First Example Of A Main Group Metal Carbonyl Complex Synthesized

In a study published in Science, Oxford chemists explain how they have prepared the first isolable complex containing the carbon monoxide molecule bonded to a main group metal. Such compounds are ubiquitous for transition metals – with the first examples having been reported over 150 years ago – but until now have remained elusive for the metals of the main group of the Periodic Table. The compound the Oxford team have synthesised, a carbonyl complex of tin, is stable close to room temperature allowing its geometric structure to be confirmed by X-ray crystallography.

Carbon monoxide (CO) is a critical feedstock for a wide range of industrial processes, and its interaction with transition metals is of primary importance to large scale purification processes (such as the Mond Process for nickel extraction) and to the catalytic production of a wide range of commodity chemicals (e.g. carboxylic acids/esters, aldehydes and hydrocarbon fuels by the Monsanto/Cativa, hydroformylation and Fischer-Tropsch processes). In contrast to transition elements, s- and p-block metal compounds that coordinate CO under ambient conditions had - until now - long remained elusive, with only a few examples being reported even at cryogenic temperatures.

Dr Maximilian Dietz, the lead author of the study, says: “Tin is usually regarded as a typical main group metal with well-defined and predictable chemistry. But we can actually show that, by fine-tuning the ligand environment, we can generate an exceptionally reactive stannylene, in which the tin centre mimics the behaviour of a transition metal. The bonding motifs we observe in the isolated tin carbonyl and carbene complexes remind us of classical transition metal chemistry. This is a striking example of how low-valent main group elements can still surprise us and expand the boundaries of chemical bonding”.

The team have shown that the coordination of CO to tin can be understood in terms of models of chemical bonding previously developed for transition metal systems, and that the reactivity of the carbon monoxide fragment at higher temperatures also mimics classical d-block chemistry. The Oxford team now hopes to further investigate the fundamental ways in which carbon monoxide and other molecules such as alkenes and hydrogen, interact more generally with main group metal compounds. Main group metals are often Earth abundant and environmentally relatively benign, ultimately making them attractive targets for future developments in catalysis.

Simon Aldridge FRS, Professor of Main Group Chemistry, says: “Carbon monoxide is a key small molecule involved in a wide range of industrial processes, and its interaction with transition metals is something that every chemistry undergraduate learns about. Extending these familiar themes to main group metal chemistry is a molecular design challenge – and one that helps us learn more about the fundamental reactive capabilities of elements from an under-exploited part of the Periodic Table”.