The first hexa-valent nickel complex
Pushing the limits of transition metal bonding
In a study published today in the Journal of the American Chemical Society, chemists from the University of Oxford report on the first stable complex of hexa-valent nickel, demonstrating that this metal can form six covalent bonds at once. The molecule synthesised, Ni(BeCp)6, has been unambiguously established using X-ray crystallography experiments and a range of other spectroscopic measurements and computational techniques.
Prior to this new report, nickel was only known to form up to four covalent bonds at once, and even this tetra-valent state remains rare and tricky to access. Nonetheless, high-valent nickel is important; tri- and tetra-valent nickel complexes are believed to be key intermediates in industrially relevant nickel-catalysed reactions, while access to the tri-valent state of nickel is vital to the function of a number of metalloenzymes in nature, including hydrogenases.
The high abundance of nickel in the Earth’s crust makes it an attractive alternative to scarce and expensive precious metals like palladium and platinum, which are central to numerous industrial catalytic processes.
Dr Josef Boronski, a Junior Research Fellow at St John’s College who led the study, said:
Nickel is an element that chemists think they know very well. This work shows that we still have much to learn about even the most familiar members of the Periodic Table.
In order to unlock the full chemical potential of nickel, a comprehensive knowledge of the range of accessible valence states is crucial. In this study, the team believe that they have synthesized a ‘textbook’ complex, which will feature in undergraduate chemistry courses in coming years.
Prof Simon Aldridge FRS from the Department of Chemistry said:
This is a molecule that challenges conventional models of chemical bonding and preconceptions about the concepts of valence and oxidation state.
As part of the study, the researchers conducted an in-depth analysis of the electronic structure of the complex. This showed that the molecule adopts an unusual geometry at nickel (C3v, rather than the octahedral geometry which is typical of six-coordinate transition metals), which lowers its overall energy. Furthermore, the researchers find evidence for aromaticity in the complex, which may further help to stabilise it.
Read the full report in the Journal of the American Chemical Society.
