The reaction [Ru(H2O)(6)](2+) + L -->(kf) [RU(H2O)(5)L](2+) + H2O was followed as a function of temperature and ethylene concentration (up to 40 MPa) using a homemade high gas pressure NMR microreactor. The reaction was first order in H2C=CH2 with 10(3)kf(298)/kg mol(-1) s(-1) = 1.22 +/- 0.06, Delta H(f)double dagger/kJ mol(-1) = 76.9 +/- 2, and Delta S(f)double dagger/J K-1 mol(-1) = -42.9 +/- 8. These results confirm previous works on mono-complex formation reactions where an Id mechanism was proposed. The reaction [Ru(H2O)5L]2+ + *L reversible arrow(kL) [Ru(H2O)(5)*L](2+) + L of exchange of L on the mono-complex was followed for L = H2C=CH2 (10(3)k(L)/kg mol(-1) s(-1) = 10.8 at 298.2 K), Me2SO (0.35 at 278.5 K), and CO (0.052 at 298.3 K); the rate-determining step is the rupture of the Ru-(HOax)-O-2 bond with trans-Ru(H2O)(4)L-2](2+) as reaction intermediate. Due to the trans effect exercised by these strong pi-accepting ligands, the ligand exchange reaction is faster than the mono-complex formation reactions. The cis-bis-complex formation reaction, [Ru(H2O)(5)L](2+) + L -->(kcis) cis-[Ru(H2O)(4)L-2](2+) + H2O, was also investigated for L = MeCN (10(3)k(cis)/kg mol(-1) s(-1) = 0.111 at 298.1 K), Me2SO (0.019 at 321.6 K), and H2C=CH2 (0.007 at 298.1 K, Delta H(cis)double dagger/kJ mol(-1) = 129.9 +/- 4, and Delta S(cis)double dagger/J K-1 mol(-1) = +92.0 +/- 11); here, too, the Ru-H2Oeq bond breaking is rate determining, but due to the decrease of the lability of water molecules cis to pi-accepting ligands, these reactions are much slower. In the case of MeCN, the reaction scheme includes the formation of the trans-bis-complex and of the mer-triscomplex. As a general rule, the rate of these complex formation reactions,of dissociative nature, can be predicted from the oxygen-17 determined water exchange rates.