Abstract. The linkage isomers of the [M(NO)(CN)5]2- (M = Fe, Ru, Os), [Ru(NO)Cl5]2-, trans-[Ru(NO)(NH3)4(L)]q+ (L = NH3, H2O, nicotinamide, imidazole, pyridine, pyrazine, NO2-, OH-, Cl-, NO2), and trans-[Ru(NO)L4(OH)]q (L = pyridine, NO2) ions are studied by density functional theory. DFT calculations show that the electronic ground-state potential surface of these nitrosyl complexes has the local minima corresponding to hapto2-NO and hapto1-O linkage isomers, of which the former is characterized by lower energy. The stationary points on the ground state potential energy surface along the GSMS2MS1 path for the [M(NO)(CN)5]2 (M = Fe, Ru, Os) and [Ru(NO)(Cl)5]2 complexes were found. The calculations of infrared spectra of linkage isomers [Ru(NO)Cl5]2 and [Ru(NO)(CN)5]2 are carried out. The analysis of quantum chemical bond orders indicates a noticeable delocalization of pi-electron density along the Ltrans–M–NO axis, and in the structures with the bent {MNO} group – to the possibility of direct intera