle (122.5°) is larger than the ideal tetrahedral angle and the F-S-F angle (93.9°) is smaller, an alternative way of expressing point 7<\/strong> above.\n <\/li>\n
    • \n The S≡N triple bond as shown above does have some reality;  it is a "banana bond<\/em>" with three connectors rather than two. Each banana bond however has only 1.25e, so the bond order of this motif is ~four (not six) but nevertheless resulting in a short S-N distance (1.406Å) with multiple character.\n <\/li>\n<\/ul>\n

      So we have achieved the same result as classical VSEPR, but using partial rather than full electron pairs to do so. We got the same result with ClF3<\/sub> before. So perhaps this variation could be called "valence shell partial electron pair repulsions<\/em>" or VSPEPR<\/strong>.<\/p>\n","protected":false},"excerpt":{"rendered":"

      The post on applying VSEPR ("valence shell electron pair repulsion") theory to the geometry of ClF3 has proved perennially popular. So here is a follow-up on another little molecue, F3SN. As the name implies, it is often represented with an S≡N bond. Here I take a look at the conventional analysis. This is as follows: Six valence electrons […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[10],"tags":[423,425,872,1472,1667,1786,2037,2320,2403,2427,2533,2554],"class_list":["post-15552","post","type-post","status-publish","format-standard","hentry","category-hypervalency","tag-chemical-bond","tag-chemical-bonding","tag-electron","tag-lone-pair","tag-molecular-geometry","tag-octet-rule","tag-quantum-chemistry","tag-stereochemistry","tag-tetrahedral-molecular-geometry","tag-theoretical-chemistry","tag-valence","tag-vsepr-theory"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/15552","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=15552"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/15552\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=15552"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=15552"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=15552"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}