HIGH-PRESSURE O-17 NMR-STUDY OF THE DIHYDROXO-BRIDGED RHODIUM(III) HYDROLYTIC DIMER - MECHANISTIC EVIDENCE FOR LIMITING DISSOCIATIVE WATER EXCHANGE PATHWAYS

Rate constants and activation parameters for water exchange on the Rh( III) hydrolytic dimer [(H2O)(4)Rh(mu-OH)(2)Rh-(H2O)(4)](4+) were deter mined using O-17 NMR spectroscopy as a function of temperature (308-32 3 K) and pressure (0.1-150 MPa). This represents the first variable-pr essure O-17 NMR study conducted on a polynuclear metal aqua ion. Two p athways for water exchange with similar rates were found for H2O coord inated at positions cis (k(cis)) and trans (k(trans)) to the bridging OH groups on the Rh(III) centers. The bridging OH groups were not foun d to exchange with the bulk solvent, indicating that they are substitu tion inert on the time scale of these experiments. The kinetic paramet ers for water exchange on the fully protonated Rh(III) dimer at [H+] = 1.0 M and mu = 2.0 M are as follows: k(trans)(298) = 8.5 x 10(-7) s(- 1), Delta H(trans) = 150 +/- 2 kJ mol(-1), Delta S*(trans) = +141 +/- 7 J K-1 mol(-1), Delta V(trans) = +8.5 +/- 0.8 cm(3) mol(-1) (+8.5 /- 0.6 cm(3) mol(-1) at [H+] = 2.0 M and mu = 3.0 M); k(cis)(298) = 5. 4 x 10(-7) s(-1), Delta H(cis) = 159 +/- 8 kJ mol(-1); Delta S*(cis) = +168 +/- 24 J K-1 mol(-1), Delta V(cis) = +10.1 +/- 0.3 cm(3) mol(- 1) (+10.9 +/- 1.0 cm(3) mol(-1) at [H+] = 2.0 M and mu = 3.0 M). In co mparison to the water exchange for [Rh(H2O)(6)](3+), the introduction of bridging OH groups in the dimer was found to labilize the bound wat ers but not to the same extent as deprotonation of the monomer. Assumi ng an expansion of the ground-state partial molar volume of 3-5 cm(3) mol(-1) due to the labilizing effect of bridging OH groups in the dime r, the kinetic results suggest a limiting dissociative pathway, D, for water exchange in both the cis and trans positions. It is postulated that dissociation of a more labile trans water molecule in the Rh(III) dimer will lead to a five-coordinate intermediate which can undergo a rapid Berry pseudorotation to account for the similarity in rate and activation parameters for exchange of both cis and trans water molecul es.