Molecules of the year? Pnictogen chains and 16 coordinate Cs.

I am completing my survey of the vote for molecule of the year candidates, which this year seems focused on chemical records of one type or another.

The first article[cite]10.1002/chem.201601916[/cite] reports striving towards creating a molecule covering a complete column of the period table. In this case, group 7, containing N, P, As, Sb, Bi and Mc. Only the first four of these were incorporated, although the prospects of extending this to five seem good (and to six extremely unlikely).  The structure of this pnictogen chain is referenced here: DOI: 10.5517/CCDC.CSD.CC1LHPJ9 and I have demurred from a calculation.

The second article[cite]10.1021/jacs.6b02590[/cite] relates to what might be called hypercoordination, and the achievement of what is felt is a maximum value of 16 to a single metal. I thought I might approach this one by searching the Cambridge structure database (CSD) by specifying any metal with a coordination number 16 as the search query. However, I was foiled in this query because the search software (Conquest) allows a maximum value of only 15! So instead I list the total number of hits retrieved for coordination numbers of 10-15: 25224, 4753, 8856, 2492, 839, 348 respectively.  

These totals have to be taken with some caution; the coordination number of what may often be very weak interactions may be often determined by human chemical perception rather than hard and fast rules. Nevertheless, the assignment of 348 molecules to having a coordination number of 15 is still a remarkably high number. If I can persuade CCDC to allow searches with 16, who knows what other candidates might emerge to rival this one, DOI: CCDC.CSD.CC1KFCQ2

The final candidate[cite]10.1039/C6SC01726F[/cite] is the only one where no measured coordinates are reported, with the title “Preparation of an ion with the highest calculated proton affinity: ortho-diethynylbenzene dianion”. There high level theoretical and computational modelling is reported to which I cannot add anything useful.

The common theme emerging of my review is that most of the candidates have crystal structures to which I have been able to occasionally add some computed quantum mechanical properties to try to tease out some other aspects of their character. It is also nice to be able to cite a persistent identifier (DOI) that leads directly to the 3D coordinates for the structures. My first ever post to this blog in 2008 addressed one solution on how such immediacy might be achieved and it is nice to see this now as a mainstream aspect of chemical publishing.

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2 Responses to “Molecules of the year? Pnictogen chains and 16 coordinate Cs.”

  1. Shant Shahbazian says:

    I am a bit perplexed with what you mean by “coordination” number. If there is no need to be “chemical/covalent” bonds between the central atoms and the neighbor atoms then the concept of coordination number is simply something “geometrical” without necessarily a chemical meaning. Then, why bothering at all about the large coordination numbers? Maybe I am missing something here but I personally think that the large electron sharing capacity of the central atom is making the large coordination number attractive and worthwhile to be considered theoretically. If the problem is simply “geometrical”, then I guess large atoms from 6 and 7 row of the PT which are surrounded by hydrogen atoms and under hydrostatic pressures are the best candidates for large coordination numbers.

  2. Henry Rzepa says:

    The terms “coordination” and “bonds” often diffuse into each other. Thus in the C&EN article highlighting this paper, both terms are used, as in “a central cesium atom is coordinated by an unprecedented 16 fluorine atoms” followed by “allowed the researchers to go beyond 12 bonds in a complex for the first time” and “It also was the first time scientists achieved 16 bonds to one metal“. Often the term “valency” and “bond” are also diffused, resulting in e.g. “hypercoordination” vs “hypervalency”. It can get rather messy, and in a sense arguments around what the precise meaning of any of these terms can go round in circles.

    I did look up the approximate ionic radii of Cs and F, and it seems an “ionic bond” between the two has a distance of ~3.0Å. Some of the distances in the reported structure can reach 3.6Å, i.e. not all the Cs…F “interactions” are equal. So rather than describe all 16 of them as bonds, the term coordination is perhaps rather less loaded, but still quite semantically lossy.

    This semantic fog is often thickened by the use of terms such as “hydrogen bond“, which itself has a spectrum of lengths ranging from longer forms which are probably better described as strong dispersion interactions to shorter forms that are clear covalent bonds in their own right. Whole books have been written on these topics of course.

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