Ruthenium complexes containing 2-(2-nitrosoaryl)-pyridine ((ONN)-N-Lambda) and tetradentate thio ether 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4), [Ru((ONN)-N-Lambda)([14]aneS4)](2+) [(ONN)-N-Lambda = 2-(2-nitrosophenyl)pyridine (2a), 10-nitrosobenzo[h]quinoline (2b), 2-(2-nitroso-4-methylphenyl)pyridine, (2c), 2-(2-nitrosophenyl)-S-(trifluoromethyl)pyridine (2d)] and analogues with the 1,4,7-trithiacydononane ([9]aneS3)/tert-butylisocyanide gand set, (Ru((ONN)-N-Lambda)([9]aneS3)(C (NBu)-Bu-t)](2+) (4a and 4b), have been prepared by insertion of a nitrosonium ion (NO+) into the Ru-aryl bond of cyclometalated ruthenium(II) complexes. The molecular structures of the (ONN)-N-Lambda-ligated complexes 2a and 2b reveal that (i) the (ONN)-N-Lambda ligands behave as bidentate chelates via the two N atoms and the bite angles are 86.84(18)-87.83(16)degrees and (ii) the Ru-N-NO and N-O distances are 1.942-(5)-1.948(4) and 1.235(6)-1.244(5) angstrom, respectively. The Ru-N-NO and N-O distances, together with v(N=O), suggest that the coordinated (ONN)-N-Lambda ligands in this work are neutral moiety (ArNO)(0) rather than monoanionic radical (ArNO)(center dot-). or dianion (ArNO)(2-) species. The nitrosated complexes 2a-2d show moderately intense absorptions centered at 463-484 nm [epsilon(max) = (5-6) x 10(3) dm(3) mol(-1) cm(-1)] and a clearly discriminable absorption shoulder around 620 nm (epsilon(max) = (6-9) x 10(2) dm(3) mol(-1) cm(-1)), which tails up to 800 nm. These visible absorptions are assigned as a mixing of d(Ru) -> (ONN)-N-Lambda metal-to-ligand charge-transfer and (ONN)-N-Lambda intraligand transitions on the basis of time-dependent density functional theory (TD-DFT) calculations. The first reduction couples of the nitrosated complexes range from -0.53 to -0.62 V vs Cp2Fe+/0, which are 1.1-1.2 V less negative than that for [Ru(bpy)([14]aneS4)](2+) (bpy = 2,2'-bipyridine). Both electrochemical data and DFT calculations suggest that the lowest unoccupied molecular orbitals of the nitrosated complexes are (ONN)-N-Lambda-centered. Natural population analysis shows that the amount of positive charge on the Ru centers and the [Ru([14]aneS4)] moieties in 2a and 2b is larger than that in [Ru(bpy)([14]aneS4)](2+). According to the results of the structural, spectroscopic, electrochemical, and theoretical investigations, the (ONN)-N-Lambda ligands in this work have considerable pi-acidic character and behave as better electron acceptors than bpy.