Four chelating di(N-heterocyclic carbene) palladium(II) complexes have been prepared and used as catalysts in the Heck reaction of aryl bromides at very low Pd concentration ("omeopathic conditions"). They differ in the nature of the nitrogen substituent, of the bridge between the carbene units or of the heterocyclic system. The obtained results indicate that the catalysts perform well with activated aryl bromides, but less efficiently than simple palladium(II) acetate in the case of deactivated aryl bromides. Such an inhibiting effect of the dicarbene ligands can be correlated with their ability to form inactive dimers and/or bischelate, tetracarbene palladium(II) derivatives in the course of the reaction and appears to depend on their steric features, with catalytic activity increasing with ligand bulkiness. (c) 2005 Elsevier B.V. All rights reserved.