Archive for the ‘Hypervalency’ Category
Saturday, January 23rd, 2010
So ingrained is the habit to think of a bond as a simple straight line connecting two atoms, that we rarely ask ourselves if they are bent, and if so, by how much (and indeed, does it matter?). Well Hursthouse, Malik, and Sales, as long ago as 1978, asked just such a question about the unlikeliest of bonds, a quadruple Cr-Cr bond, found in the compound di-μ-trimethylsilylmethyl-bis-[(tri-methylphosphine) (trimethylsilylmethyI)chromium(II)[1]. They arrived at this conclusion by looking very carefully at how the overlaps with the Cr d-orbitals might be achieved.
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References
- M.B. Hursthouse, K.M.A. Malik, and K.D. Sales, "Crystal and molecular structure of di-µ-trimethylsilylmethyl-bis[(tri-methylphosphine)(trimethylsilylmethyl)chromium(
<scp>II</scp>
)](4 Cr–Cr)", J. Chem. Soc., Dalton Trans., pp. 1314-1318, 1978. https://doi.org/10.1039/dt9780001314
Tags:bond, Hypervalency, Interesting chemistry, quadruple
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Friday, January 1st, 2010
In the previous post, the molecule F3S-C≡SF3 was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal/mol, ν 284i cm-1) to interconversion of the two valence-bond forms. So an interesting extension of this phenomenon is shown below:
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Tags:aromaticity, Hypervalency, Interesting chemistry, pericyclic, South Carolina
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Wednesday, December 30th, 2009
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 which three electron pairs would all 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 of the units replaced by a CF3 group or a SF5 replacing the SF3[1]
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References
- B. Poetter, K. Seppelt, A. Simon, E.M. Peters, and B. Hettich, "Trifluoroethylidynesulfur trifluoride, CF3C.tplbond.SF3, and its dimer", Journal of the American Chemical Society, vol. 107, pp. 980-985, 1985. https://doi.org/10.1021/ja00290a038
Tags:animation, calculated free energy barrier, Hypervalency, inorganic chemist, Interesting chemistry
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Sunday, December 6th, 2009
In the previous two posts, a strategy for tuning the nature of the CS bond in the molecule HO-S≡C-H was developed, based largely on the lone pair of electrons identified on the carbon atom. By replacing the HO group by one with greater σ-electron withdrawing propensity, the stereo-electronic effect between the O-S bond and the carbon lone pair was enhanced, and in the process, the SC bond was strengthened. It is time to do a control experiment in the other direction. Now, the HO-S group is replaced by a H2B-S group. The B-S bond, boron being very much less electronegative than oxygen, should be a very poor σ-acceptor. In addition, whereas oxygen was a π-electron donor (acting to strengthen the S=C region), boron is a π-acceptor, and will also act in the opposite direction. So now, this group should serve to weaken the S-C bond.
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Tags:Hypervalency, Interesting chemistry
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Saturday, December 5th, 2009
In my first post on this theme, an ELF (Electron localization function) analysis of the bonding in the molecule HO-S≡C-H [1] was presented. This analysis identified a lone pair of electrons localized on the carbon (integrating in fact to almost exactly 2.0) in addition to electrons in the CC region. This picture seems to indicate that the triple bond splits into two well defined regions of electron density (synaptic basins). In a comment to this post, I also pointed out that an NBO analysis showed a large interaction energy between the carbon lone pair and the S-O σ* orbital, characteristic of anomeric effects (in eg sugars). This latter observation gives us a handle on possibly tweaking the effect. Thus if the S-O σ* orbital can be made a better electron acceptor, then its interaction with the lone pair could be enhanced.
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References
- P. Schreiner, H. Reisenauer, J. Romanski, and G. Mloston, "A Formal Carbon–Sulfur Triple Bond: HCSOH", Angewandte Chemie International Edition, vol. 48, pp. 8133-8136, 2009. https://doi.org/10.1002/anie.200903969
Tags:Hypervalency, Interesting chemistry, large interaction energy
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Tuesday, December 1st, 2009
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:
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Tags:Hypervalency, Interesting chemistry, metal, metal atom, metal centres, Mg atom
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Monday, October 5th, 2009
We have seen in the series of posts on the topic of hypervalency how the first row main group elements such as Be, B, C and N can sustain apparent hypercoordination and arguably hypervalency. The latter is defined not so much by expanding the total valence shell of electrons surrounding the hypervalent atom beyond eight, but in having more than four well defined bonds to it, as quantified by AIM and ELF analysis. The previous post made the suggestion of how a compound involving hypervalent boron could also sustain a genuine bond to the rare gas helium. It is surely time to seek evidence that this type of bonding can be sustained in reality. Fortunately, a crystal structure of a reasonably analogous compound IS available (DOI: 10.1016/0022-328X(94)05089-T).
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Tags:Hypervalency, rare gas helium, YOCVIV
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Saturday, October 3rd, 2009
Quite a few threads have developed in this series of posts, and following each leads in rather different directions. In this previous post the comment was made that coordinating a carbon dication to the face of a cyclopentadienyl anion resulted in a monocation which had a remarkably high proton affinity. So it is a simple progression to ask whether these systems may in turn harbour a large affinity for binding not so much a H+ as the next homologue He2+?
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Tags:AIM, bonding, Cambridge, chemical bonds, double-hybrid, ELF, gas phase, Hypervalency, hypervalent, Interesting chemistry, Krapp and co
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Saturday, October 3rd, 2009
The previous posts have seen how a molecule containing a hypervalent carbon atom can be designed by making a series of logical chemical connections. Another logical step is to investigate whether the adjacent atoms in the periodic table may exhibit similar effects (C2+ ≡ B+ ≡ N3+ ≡ Be ≡ O4+). So here are reported some results (B3LYP/6-311G(d,p) ) for boron, beryllium and nitrogen, for the general tetramethyl substituted system shown below
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Tags:Hypervalency, Interesting chemistry, logical chemical connections
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