New ruthenium(II) complexes having a tetradentate ligand such as tris(2-pyridylmethyl) amine (TPA), tris[2-(5-methoxycarbonyl) pyridylmethyl] amine [5-(MeOCO)(3)-TPA], tris(2-quinolylmethyl) amine (TQA), or bis(2-pyridylmethyl)glycinate (BPG) have been prepared. The reaction of the ligand with [RuCl2(Me2SO4] resulted in a mixture of trans and cis isomers of the chloro(dimethyl sulfoxide-kS) ruthenium(II) complexes containing a TPA or a BPG, whereas a trans(Cl, N-amino) isomer was selectively obtained for 5-( MeOCO)(3)-TPA and TQA. The trans and cis isomers of the [RuCl(TPA)(Me2SO)](+) complex were easily separated by fractional recrystallization. The molecular structures of trans- and cis(Cl,N-amino)-[RuCl(TPA)(Me2SO)](+) complexes and the trans(Cl, N-amino)-[RuCl{5-(MeOCO)(3)-TPA}(Me2SO)](+) complex have been determined by X-ray structural analyses. The reaction of TPA with [RuCl2(PhCN)(4)] gave a single isomer of the chloro(benzonitrile) ruthenium(II) complex, whereas the bis(benzonitrile) ruthenium(II) complex was obtained with BPG. The cis(Cl, N-amino)-[RuCl(TPA)(Me2SO)](+) complex is thermodynamically much less stable than the trans isomer and isomerizes in dimethyl sulfoxide at 65 - 100 degrees C. Oxygenation of alkanes catalyzed by these ruthenium(II) complexes has been examined. The chloro(dimethyl sulfoxide-kS) ruthenium(II) complexes with TPA and its derivatives using m-chloroperbenzoic acid as a cooxidant showed high catalytic ability. Adamantane was efficiently and selectively oxidized to give 1-adamantanol up to 88%. The chloro(dimethylsulfoxide-kS)ruthenium(II) complex with 5-(MeOCO)(3)-TPA was found to be the most active catalyst among the complexes examined.