A class of anionic and neutral complexes of ruthenium (II) containing nitrogen oxide (NO) and optionally a nitrogen ligand is described; a process for their preparation is also described. The preparation process includes the use of starting complexes of ruthenium (III) which are reacted with suitable reagents so as to obtain complexes containing NNO coordinated to ruthenium (II). Additional substitution reactions allow the introduction of new groups that coordinate to the ruthenium atom, among which some nitrogen ligands.