NMR KINETIC-STUDY OF LIGAND-EXCHANGE ON A (MU-OXO)BIS(MU-ACETATO)DIRUTHENIUM(III) COMPLEX - A SPECIAL KINETIC CASE OF INTRODUCTION OF A MORE LABILE LIGAND

We have prepared the new [Ru-2(mu-O)(mu-O2CCH3)(2)(4-pic)(6)](PF6)(2) complex and followed the kinetics of exchange of the 4-picoline ligand s, trans to the oxo bridge, with dimethyl sulfoxide (DMSO-d(6)) by H-1 NMR at 30, 40, 50, and 60 degrees C. The H-1 NMR spectra of this comp lex in DMSO gradually change in the acetyl methyl region as solvent su bstitution of the 4-picoline trans to the oxo bridge takes place. This substitution is in accordance with the lability of the ligands trans to the Ru-III-O-Ru-III bridge in this kind of complex. The kinetic dat a fit in a series of double reversible reactions, pseudo first order i n the forward direction and second order in the reverse direction, wit h Delta H double dagger = 115, 105, 116, and 106 kJ mol(-1) and Delta S double dagger = 35.7, 17.2,38.8, and 16.9 J K-1 mol(-1) associated w ith k(1), k(-1), k(2), and k(-2) rate constants, respectively. [GRAPHI CS] A, B and C stand for the nonexchanged, monoexchanged, and diexchan ged complexes, respectively. The large Delta H double dagger and posit ive Delta S double dagger values agree with the dissociative nature of the mechanism in both forward and reverse reactions. The differences in H-1 and C-13 NMR chemical shifts of the three complexes are explain ed in terms of the effect of the ligand exchange on the N-Ru bond leng ths and the geometry of the dimetal core. It is evident from our data that the exchange on one metallic site modifies the reactivity at the other site in this kind of dinuclear complex.