The complexes [RuCp(8MTT-κS)LL′] and [RuCp(8MTT-κN7)LL′] (8MTT=8-methylthio-theophyllinate; L,L′=PTA, mPTA; L=mPTA, L′=PPh3; PTA=1,3,5-triaza-7-phosphaadamantane, mPTA=N-methyl-1,3,5-triaza-7-phosphaadamantane) have been investigated by DFT theoretical methods. Structures, infrared spectra and thermodynamical properties in gas phase, water and ethanol of the studied complexes have been determined at B3LYP/DZVP level of theory. Complexes [RuCp(8MTT-κS)(PTA)2] and [RuCp(8MTT-κS)(mPTA)(PPh3)]+ are slightly stabilized by favourable Gibbs free energy respect to [RuCp(8MTT-κN7)(PTA)2] and [RuCp(8MTT-κN7)(mPTA)(PPh3)]+ in gas phase while in water and ethanol the calculated Gibbs free energy showed that the 8MTT-κS-complexes are quite more stable than the corresponding 8MTT-κN7-complexes. However, the [RuCp(8MTT-κN7)(mPTA)2]2+ is more stable than the corresponding κS-complex in gas phase while in water remains less stable. These theoretical results are in agreement with obtained until now experimental results as two new 8MTT-κS-complexes, [RuCp(8MTT-κS)(mPTA)(PPh3)](CF3OSO2) (1) and [RuCp(8MTT-κS)(mPTA)2]Cl(CF3OSO2)·1.5H2O (2·1.5H2O) were synthesized and characterized by single crystal X-ray diffraction.