Archive for the ‘Hypervalency’ Category
Friday, October 2nd, 2009
The previous post talked about making links or connections. And part of the purpose for presenting this chemistry as a blog is to expose how these connections are made, or or less as it happens in real time (and not the chronologically sanitized version of discovery that most research papers are). So each post represents an evolution or mutation from the previous one. To recapitulate, we have seen how the idea of cyclopentadienyl anion as a ligand for a dipositive carbon atom has evolved. Let us move in yet another direction; the cyclobutadienyl dianion. This ligand has recently been shown to bind Mg2+ (DOI: 10.1002/ejic.200800066), so why not He2+? And picking up again the previous theme, we will then protonate the bound complex. The result now is a monocation, and it has the C4v-symmetric structure shown below (DOI: 10042/to-2438). This bears some resemblance to pyramidane, a neutral C5H4 compound with hemispherical carbon reported in 2001[1] which is also a stable minimum in the potential energy surface.
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References
- J.P. Kenny, K.M. Krueger, J.C. Rienstra-Kiracofe, and H.F. Schaefer, "C<sub>5</sub>H<sub>4</sub>: Pyramidane and Its Low-Lying Isomers", The Journal of Physical Chemistry A, vol. 105, pp. 7745-7750, 2001. https://doi.org/10.1021/jp011642r
Tags:Hypervalency, Interesting chemistry, potential energy surface, pseudo
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Friday, October 2nd, 2009
Science is about making connections. And these can often be made between the most unlikely concepts. Thus in the posts I have made about pentavalent carbon, one can identify a series of conceptual connections. The first, by Matthias Bickelhaupt and co, resulted in the suggestion of a possible frozen SN2 transition state. They used astatine, and this enabled a connection to be made between another good nucleophile/nucleofuge, cyclopentadienyl anion. This too seems to lead to a frozen Sn2 transition state. The cyclopentadienyl theme then asks whether this anion can coordinate a much simpler unit, a C2+ dication (rather than Bickelhaupt’s suggestion of a (NC)3C+ cation/radical) and indeed that complex is also frozen, again with 5-coordinate carbon, and this time with five equal C-C bonds. So here, the perhaps inevitable progression of ideas moves on to examining the properties of this complex, the outcome being a quite counter-intuitive suggestion which moves us into new territory.
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Tags:CHs, Hypervalency, Interesting chemistry, Matthias Bickelhaupt
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Wednesday, September 30th, 2009
In the previous two posts, I noted the recent suggestion of how a stable frozen SN2 transition state might be made. This is characterised by a central carbon with five coordinated ligands. The original suggestion included two astatine atoms as ligands (X=At), but in my post I suggested an alternative which would have five carbon ligands instead (X=cyclopentadienyl anion).
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Tags:Ge, Hypervalency, Interesting chemistry
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Tuesday, September 22nd, 2009
The bimolecular nucleophilic substitution reaction at saturated carbon is an icon of organic chemistry, and is better known by its mechanistic label, SN2. It is normally a slow reaction, with half lives often measured in hours. This implies a significant barrier to reaction (~15-20 kcal/mol) for the transition state, shown below (X is normally both a good nucleophile and a good nucleofuge/leaving group, such as halide, cyanide, etc. Y can have a wide variety of forms).
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Tags:ACS, computational chemist, Ethan L. Fisher, Hypervalency, Interesting chemistry, potential systems, Salt Lake City, Steve Bachrach, Tristan H. Lambert, United States
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