Kinetics and mechanism of substitution reactions of (dimethylsulfoxide)pentaamminecobalt(III) ion in dimethylsulfoxide solvent: complications due to redox and conjugate base processes

Simple anation and solvent exchange reactions of Co(NH3)5DMSO3+ in DMSO are accompanied by reactions in which the conjugate base of this complex undergoes either internal redox to cobalt(II) or relatively rapid substitution. These conjugate base reactions are eliminated by addition of H+, although a minor redox pathway persists within ion pairs of Co(NH3)5DMSO3+ with chloride or bromide. The latter pathway is evidently not mechanistically related to the solvent exchange or anation reactions, which proceed by dissociative interchange (Id) according to the activation parameters (volume of activation = +10 cm3 mol−1 for DMSO exchange; enthalpy of activation = 123, 121, and ∼126 kJ mol−1 for DMSO exchange and bromide and chloride anation respectively). Enthalpies of activation for solvent exchange and for the limiting anation rate are shown to be better criteria of mechanism than the corresponding rate coefficients.

Phosphato complexes of cobalt(III). V. The anation of diaquobis(ethylenediamine) cobalt(III) by orthophosphates

The anation reaction of [Co en2(OH2)2]3+ by PO43-, and their protonic analogues, is shown to proceed in two kinetic steps. The first is the rapid equilibrium formation of ion-pairs, followed by a slow substitution step. Ion-pair formation causes a significant shift in the cis-trans equilibrium position at pH 7. The anation rate exhibits a sharp maximum around pH 7, and kinetic studies demonstrate that the anation proceeds predominantly via the trans-[Co en2OH.OH2]2+.HPO2-4ion-pair. The rate of inner-sphere substitution from this ion-pair (k = 0.255 min-1 at 47.9�, ΔH? = 30.0 � 0.6 kcal mole-1 =, ΔS? = +(19 � 3) e.u.) is 10-20 times slower than the estimated rate of water exchange with trans-[Co en2OH.OH2I2+. Specific structures are assigned to the ion-pairs and these structures influence rates of substitution. At high phosphate concentrations, there is spectral evidence for the slower formation of diphosphate complexes. The relevance of these kinetic observations to the generation and preparation of phosphato complexes is discussed.