Ru(II) complexes with diazabutadiene (R-DAB) ligands have been prepared. The reaction of RuCl 3 •nH 2 O with P(p-tolyl) 3 gave a [RuCl 2 {P(p-tolyl) 3 }] precursor, whose reactions with R-DAB in toluene gave dinuclear trichloro-bridged Ru(II) complexes [Ru 2 Cl 3 (P(p-tolyl) 3 ) 2 (R-DAB) 2 ](BF 4 ) which have been characterized by spectroscopic methods. In addition, one of the complexes was characterized using X-ray crystallography. Meanwhile, two mononuclear Ru(II) complexes [RuCl 2 (P(p-tolyl) 3 ) 2 (R-DAB)] were obtained from the reactions of the [RuCl 2 {P(p-tolyl) 3 }] precursor with R-DAB ligands in THF. The two trans-mononuclear complexes were characterized by X-ray crystallography and solid-state 31 P NMR. A temperature-dependent 31 P NMR study was carried out to monitor the formation of dinuclear and mononuclear complexes.