Dalton Transactions’ sleeping beauty and the power of tau

Dalton Transactions has been a home for high quality inorganic, organometallic and bioinorganic chemistry research since the late 1960s. One paper in particular has received resounding attention from the community and is one of the most highly cited inorganic papers of all time.

In 1984, ‘Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane]copper(II) perchlorate’ was published in the Journal of the Chemical Society, Dalton Transactions. This work was the result of a collaboration between the Addison lab at Drexel University, USA, and the Reedijk lab at Leiden University, the Netherlands. It outlines a model for the active site of the Type-1 copper protein azurin, and was the first publication to introduce the τ (tau) parameter as a structural descriptor for 5-coordinate compounds.

τ (tau) symbol
5-coordinate compound

Early citations of the work were related to copper proteins and models for them, and it was used for electron paramagnetic resonance (EPR) vs. structure correlation. However, it was not until the early 1990s that this ‘sleeping beauty’, a term coined1 for papers whose importance lies dormant for many years before they are recognized and citations start to rise, was awoken. The τ parameter, now also known as the geometry index or structural parameter, is what led to the paper’s rise in popularity. Researchers began to adopt the parameter to determine the coordination center geometry of a given molecule of interest. It has a value between 0 and 1, and when expressed in the extremes, this indicates that a molecule will either be square pyramidal or trigonal bipyramidal, respectively. The team later also developed the ‘disphenoidality’ parameter (φt) for four-coordination.

With over five and a half thousand citations to date, and a continued steady rise of approximately 10 citations a week, the significance and appreciation for this parameter is clearly shown by the community. When describing his work, Addison explains his motivation for inventing the τ -parameter “I simply couldn’t figure out how to compare differently ‘irregular’ pentacoordinate centres easily using metrics such as the Muetterties/Guggenberger2 parameter sets” and adds “I suppose this has become our “iPod” paper – the thing that people wanted, but didn’t appreciate beforehand how much they wanted it!  But a difference is that Steve Jobs actually realised in advance that people would find the iPod useful”. Addison goes on to conclude “I think it provides a demonstration of the utility of simplicity for helping in understanding otherwise complicated ideas.”

Dalton Transactions is proud to be the home of this pioneering work and eagerly awaits the break in slumber of today’s sleeping beauties.


1. Q. Ke, E. Ferrara, F. Radicchi, & A. Flammini,  Proc. Natl Acad. Sci., 2015, 112 (24), 7426-7431, DOI10.1073/pnas.1424329112

2. E.L. Muetterties & L.J. Guggenberger, J. Am. Chem. Soc., 1974, 96 (6), 1748-1756, DOI: 10.1021/ja00813a017

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