The cover image refers to research of HIMS chemists in cooperation with scientists from ETH Zürich on the catalytic activity of a ruthenium diolefin diazadiene (trop2dad) complex. Referring to the possible oxidation states of the complex and paraphrasing the iconic movie title, the chemists present 'The Innocent, the Guilty and the Suspicious'.

Poorly defined oxidation states

As is the case with the trop2dad ligand and ruthenium, certain redox-active organic ligands display a redox-potential that is in the same range as that of transition metal they are bound to. In that case oxidation states can become poorly defined, as both the metal and ligand can get oxidized or reduced in a chemical reaction. As a result, for such systems the 'real' oxidation state of the metal differs from the state predicted by common rules of formal oxidation state counting. For this reason, such ligands are often called to be 'non-innocent'.

Interestingly, non-innocent ligands not only give rise to intriguing electronic structures, but can also trigger unexpected ligand-centered reactivity. The trop2dad molecule is an example pur sang of this. Several resonance structures contribute to the electronic structure of [Ru(trop2dad)], which shows both ligand- and metal-centered reactivity.

Furthermore, reaction-profile calculations as presented in the ChemEurJ publication show that trop2dad ligand radical contributions give rise to lower barriers in reactions involving the 'dad' moiety with open-shell singlet character. Hence, the 'suspicious' trop2dad ligand is neither fully 'innocent' nor completely 'guilty'.

Wanted: dead or alive!

To the researchers this situation brought to mind the famous Spaghetti-Western, wherein the conception of 'good guys' and 'bad guys' becomes fully diffuse along the movie plot. They therefore decided to design a Spagheti-Western-style cover referring to their article. It is a wink to “Il buono, il brutto, il cattivo” ("The good, the bad and the ugly"), one of the greatest and most influential westerns of all times.

Article:

V. Sinha, B. Pribanic, B. de Bruin, M. Trincado, H. Grützmacher: Ligand- and Metal-Based Reactivity of a Neutral Ruthenium Diolefin Diazadiene Complex: The Innocent, the Guilty and the Suspicious. Chemistry, A European Journal, DOI: 10.1002/chem.201705957

Cover explanation: DOI: 10.1002/chem.201800439