An O-bonded sulphito complex, Rh(OH₂)₅(OSO₂H)²⁺, is reversibly formed in the stoppedflow time scale when Rh(OH₂)₆³⁺ and SO₂/HSO₃⁻ buffer (1 <pH< 3) are allowed to react. For Rh(OH₂)₅OH₂₊+ SO₂ □ Rh(OH₂)₅(OSO₂H)²⁺ (k₁/k_-1), k₁ = (2.2 ±0.2) × 10³ dm³ mol⁻¹ s⁻¹, k₋1 = 0.58 ±0.16 s⁻¹ (25°C,I = 0.5 mol dm⁻³). The protonated O-sulphito complex is a moderate acid (K_d = 3 × 10⁻⁴ mol dm⁻³, 25°C, I= 0.5 mol dm⁻³). This complex undergoes (O, O) chelation by the bound bisulphite withk= 1.4 × 10⁻³ s⁻¹ (31°C) to Rh(OH₂)₄(O₂SO)⁺ and the chelated sulphito complex takes up another HSO₃⁻ in a fast equilibrium step to yield Rh(OH₂)₃(O₂SO)(OSO₂H) which further undergoes intramolecular ligand isomerisation to the S-bonded sulphito complex: Rh(OH₂)₃(O₂SO)(OSO₂)^- → Rh(OH₂)₃(O₂SO)(SO₃)⁻ (k_iso = 3 × 10⁻⁴ s⁻¹, 31°C). A dinuclear (μ-O, O) sulphite-bridged complex, Na₄[Rh₂(μ-OH)₂(OH)₂(μ-OS(O)O)(O₂SO)(SO₃) (OH₂)]5H₂O with (O, O) chelated and S-bonded sulphites has been isolated and characterized. This complex is sparingly soluble in water and most organic solvents and very stable to acid-catalysed decomposition

The bis phenoxide forms of (1,2)bis(2-hydroxybenzamido)ethane(I), (1,5)bis(2-hydroxybenzamido) 3-azapentane(II), (1,3)bis(2-hydroxybenzamido)propane(III), and (1,8)bis(2-hydroxybenzamido)3,6-diazaoctane(IV) undergo facile hydrolysis of one of the amide groups (0.02 ≤ [OH⁻]$_T$ (mol dm^-3) ≤ 0.5, 10% MeOH (v/v) + H₂O medium) without exhibiting [OH⁻] dependence. The reactivity trend follows I ∼ II > > III ∼ IV with low activation enthalpy {$25.7\pm 2.8 ≤ \Delta H^{\neq}$(kJ mol^-1) $≤ 64.8\pm 7.0$}). The high negative and comparable values of activation entropy {$-234 \pm 8 ≤ \Delta S^{\neq}$(J K^-1 mol^-1) $≤ −127 \pm 20$} are consistent with closely similar, and ordered transition states which can be assembled by favourably oriented phenoxide groups. The solvent kinetic isotope effect for I, $k^{H2O}/k^{D2O+H2O} \sim 1$ (20 and 50 volume% D₂O), indicates that proton transfer is not involved as a part of the rate controlling process. The observed slowing down of the rate of this reaction for I in the micellar pseudo phase of CTABr also supports the proposed mechanism. Under premicellar conditions, however, rate acceleration is observed, a consequence believed to be associated with the capping effect of the hydrophobic tail of the surfactant cation forming the reactive ion-pair, CTA⁺, (I-2H)²⁻ exclusively in the aqueous pseudo phase.

The kinetics of oxidation of glyoxylic acid (HGl) by Mn^III(salen)(OH₂)$^+_2$ ((H₂salen = N,N'- bis(salicylidene)ethane-1,2-diamine) is investigated at 30.0-45.0°C, 1.83 ≤ pH ≤ 6.10, I = 0.3 mol dm^-3(NaClO₄). The products are identified as formic acid, CO₂ and Mn^II with the reaction stoichiometry, |𝛥[Mn^III]/𝛥[HGl]| = 2. The overall reaction involves fast equilibrium pre-association of Mn^III(salen)(OH₂)$^+_2$ with HGl and its conjugate base Gl⁻ forming the corresponding inner sphere complexes (both HGl and Gl⁻ being the monohydrate gem-diol forms) followed by the slow electron transfer steps. In addition, the second order electron transfer reactions involving the inner-sphere complexes and HGl/Gl⁻ are also observed. The rate, equilibrium constants and activation parameters for various steps are presented. Mn^III(salen)(OH₂)(Gl) is virtually inert to intra molecular electron transfer while the process is facile for Mn^III(salen)(OH₂)(HGl)⁺ (10$^5k_{et} = 2.8 \pm 0.3$ s^-1 at 35.0°C) reflecting the involvement of proton coupled electron transfer mechanism in the latter case. A computational study of the structure optimization of the complexes, trans-Mn^III(salen)(OH₂)$^+_2$, trans-MnIII(salen)(OH₂)(Gl), and trans- Mn^III(salen)(OH₂)(HGl)⁺ (all high spin Mn^III(d⁴) systems), reveals strongest axial distortion for the (aqua)(Gl) complex ; HGl bound to Mn^III centre by the C=O function of the carboxyl group in the (aqua)(HGl) complex facilitates the formation of a hydrogen bond between the proton of the carboxyl group and the coordinated phenoxide moiety ((O-H…O hydrogen bond distance 1.745 Å) and the gem-diols are not involved in H-bonding in either case. A rate comparison for the second order paths: Mn^III(salen)(OH₂)(HGl)/Gl)^+/0 + HGl/Gl⁻ → products, shows that HGl for the (aqua)(HGl) complex is a better reducing agent than Gl⁻ for the (aqua)(Gl) complex ($k_{\text{HG}} \tilde 5 k_{\text{Gl}}$). The high values of activation enthalpy (𝛥H$^\neq$ = 93-119 kJ mol^-1) are indicative of substantial reorganization of the bonds as expected for inner-sphere ET process.