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 H2CCH2 with 103 k f 298/kg mol-1 s-1 = 1.22 ± 0.06, ΔH f ⧧/kJ mol-1 = 76.9 ± 2, and ΔS f ⧧/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 = H2CCH2 (103 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−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 (103 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, ΔH cis ⧧/kJ mol-1 = 129.9 ± 4, and ΔS cis ⧧/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.

Ligand exchange and complex formation kinetics studied by NMR exemplified on fac-[(CO)3M(H2O)]+ (M=Mn, Tc, Re)

“…In the case of coupling to a nucleus with an electric quadrupole moment (I > 1/2) the shape of the multiplet is influenced by the relaxation of that nuclear spin due to quadrupolar relaxation [45]. If the nuclear quadrupole moment is large as for example for 55 Mn and 185/187 Re quadrupolar relaxation of the metal nuclear spin is fast. The multiplet of the observed 17 O spin collapses completely and a single Lorentzian line shape is observed despite the coupling.…”

“…Using a mathematical description developed for complex formation [55] 12 CO are shown in Fig. 10 [54].…”

“…To be able to compare the water exchange rate constant k ex to k i a statistical factor f/n c has to be considered [55], where f is the number of molecules in the second coordination sphere and n c is the number of equivalent water molecules in the first sphere. The rate constants to compare are therefore k ex and k i (Eq.…”

“…whereas the value of n c = 3 is evident in fac-[(CO) 3 M(H 2 O) 3 ] + compounds the value for f is less clear and a value of f = 12 is generally assumed [55]. The values of K os can be estimated with the Fuoss-Eigen equation [65,66] and typical values are 0.24 M −1 for neutral ligands and 1.1 M −1 for oppositely charged ligands.…”

Aspects of Aqueous Ruthenium(II) Chemistry

“…c ) Rate constant, k ex , for the exchange of a particular H 2 O molecule[1]. d ) From[18]. e ) From[19].…”