Mechanism of Complex Formation of Ruthenium(II) Aquacomplexes with H₂CCH₂, MeCN, Me₂SO, and CO: Metal−Water Bond Rupture as Rate-Determining Step

Abstract

The reaction [Ru(H2O)6]2+ + L [Ru(H2O)5L]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 H2CCH2 with 103kf298/kg mol-1 s-1 = 1.22 ± 0.06, ΔHf⧧/kJ mol-1 = 76.9 ± 2, and ΔSf⧧/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 [Ru(H2O)5*L]2+ + L of exchange of L on the mono-complex was followed for L = H2CCH2 (103kL/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−H2Oax bond with trans-[Ru(H2O)4L2]2+ as reaction intermediate. Due to the trans effect exercised by these strong π-accepting ligands, the ligand exchange reaction is faster than the mono-complex formation reactions. The cis-bis-complex formation reaction, [Ru(H2O)5L]2+ + L cis-[Ru(H2O)4L2]2+ + H2O, was also investigated for L = MeCN (103kcis/kg mol-1 s-1 = 0.111 at 298.1 K), Me2SO (0.019 at 321.6 K), and H2CCH2 (0.007 at 298.1 K, ΔHcis⧧/kJ mol-1 = 129.9 ± 4, and ΔScis⧧/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 π-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.