The previous post demonstrated the simple iso-electronic progression from six-coordinate carbon to five coordinate nitrogen. Here, a further progression to oxygen is investigated computationally. The systems are formally constructed from a cyclobutadienyl di-anion and firstly the HO ⁵⁺ cation, giving a tri-cationic complex. There are no examples of the resulting motif in the Cambridge structure database.

A few years back I followed a train of thought here which ended with hexacoordinate carbon, then a hypothesis rather than a demonstrated reality. That reality was recently confirmed via a crystal structure, DOI:10.5517/CCDC.CSD.CC1M71QM[cite]10.1002/anie.201608795[/cite]. Here is a similar proposal for penta-coordinate nitrogen. First, a search of the CSD (Cambridge structure database) for such nitrogen.

It is not only the non-classical norbornyl cation that has proved controversial in the past. A colleague mentioned at lunch (thanks Paul!) that tri-coordinate group 14 cations such as R ₃ Si ⁺ have also had an interesting history.[cite]10.1021/ja990389u[/cite] Here I take a brief look at some of these systems. Their initial characterisations, as with the carbon analogues, was by ²⁹ Si NMR.

The example a few posts back of how methane might invert its configuration by transposing two hydrogen atoms illustrated the reaction mechanism by locating a transition state and following it down in energy using an intrinsic reaction coordinate (IRC). Here I explore an alternative method based instead on computing a molecular dynamics trajectory (MD). I have used ethane instead of methane, since it is now possible to

The Wikipedia entry on peroxydisulfate is quite short (as of today). But I suspect this article may change things.[cite]10.1038/s41559-017-0083[/cite]. A search of the Cambridge structure database reveals around 18 high quality crystal structures containing this species are known, many as metal salts.

A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. Pyrophoric iron for example is prepared by heating anhydrous iron (II) oxalate in a sealed test tube ( i.e. to 600° or higher). When the tube is broken open and the contents released, a shower of sparks forms. Not all metals do this;

In the previous post, I found intriguing the mechanism by which methane (CH ₄ ) inverts by transposing two of its hydrogens. Here I take a look at silane, SiH ₄ . It appears it is a three-stage process! Firstly, silane eliminates molecular hydrogen to form a molecular complex between H _{2 and SiH 2} (DOI: 10.14469/hpc/2290). The barrier (~60 kcal/mol) is very much lower than with methane.

This is a spin-off from the table I constructed here for further chemical examples of the classical/non-classical norbornyl cation conundrum. One possible entry would include the transition state for inversion of methane via a square planar geometry as compared with e.g. NiH ₄ for which the square planar motif is its minimum.

This is another of those posts that has morphed from an earlier one noting the death of the great chemist George Olah.

George Olah passed away on March 8th. He was part of the generation of scientists in the post-war 1950s who had access to chemical instrumentation that truly revolutionised chemistry.

A few years back, I did a post about the Pirkle reagent[cite]10.1039/c39910000765[/cite] and the unusual π-facial hydrogen bonding structure[cite]10.1039/P29940000703[/cite] it exhibits. For the Pirkle reagent, this bonding manifests as a close contact between the acidic OH hydrogen and the edge of a phenyl ring; the hydrogen bond is off-centre from the middle of the aryl ring.