75 years after the discovery of hydroformylation, cobalt catalysts are now undergoing a renaissance in hydrogenation reactions. We have evaluated arene metalates in which the low-valent metal species is--conceptually different from heteroatom-based ligands--stabilized by π coordination to hydrocarbons. Potassium bis(anthracene)cobaltate 1 and -ferrate 2 can be viewed as synthetic precursors of quasi-"naked" anionic metal species; their aggregation is effectively impeded by (labile) coordination to the various π acceptors present in the hydrogenation reactions of unsaturated molecules (alkenes, arenes, carbonyl compounds). Kinetic studies, NMR spectroscopy, and poisoning studies of alkene hydrogenations support the formation of a homogeneous catalyst derived from 1 which is stabilized by the coordination of alkenes. This catalyst concept complements the use of complexes with heteroatom donor ligands for reductive processes.

Iron(0) nanoparticles in ionic liquids (ILs) have been shown to catalyse the semi-hydrogenation of alkynes. In the presence of a nitrile-functionalised IL or acetonitrile, stereoselective formation of (Z)-alkenes was observed. The biphasic solvent system allowed facile separation and re-use of the catalyst.

On the move: A highly practical olefin isomerization protocol under iron catalysis has been developed. The standard conditions involve reaction of the olefin at room temperature in THF with a simple pre‐catalyst system. Allylbenzenes give propenylbenzenes; 1‐alkenes can be selectively converted to 2‐alkenes; isolated dienes give conjugated dienes; (Z)‐olefins give (E)‐olefins. The synthesis of propenyl‐benzenes has also been realized by an iron‐catalyzed allylation‐isomerization domino reaction in good yields.

The scope and mechanism of a practical protocol for the iron-catalyzed hydrogenation of alkenes and alkynes at 1 bar H2 pressure were studied.

Ironing it out: The simple pre‐catalyst system FeCl3/EtMgCl was applied to the hydrogenation of various alkenes and alkynes at room temperature. Domino iron‐catalyzed allylation/hydrogenation reactions were performed under 1–4 bar H2. Spectroscopic studies determined the heterogeneous nature of the active catalyst species, its oxidation state (Fe0), and the local structure and size of the formed particles.