This study describes the structural investigation of a highly versatile ruthenium-NHC (N-heterocyclic carbene) catalyst complex, which has been established for the asymmetric hydrogenation of various aromatic compounds. A complex containing an unusual doubly deprotonated NHC ligand was isolated and identified as the precatalyst to this complex. When its subsequent reactivity was monitored, two additional precatalysts, featuring partially hydrogenated naphthyl substituents, were characterized spectroscopically. Ligand hydrogenation appears to be a key activation process en route to the active catalyst.

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

Starting from the novel chlorido precursor trans‐[RuCl2(IMe)4] (1, IMe = 1,3,4,5‐tetramethylimidazol‐2‐ylidene), hydridoruthenium complexes trans‐[RuH2(IMe)4] (2) and [RuH(IMe)4][BEt4] (3BEt4) have been synthesized. Complex 2 was isolated from the reaction of 1 with LiAlH4, while ionic compound 3 was obtained when LiBHEt3 was used as the hydride source. Complexes 1–3 have been characterized by X‐ray crystallography, multinuclear NMR, IR, UV/Vis spectroscopy and mass spectrometry. Neutral dihydride 2 displays a tetragonal bipyramidal geometry with four carbene groups coordinated to ruthenium in equatorial positions and two apical hydrogen atoms. DFT calculations suggest that the trans structure observed for 2 is preferred over a cis arrangement for steric reasons. The ruthenium atom of 3 has a tetragonal pyramidal coordination environment with a vacant coordination site sterically protected by the Me substituents of the ligands. Thus, compound 3 should be an attractive target for future coordination studies.

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