Non-classical ruthenium hydride complexes are promising lead structures for the C À H bond activation and functionalization of aromatic compounds. In the present paper, the preparation and crystallographic characterisation of the first organometallic complexes bearing dihydrogen ligands and N-heterocyclic carbene ligands in the same coordination sphere are described. The mixed phosphine/ carbene complex [(IMes)Ru(H) 2 (H 2 ) 2 (PCy 3 )] (IMes 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene; 3a) shows a unique reactivity pattern in the inter-and intramolecular activation of C À H bonds. In particular, complex 3a effects a rapid and remarkably selective intermolecular activation of sp 2 C À H bonds in simple aromatic compounds at room temperature.

The synthesis and characterisation of nonclassical ruthenium hydride complexes containing bidentate PP and tridentate PCP and PNP pincer‐type ligands are described. The mononuclear and dinuclear ruthenium complexes presented have been synthesised in moderate to high yields by the direct hydrogenation route (one‐pot synthesis) or in a two‐step procedure. In both cases [Ru(cod)(metallyl)2] served as a readily available precursor. The influences of the coordination geometry and the ligand framework on the structure, binding, and chemical properties of the MH2 fragments were studied by X‐ray crystal structure analysis, spectroscopic methods, and reactivity towards N2, D2, and deuterated solvents.

The water‐soluble complex derived from Rh(CO)2(acac) and human serum albumin (HSA) proved to be efficient in the hydroformylation of several olefin substrates. The chemoselectivity and regioselectivity were generally higher than those obtained by using the classic catalytic systems like TPPTS‐Rh(I) (TPPTS=triphenylphosphine‐3,3′,3″‐trisulfonic acid trisodium salt). Styrene and 1‐octene, for instance, were converted in almost quantitative yields into the corresponding oxo‐aldehydes at 60 °C and 70 atm (CO/H2=1) even at very low Rh(CO)2(acac)/HSA catalyst concentrations. The possibility of easily recovering the Rh(I) compound makes the system environmentally friendly. The circular dichroism technique was useful for demonstrating the Rh(I) binding to the protein and to give information on the stability in solution of the catalytic system. Some other proteins have been used to replace HSA as complexing agent for Rh(I). The results were less impressive than those obtained using HSA and their complexes with Rh(I) were much less stable.