Synthesis, characterization, X-ray structural determination and theoretical study of the complexes [RuCp(8MTT-κS)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)