An I_a mechanism was assigned for water exchange on the hexaaquaions Rh(OH₂)₆³⁺ and Ir(OH₂)₆³⁺ on the basis of negative ΔV_‡ experimental values (−4.2 and −5.7 cm³ mol^-1, respectively). The use of ΔV_‡ as a mechanistic criterion was open to debate primarily because ΔV_‡ could be affected by extension or compression of the nonparticipating ligand bond lengths on going to the transition state of an exchange process. In this paper, volume and energy profiles for two distinct water exchange mechanisms (D and I_a) have been computed using quantum chemical calculations which include hydration effects. The activation energy for Ir(OH₂)₆³⁺ is 32.2 kJ mol^-1 in favor of the I_a mechanism (127.9 kJ mol^-1), as opposed to a D pathway; the value for the I_a mechanism being close to ΔH_‡ and ΔG_‡ experimental values (130.5 kJ mol^-1 and 129.9 kJ mol^-1 at 298 K, respectively). Volumes of activation, computed using Connolly surfaces and for the I_a pathway (ΔV_‡calc = −3.9 and −3.5 cm³ mol^-1, respectively, for Rh³⁺ and Ir³⁺), are in agreement with the experimental values. Further, it is demonstrated for both mechanisms that the contribution to the volume of activation due to the changes in bond lengths between Ir(III) and the spectator water molecules is negligible: −1.8 for the D, and −0.9 cm³ mol^-1 for I_a mechanism. This finding clarifies the debate about the interpretation of ΔV_‡ and unequivocally confirms the occurrence of an I_a mechanism with retention of configuration and a small a character for both Rh(III) and Ir(III) hexaaquaions.

The C₂-symmetric electron-poor ligand (R)-BINOP-F (4) was prepared by reaction of (R)-BINOL with bis(pentafluorophenyl)-phosphorus bromide in the presence of triethylamine. The iodo complex [CpRu((R)-BINOP-F)(I)] ((R)-6) was obtained by substitution of two carbonyl ligands by (R)-4 in the in situ-prepared [CpRu(CO)₂H] complex followed by reaction with iodoform. Complex 6 was reacted with [Ag(SbF₆)] in acetone to yield [CpRu((R)-BINOP-F)(acetone)][SbF₆] ((R)-7). X-ray structures were obtained for both (R)-6 and (R)-7. The chiral one-point binding Lewis acid [CpRu((R)-BINOP-F)][SbF₆] derived from either (R)-7 or the corresponding aquo complex (R)-8 activates methacrolein and catalyzes the Diels−Alder reaction with cyclopentadiene to give the [4 + 2] cycloadduct with an exo/endo ratio of 99:1 and an ee of 92% of the exo product. Addition occurs predominantly to the methacrolein C_α-Re face. In solution, water in (R)-8 exchanges readily. Moreover, a second exchange process renders the diastereotopic BINOP-F phosphorus atoms equivalent. These processes were studied by the application of variable-temperature ¹H, ³¹P, and ¹⁷O NMR spectroscopy, variable-pressure ³¹P and¹⁷O NMR spectroscopy, and, using a simpler model complex, density functional theory (DFT) calculations. The results point to a dissociative mechanism of the aquo ligand and a pendular motion of the BINOP-F ligand. NMR experiments show an energy barrier of 50.7 kJ mol^-1 (12.2 kcal mol^-1) for the inversion of the pseudo-chirality at the ruthenium center.

The hexaaqua complex of ruthenium(II) represents an ideal starting material for the synthesis of isostructural compounds with a [Ru(H2O-ax)(H2O-eq)4L]2+ general formula. We have studied a series of complexes, where L = H2O, MeCN, Me2SO, H2CCH2, CO, and F2CCH2. We have evaluated the effect of L on the cyclic voltammetric response, on the rate and mechanism of exchange reaction of the water molecules, and on the structures calculated with the density functional theory (DFT). As expected, the formal redox potential, E°‘(+2/+3), increases with the Ï€-accepting capabilities of the ligands. For L = N2, the oxidation to Ru(III) is followed by a fast substitution of dinitrogen by a solvent molecule, revealing the poor stability of the Ru(III)−N2 bond. The water exchange reactions have been followed by 17O NMR spectroscopy. The variable-pressure and variable-temperature kinetic studies made on selected examples are all in accordance with a dissociative activation mode for exchange. The positive activation volumes obtained for the axial and equatorial water exchange reactions on [Ru(H2O)5(H2CCH2)]2+ (ΔVax and ΔVeq = +6.5 ± 0.5 and +6.1 ± 0.2 cm3 mol-1) are the strongest evidence of this conclusion. The increasing cis-effect series was established according to the lability of the equatorial water molecules and is as follows: F2CCH2 CO < Me2SO < N2 < H2CCH2 < MeCN < H2O. The increase of the lability is accompanied by a decrease of the E°‘ values, but no change was found in the calculated Ru−H2Oeq bond lengths. The increasing trans-effect series, established from the lability of the axial water molecule, is the following: N2 MeCN < H2O < CO < Me2SO < H2CCH2 < F2CCH2. A variation of the Ru−H2Oax bond lengths is observed in the calculated structures. However, the best correlation is found between the lability and the calculated Ru−H2Oax bond energies. It appears, also, that a decrease of the electronic density along the Ru−Oax bond and the increase of the lability can be related to an increase of the Ï€-accepting capability of the ligand. For L = N2, the calculations have shown that the Ru(II)−N2 bond is weak. Consequently, the water exchange reaction proceeds through a different mechanism, where first the N2 ligand is substituted by one water molecule to produce the hexaaqua complex of Ru(II). The water exchange takes place on this compound before re-formation of the [Ru(H2O)5N2]2+ complex.

From our combined experimental and computer modeling study we found a structurally and kinetically well-defined second coordination shell around chromium(III) ions in aqueous solution. Strong hydrogen binding due to polarization of first coordination sphere water molecules leads to a mean coordination number of 12.94 water molecules in the second shell and to short first shell hydrogen−second shell oxygen distances of about 1.4 Å. The experimentally measured exchange rate constant of = (7.8 ± 0.2) × 109 s-1 (ΔH = 21.3 ± 1.1 kJ mol-1, ΔS = +16.2 ± 3.7 J K-1 mol-1) corresponds to a lifetime of 128 ps for one water molecule in the second coordination shell and compares very well with a lifetime of 144 ps as observed from molecular dynamics simulation of a [Cr(H2O)6]3+ complex in aqueous solution. The geometry and the partial atomic charges of [Cr(H2O)6]3+ were determined by density functional theory (DFT) calculations. Water exchange from the second coordination shell to the bulk of the solution proceeds between a H2O sitting in the second shell and an adjacent one which just entered this shell from the bulk. By a small rotation of the first coordination shell water molecule, one of its two hydrogen bonds jumps to the entered water molecule and the one which lost its hydrogen bond leaves the second shell of the [Cr(H2O)6]3+. This associative reaction mode is a model for water exchange between water molecules which are bound by strong hydrogen bonds, as in the case for strongly polarizing 3+ ions such as Al3+ or Rh3+. Furthermore, the exchange phenomenon between second sphere and bulk water involving only two adjacent water molecules is strongly localized and independent of other water molecules of the second shell. In this respect it may be considered as a starting point for a study of water exchange on a protonated metal oxide surface.