LINKAGE ISOMERIZATION-REACTIONS OF (ACETONE)PENTAAMMINERUTHENIUM(III) AND (ACETONE)PENTAAMMINERUTHENIUM(II) COMPLEXES

A complete kinetic and thermodynamic analysis of the linkage isomerizations of [Ru(NH3)5(acetone)]3+/2+ is presented. Each oxidation state exists as two linkage isomers in which acetone is bound via O in an eta-1 fashion (favored by Ru(III)) and via C and O of the carbonyl group in an eta-2 fashion (favored by Ru(II)). The [Ru(NH3)5(eta-2-OC(CH3)2)]2+ and [Ru(NH3)5(eta-1-OC(CH3)2)]3+ complexes were characterized by spectroscopic methods (IR, UV/vis, and NMR), and the kinetic and thermodynamic data for these isomerizations were obtained by electrochemical methods. In acetone, the [Ru(NH3)5(eta-2-OC(CH3)2)]3+/2+ couple is observed at E1/2 = -337 mV as ferrocenium/ferrocene (Fc+/Fc) at 25-degrees-C, while the [Ru(NH3)5(eta-1-OC(CH3)2)]3+/2+ couple is observed at E1/2 = -478 mV (vs Fc+/Fc at 25-degrees-C). For the eta-1 --> eta-2 linkage isomerization of [Ru(NH3)5(acetone)]2+, the rate and equilibrium constants at 25-degrees-C in acetone and the enthalpy and entropy of activation are 18 +/- 1 s-1, 16 +/- 1, 63 +/- 1 kJ mol-1, and -10 +/- 2 J K-1 mol-1, respectively. Similarly, for the eta-2 --> eta-1 linkage isomerization of [Ru(NH3)5(acetone)]3+, they are 0.46 +/- 0.03 s 1, 14 +/- 1, 48 +/- 2 kJ mol-1, and -91 +/- 7 J K-1 mol-1, respectively. The rate constant for the exchange of coordinated acetone with free acetone in the Ru(III) complex is (6.9 +/- 0.5) X 10(-6) s-1, at 0-degrees-C. This is much slower than the Ru(III) eta-1 --> eta-2 isomerization rate, (4.3 +/- 0.4) X 10(-3) s-1, at the same temperature, indicating an intramolecular isomerization. In a 50% v/v acetone/acetonitrile mixture, at 3-degrees-C, the Ru(II) eta-1 --> eta-2 isomerization rate is 0.3 s-1, while the rate constant for the substitution of the coordinated acetone is 0.4 s-1. Therefore, it was not possible to distinguish between an intra- and an intermolecular process. IR spectroscopy and other evidence indicate that the eta-2 complex is stabilized by both pi-back-bonding and sigma-bonding via the ketone double bond, with the latter being more important. At -23-degrees-C, a second ECE process is observed involving the [Ru(NH3)5(OSO2CF3)]2+/+ couple (E1/2 = -494 mV vs Fc+/Fc) and the eta-2-acetone complex; the rate constant for the substitution of the triflate ligand, in the Ru(II) complex, by acetone was found to be 1.2 +/- 0.2 s-1, compared to 0.109 s-1 for the Ru(II) linkage isomerization under the same conditions. The analogous hexafluoroacetone complexes exist entirely in the eta-2 form and have redox characteristics similar to those of the eta-2-acetone complexes