Ever since the discovery of the first homoleptic metal carbonyl complex Ni(CO)4 over 130 years ago, chemists and physicists of many different fields laid their focus on the understanding and application of this peculiar family of compounds: a molecule of carbon monoxide, covalently bound to a metal center in mostly unusual oxidation states. Why does Ni(CO)4 not spontaneously decompose into Ni-metal and four molecules of very stable CO but rather forms a liquid compound that can even be distilled?
Today, carbon monoxide proves to be one of the most important ligands in transition metal chemistry, with more than 25000 publications (search topic “metal carbonyl” in Web of Science database) so far and an average of about 800 publications a year in the past decades.
In order for a better understanding of the nature of what keeps molecules together, it is in the focus of our group to push and challenge these boundaries. Inspired by observations and discoveries in the gas phase, we try to gain access to and stabilize highly reactive cations as salts of weakly coordinating anions (WCAs) in the condensed phase. At best, these species are even stable at ambient temperature under inert conditions. Therefore, it is possible to fully characterize their properties and look into their chemical behavior. Our approach of using (more or less) regular standard laboratory equipment and Schlenk-techniques – avoiding extremely corrosive superacidic systems like HSO3F/SbF5 – allows a wide scientific community to contribute to the understanding of these fundamentally new compounds.
Homoleptic transition metal carbonyl complexes are the most simple and fundamental of their kind and their cations are even smaller in number than the neutral or anionic analogs. This is mostly due to the fact, that an electron poor metal center tends to keep its valence electrons and thus contributes less to no π backbonding to the metal-CO bond, resulting in labile or unstable complexes. As a result, Mn was deemed to be lightest of the 3d-metals to form a stable carbonyl cation in form of the 18 electron species [Mn(CO)6]+.
Overview of isolated homoleptic carbonyl cations (green) and our work (yellow).
However, the right combination of a strong oxidant with a WCA allowed the oxidation of Cr(CO)6 and isolation of the room-temperature stable radical salt [Cr(CO)6]•+[WCA]–. Usually though, the oxidant [NO]+ leads to an inevitable coordination of the liberated NO(g)/(solv) to the metal centre. With the suitable reaction conditions, this coordination can be suppressed or promoted, yielding either [Cr(CO)6]•+ or [Cr(CO)5(NO)]+ selectively.
Via this relatively simple method, [Cr(CO)6]•+ as the parent compound of the formerly unknown family of homoleptic metal carbonyl radical cations is accessible on a multi-gram scale. The title compound shows surprising stability and is closely related to the isoelectronic and isostructural V(CO)6 with the same D3d symmetric ground state, which is supported by experimental IR, Raman, and EPR spectroscopic investigations, as well as magnetic measurements and calculated DFT/full ab initio data.
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