All iNCSlusive

Figure: Periodic table of thiocyanates, showing the state of our knowledge

There are 118 elements and we are now confident that there will be no more stable elements discovered. Equally, we’re fairly confident that there won’t be many (if any) binary compounds made with stable elements. You might assume that this settled state of knowledge is correct for other simple compounds; however, this isn’t true.

In this review, I summarise what is known about the solid-state chemistry of the simple anionic molecule thiocyanate bound to metals. Thiocyanate is a really ubiquitous anion, and it’s even widely used in school chemistry demonstrations as it can be used to make quite convincing fake blood with iron(III), and also for the spectacular ‘Pharoah’s serpent’ decomposition of mercury thiocyanate (which is perhaps less common nowadays due to the toxicity of mercury!). Despite this, we only know the structures of the thiocyanate compounds of less than 40% of stable metal, and about a quarter of metals have never been reported to form a binary thiocyanate compound at all. This paper aims to be a useful resource for chemists investigating thiocyanate chemistry, and perhaps for those interested in finding new materials too.

This was an interesting paper to write: although I have previously done a lot of work on thiocyanate compounds, a comprehensive review like this requires deeper reading. I read papers both more than 200 years old and published last year; written in English, German, Russian, Polish, Japanese and Chinese (thank you Google Translate!) and from journals published in eleven different countries. I enjoyed doing this work, and as a result I gained a broader perspective on thiocyanates: I read papers on cocaine, solar cells and mineralogy. It was also interesting to see a broader range of older publications than the star papers which remain ubiquitous in contemporary citation lists. These papers included many complex crystal structures, determined with great precision before everyone had the power in our pockets to brute-force the ribosome; but equally, there were a handful of structures which were obviously rubbish. The syntheses were also of variable quality: the preparation of Fe[Pt(SCN)₆] from 1855 worked a charm (once the atomic masses were corrected for the fact that nitrogen is a dimer); but other preparations report anhydrous binary thiocyanates with clear X-H bonds in the infrared spectra.

There were some challenges associated with reviewing this wider range of literature. The digital native assumption that all the world’s knowledge is at your fingertips is wrong: even at a research intensive western university I struggled to locate many of the relevant papers. I had to rely on colleagues, colleagues’ colleagues and kind souls from Twitter to get my hands on some articles. Digitization of older (pre-2000) chemical texts is patchy outside of the current big publishers: even finding seminal books by Liebig was hard; and finding papers from defunct European journals online is impossible. I relied on the fact that university libraries still retain older journals as hard copies, even if they are not consulted every day: the University of Nottingham has a complete run of the Russian Journal of Inorganic Chemistry (complete with photographs of Lenin) which was invaluable; and the University of Cambridge has the accounts of the Bulgarian Academy of Sciences. Other books I needed to buy second-hand: including the key text: Golub, Köhler & Skopenko. This digital gap means that despite the increasing focus on data caused by machine learning, there’s a lot of hard-won historic data sitting in undigitized stacks that is potentially at risk of being lost.

Searching for these older papers was also trickier: the ICSD was invaluable as a source of crystal structures and their database is comprehensive for well characterised materials, as was Google Scholar. Other chemical databases, such as Scifinder and Reaxys were of some use, but required careful investigation as they are not as optimised for inorganic materials. The existing references from previous reviews were also very important, as much of the Soviet literature was not well indexed otherwise, and I should also thank the reviewers, who was found reference to the rare earth thiocyanates in a journal published by Tomsk State University that as far as I can tell has not been cited in the chemical literature before. I also looked in some of the large databases of predicted structures. These, surprisingly, did not provide much extra value: perhaps because thiocyanates are relatively rare in source databases or because thiocyanate compounds are not necessarily stable with respect to the elements. Computational materials discovery still seems to have a blind spot for molecular inorganic materials and so while Google/Microsoft/Facebook/Tencent improve their algorithms, literature on paper remains our best guide!