Thermal reactions between [RuCl2(diene)](n) (diene = 2,5-norbornadiene, nbd; 1,5-cyclooctadiene, cod) with an excess of N,N,N-',N-'-tetramethylethylene diamine (tmeda) afforded derivatives [RUCl2(diene)(tmeda)] (diene = nbd, 1; cod, 2) as a mixture of cis and trans isomers. When thermolysis was performed under H-2 mixtures of hydride species [RuCI(H)(diene)(tmeda)] (diene = nbd, 3; cod, 4) and the bis-tmeda adduct trans-[RuCl2(tmeda)(2)] (5) were obtained in different ratios depending upon the reaction conditions and reaction times. Heating polymeric Ru(II) precursors in toluene in the presence of a 5-fold excess of the bulkier N,N,N-',N-'-tetraethylethylene diamine (teeda) resulted in a rare diamine dealkylation process with formation of trans-[RUCl2(nbd)(Et2NCH2CH2NHEt)] (6) and trans-[RUCl2(cod)(EtHNCH2CH2NHEt)] (7) in high yields. The presence of N-H functionalities in the coordinated diamine ligands of 6 and 7 was unambiguously established by single-crystal X-ray diffraction studies. The dealkylation process of the teeda ligand seems to proceed intramolecularly as shown by solution NMR studies performed with the soluble Ru(II) precursors trans-[RUCl2(amine)2(diene)] (diene = nbd, amine = morpholine, 9; diene = cod, amine = Et2NH, 10). The above complexes [RUCl2(diene)(diamine)] have been tested as precatalysts in the hydrogenation of ketones both for transfer as well as direct hydrogenation, the latter route being the most effective.