The half-sandwich complexes [Cp*Co(bpy)(H2O)](PF6)(2), (M = Rh, Ir), [L(2-4)Ru(bpy)Cl]Cl, and [Cp*lr(OH)(3)IrCp*]OH.11H(2)O were prepared and characterized (Cp* = L(1) = eta(5)-pentamethylcyclopentadienyl anion; bpy = 2,2'-bipyridine; L(2) = eta(6)-benzene; L(3) = eta(6)-cymene; L(4) = eta(6)-hexamethylbenzene). X-ray structure analyses of [Cp*Rh(bpy)Cl]ClO4 (=C20H23Cl2N2O4Rh; orthorhombic, Pmn2(1); a = 12.720(5), b = 8.141(5), c = 10.504(5) Angstrom; Z = 2; R(w) = 0.0352), and [Cp*Ir(OH)(3)IrCp*]OH.11H(2)O (=C20H56Ir2O15; orthorhombic, Pnma; a = 17.43(2), b = 17.93(2), c = 10.52(1) A; Z = 4; R(w) = 0.0317) were carried out. Complexes ECp*Rh(bpy)Cl]ClO(4)p and [Cp*Ir(bpy)Cl]ClO4 and isostructural. The dinuclear triply OH-bridged complex [Cp*Ir(OH)(3)IrCp*]OH.11H(2)O is isostructural with [Cp*Rh(OH)(3)RhCp*]OH.11H(2)O (Nutton et al. J. Chem. Sec., Dalton Trans. 1981, 1997) with the two Cp* ligands being orientated in a coplanar fashion. The pK(a)'s (298 K, I = 0.5 M (NaClO4)) of the coordinated water in the monoaqua species [Cp*M(bpy)(H2O)](2+) were found to be 8.4 (Co), 8.2 (Rh), and 7.5 (Ir). The coordinated water in the species [L(2-4)RU(bpy)(H2O)](2+) is slightly more acidic (pK(a) = 6.9 (L(2)) 7.2 (L(3)), 7.3 (L(4))). The vis absorption characteristics of the complex cations [Cp*(bpy)X](2+/+) (M = Co, Rh, Ir) and [L(2-4)Ru(bpy)X](2+/+) are reported for X = H2O, SCN, I, Br, N-3, thiourea, N-methylimidazole. Stopped-flow spectrophotometry was used to study the anation kinetics of the species [Cp*M(bpy>(H2O)](2+) (M = CO, Rh, Ir) and [L(2-4)Rh(bpy)(H2O)](2+) in aqueous solution at pH 4.8 for a variety of monodentate anionic and neutral ligands X at variable temperature. The kinetics follow a second-order rate law, rate = k(x)[X][complex]. On the basis of the Eigen-Wilkins mechanism, the second-order rate constants k(X) were corrected for outer-sphere complex formation to obtain the rate constants for the interchange step, k(i), according to ki = k(X)/K-os (K-os was calculated). Rate constants k(i) are nearly independent of the nature of the entering ligand X. The data for k(i)(average) (=mean of k(i) for all of the nucleophiles X studied) thus obtained range from k(i)(average) = 0.068 +/- 0.038 s(-1) for [L(2)Ru(bpy)(H2O)](2+) to k(i)(average) = 1590 +/- 760 s(-1) for [Cp*Rh(bpy)(H2O)](2+). For the homologous series [Cp*M(bpy)(H2O)](2+) (M = Co, Rh, Ir) the order for k(i)(average) (s(-1)) is found to be Co:Rh:Ir = 0.60:1590:219 at 293 K. The data obtained for k(i)(average) can be taken as a good approximation for the rate of water exchange; i.e., k(i)(average) kex. O-17-NMR techniques were used to study the water exchange in [Cp*Rh(H2O)(3)](2+) and [Cp*Ir(H2O)(3)](2+) at variable temperature and pressure. The kinetic data for k(ex)(298) (s(-1)), Delta H-double dagger (kJ mol(-1)), Delta S-double dagger (J k(-1) mol(-1)), and Delta V-double dagger (cm(3) mol(-1)) are 1.6 x 10(5), 65.6, +75.3, and +0.6 for [Cp*Rh-(H2O)(3)](2+) and 2.53 x 10(4), 54.9, +23.6, and +2.4 for [Cp*Ir(H2O)(3)](2+). The ratio k(ex)(Rh)/k(ex)(Ir) for the species [Cp*M(H2O)(3)](2+) is very close to the ratio k(i)(average)(Rh)/k(i)(average)(Ir) for the species [Cp*M(bpy)(H2O)](2+). The kinetic findings support the operation of an (dissociative) interchange mechanism (I-(d) for the anation of the species [LM(bpy)(H2O)](2+) (M = Cb, Rh, Ir; L = L(1)-L(4)) as well as for the water exchange in [Cp*M(H2O)(3)](2+) (M = Rh, Ir). The rate-enhancing effect of pi-arene ligands L on the water exchange in the half-sandwich cations [LM(H2O)(3)](2+) and [LM(bpy)(H2O)](2+) is discussed.
