The rate constants and activation parameters for water exchange on hexaaqua and monohydroxy pentaaqua iridium(III) have been determined by 17O NMR spectroscopy as a function of temperature (358−406 K) and pressure (0.1−210 MPa) at several acidities (0.5−5.0 m). Noncoordinating trifluoromethanesulfonate (CF3SO3 -) was used as the counterion. The observed rate constant was of the form k = k 1 + k 2/[H+], where the subscripts 1 and 2 refer to the exchange pathways on [Ir(H2O)6]3+ and [Ir(H2O)5(OH)]2+, respectively. The kinetic parameters obtained are summarized as follows:  k 1 298 = (1.1 ± 0.1) × 10-10 s-1, ΔH 1 ⧧ = 130.5 ± 0.6 kJ mol-1, ΔS 1 ⧧ = +2.1 ± 1.7 J K-1 mol-1, and ΔV 1 ⧧ = −5.7 ± 0.5 cm3 mol-1; k 2 298 = (1.4 ± 0.6) × 10-11 m s-1, ΔH 2 ⧧ = 138.5 ± 4.5 kJ mol-1, ΔS 2 ⧧ = +11.5 ± 11.6 J K-1 mol-1, and ΔV 2 ⧧ = −0.2 ± 0.8 cm3 mol-1. The value obtained for k 1 298 corresponds to a residence time of ca. 300 years. The pK a 298 and the volume change ΔV a 0 associated with the first hydrolysis of [Ir(H2O)6]3+ were determined by potentiometric and high-pressure spectrophotometric methods to be 4.45 ± 0.03 and −1.5 ± 0.3 cm3 mol-1, respectively. Utilizing the relation k 2 = k OH K a1, values for the first-order rate constant and the corresponding activation volume for [Ir(H2O)5(OH)]2+ were estimated to be k OH 298 = 5.6 × 10-7 s-1 and ΔV OH ⧧ = +1.3 cm3 mol-1, respectively. These data are supportive of an associative interchange (Ia) mechanism for water exchange on [Ir(H2O)6]3+, but of an interchange (I) mechanism on the deprotonated species [Ir(H2O)5(OH)]2+. These mechanistic results have also been compared to those reported for other trivalent metal ions.

Solvent exchange on the half-sandwich organic solvates [(eta(5)-C(5)Me(5))M(S)(3)](2+) (M = Rh, S = MeCN (1) or Me(2)SO (3); and M = Ir, S = MeCN (2) or Me(2)SO (4)) has been investigated as a function of temperature, pressure, and concentration of free solvent by (1)H NMR line-broadening techniques in CD(3)CN and/or CD(3)NO(2). The exchange rates span several orders of magnitude, from k(ex)(298) = 8.8 x 10(-)(2) s(-)(1) for 2 to 3.6 x 10(3) s(-)(1) for 3, as a result of changes in the electronic and steric properties of the ligands. Nevertheless, the volume of activation remains consistently positive for compounds 1-4 with values ranging from +0.8 to +3.3 cm(3) mol(-)(1). In combination with the positive activation entropies obtained and the first-order rate law established for these systems, it was concluded that regardless of the nature of the ligand the solvent exchange process on 1-4 proceeds via a dissociative D mechanism. Of note, the intermolecular exchange with free Me(2)SO on 4 takes place exclusively from a conformational isomer of 4 (structure 4.2), which is itself in equilibrium with a second, more compact conformer (structure 4.1).

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Chemical or electrochemical reduction of the cationic rhenium aryldiazenido complexes [Cp'Re(L~)(L2)(p-N2CsH40Me)l[BF4] (lb-7b) generates the corresponding neutral dinitrogen complexes la-7a of the type Cp'Re(Ll)(L2)(N2) ((a) Cp' = Cp; L1 = L2 = CO ( l a ) ; (b) Cp' = Cp*, LI = CO, L2 = PMe3 (3a) or P(OMe)3 (4a); and (c) Cp' = Cp*, L1= L2 = CO (2a), PMe3(Sa), dmpe (gal, or P(OMe)3 (7a)). Evidence is presented in support of a proposed mechanism which involves one-electron reduction of the aryldiazenido complex cation in lb-7b to the neutral 19-electron intermediate, followed by C-N bond homolysis to give la-7a plus the p-methoxyphenyl radical that proceeds to form anisole by hydrogen atom abstraction from the solvent. The cathodic reduction peak potentials observed for lb-7b by cyclic voltammetry are increasingly negative in the sequence of coligands (PMe3)2 (Sb, -1.89 V) > dmpe (6b, -1.74 V) > [P(OMe)312 (7b, -1.41 V) > (CO)(PMe3) (3b, -1.24 V) > (CO)[P(OMe)31(4b, -0.98 V) > (C0)2 (2b, -0.62 V). The value for the dicarbonyl Cp* complex 2 b is more negative than for the corresponding Cp complex l b (-0.46 V). These potentials are a good indication of the success of chemical reduction: all lb-7b are reduced to la-7a by N a g in THF, whereas CpzCo, which has a smaller reduction potential than N a g , only reduces lb, 2b, and 4b. The reaction of 2b with NaBH4 is shown by full spectroscopic characterization to produce the aryldiazene complex Cp*Re(CO)z@-NHNCsH4OMe), which is thermally unstable and yields the dinitrogen complex 2a and anisole.

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