The water exchange process on [(CO)(3)Re(H(2)O)(3)](+) (1) was kinetically investigated by (17)O NMR. The acidity dependence of the observed rate constant k(obs) was analyzed with a two pathways model in which k(ex) (k(ex)(298) = (6.3 +/- 0.1) x 10(-3) s(-1)) and k(OH) (k(OH)(298)= 27 +/- 1 s(-1)) denote the water exchange rate constants on 1 and on the monohydroxo species [(CO)(3)Re(I)(H(2)O)(2)(OH)], respectively. The kinetic contribution of the basic form was proved to be significant only at [H(+)] < 3 x 10(-3) M. Above this limiting [H(+)] concentration, kinetic investigations can be unambiguously conducted on the triaqua cation (1). The variable temperature study has led to the determination of the activation parameters Delta H(++)(ex) = 90 +/- 3 kJ mol(-1), Delta S(++)(ex) = +14 +/- 10 J K(-1) mol(-1), the latter being indicative of a dissociative activation mode for the water exchange process. To support this assumption, water substitution reaction on 1 has been followed by (17)O/(1)H/(13)C/(19)F NMR with ligands of various nucleophilicities (TFA, Br(-), CH(3)CN, Hbipy(+), Hphen(+), DMS, TU). With unidentate ligands, except Br(-), the mono-, bi-, and tricomplexes were formed by water substitution. With bidentate ligands, bipy and phen, the chelate complexes [(CO)(3)Re(H(2)O)(bipy)]CF(3)SO(3) (2) and [(CO)(3)Re(H(2)O)(phen)](NO(3))(0.5)(CF(3)SO(3))(0.5).H(2)O (3) were isolated and X-ray characterized. For each ligand, the calculated interchange rate constants k'(i) (2.9 x 10(-3) (TFA) < k'(I) < 41.5 x 10(-3) (TU) s(-1)) were found in the same order as the water exchange rate constant k(ex), the S-donor ligands being slightly more reactive. This result is indicative of I(d) mechanism for water exchange and complex formation, since larger variations of k'(i) are expected for an associatively activated mechanism.

The photoreduction of H+ to H2 by sunlight and colloidal TiO2 in the presence of the rhodium(III) complexes 1 (ppy = oligopyridine ligand, e.g., bipyridyl; L=H2O, Cl⊖, Br⊖, I⊖; n = 1, 2) is possible without the participation of colloidal platinum and is not restricted to alkaline solution. The complexes are thereby reduced, via the RhII stage, to RhI complexes, which then undergo transformation to the RhIII complexes with evolution of hydrogen. The course of the reaction was investigated by cyclic voltammetry and pulse radiolysis as well as by transient absorption spectroscopy.

Rhodium bipyridine complexesRh"'-Komplexe [Cp*Rh(ppy)L]"+ (L = H20;n = 2, oder L = C1-, OH-, n = 1; Cp* = qS-C5Me5, ppy = ?,2'-Bipyridin, 1, 4,4'-Dicarboxy-2,2'-bipyridin, 2, 4,4'-Diacetyl-2,2'-bipyridin, 3, 4,4-Dimethyl-2,2'-bipyridin, 4, 3,3'-Dicarboxy-2,2'-bipyridin, 5, o-Phenanthrolin, 6) liefern bei der Reduktion mit Na/Hg oder

The title complex [fCp*RuCI,},], 1 (Cp* = q5-C,Me,),['.21 is a much-used reagent in the preparation of Cp*Ru sandwich and, in particular, half-sandwich complexes.I3-'] Although the solubility of the complex in halogenated hydrocarbons and the molar mass in chloroform (as determined by osmometric speaks against the existence of a polymer, the compound is always seen as such. ' In the preparation from RuCI,(,,, and Cp*H in methanol ['] or ethanol,[21 1 precipitates as a red-brown, microcrystalline powder, which for some time thwarted attempts at crystallization. Crystals suitable for X-ray crystallography were finally obtained by diffusion of ether into a concentrated dichloromethane solution of 1, after purification of an ether/dichloromethane solution by chromatography on silylated silicaFor compounds 2 and 3 the chromatographic procedure is unnecessary.showed that all three compounds were dimers, as anticipated. A pecularity of the crystals of 1 was the equal distribution of forms l a and lb (Fig. l) (3.712 wI1']). This isomer should thus be paramagpetic. This interpretation is supported by solid-state CP/MAS NMR spectroscopy on 1.'''] The 'H and 13C NMR spectra each show two resonance signals (6 = 2 and 18, and 6 = 13 and 246, respectively), which can be attributed to the deformational isomers. The low-field signals show the temperature proportionality characteristic of NMR signals of paramagnetic compounds. In contrast to the IR spectrum of 4, in the case of 1 (CsT pellet) the bands assigned to the M-CI vibrational modes at 292/309 and 260/275 cm-' are split. This correlates with the different distances between the Ru atoms and the terminal chloro ligands in l a and l b (Table 1). Ru-CI-Ru angles, and Ru-Ru distances of 2.93 and 3.75 A (Fig. 2), and are located at positions 000 and 0% (la) and and 9 0 (lb) of the monoclinic unit cell. The bonding parameters of l a and l b differ enough to justify the term "deformational isomerism". The molecular geometries of the compounds 2 and 3, where crystals consist of only one form, correspond to isomer l a ( Table 1). The different Ru-Ru distances in the isomers suggest a strong exchange interaction between the unpaired spins of the d5 low-spin Ru centers['61 in la, but not in lb, where the metal-metal distance is similar to the nonbonding Rh-Rh distance in the analogous Rh complex [{Cp*RhCl,},] 4

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