With four site: Catalytic olefin hydrogenation is observed with palladium complexes when the N-heterocyclic carbene ligand is changed from the normal C2- to the unusual C4-bonding mode (see scheme). In a cis coordinating framework, the electron density at the metal center is substantially increased as a result of the exceptionally strong donor ability of C4-bound carbenes. (Chemical Equation Presented) © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.