New ruthenium(II) complexes containing eta(6)-arene-eta(1)-pyrazole ligands were synthesized and characterized by elemental analysis and spectroscopic methods. In addition, the molecular structure of di-chloro-3,5-dimethyl-1-(pentamethylbenzyl)-pyrazole-ruthenium(II), [Ru]L(3)b, was determined by X-ray diffraction studies. These complexes were applied in the transfer hydrogenation of acetophenone by isopropanol in the presence of potassium hydroxide. The activities of the catalysts were monitored by NMR. (C) 2012 Elsevier Ltd. All rights reserved.