Kinetics for complex formation between Pd(H2O)(4)(2+) and formic, butyric, DL-lactic, 2-methyllactic, methoxyacetic, malonic, succinic, oxydiacetic, L-malic, and citric acids has been studied in an aqueous acidic medium by use of variable-temperature and -pressure stopped-flow spectrophotometry. Kinetics traces for reactions between the metal ion and formic, butyric, lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acids can be described by single exponentials, which are assigned to the formation of monodentate complexes : Pd(H2O)(4)(2+)+RCOOH reversible arrow Pd(H2O)(3)OOCR+H3O+(k(1), k(-1)). Equilibrium constants K-1 for lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acid reactions calculated from spectrophotometric equilibrium measurements and from kinetics (K-1=k(1)/k(-1)) are in good agreement. There is a linear correlation between the stability constants beta(1) of the carboxylato complexes and the first dissociation constants K-al of the carboxylic acids as expressed by log beta(1) (0.48+/-0.03)pK(al)+(2.1+/-0.1). The formation rate constants k(1) are insensitive to the basicity and steric properties of the carboxylic acids at 25 degrees C, due to an excellent isokinetic relationship between Delta H-1 double dagger and Delta S-1 double dagger with an isokinetic temperature of 292 K, suggesting also that all of the carboxylic acids react via the same mechanism. Rate constants k(-1) are correlated with pK(al) of the entering carboxylic acids according to log k(-1)=(0.47+/-0.06)pK(al)-(0.7+/-0.2), indicating that the weaker the carboxylic acid, the larger k(-1). These facts together with the observation that a weak carboxylic acid is prone to form a strong complex as shown by the correlation between log beta(1) and pK(al) are interpreted in term of a proton-assisted reaction mechanism which is further supported by the ionic strength dependence of the rate constant k(-1), consistent with a direct attack of an oxonium ion on Pd(H2O)(3)OOCR(+) for the reverse reaction. High-pressure stopped-flow measurements at 25 degrees C give activation volumes Delta V-1 double dagger=-4.9+/-0.2 cm(3) mol(-1), Delta V(-1)double dagger=-2.7+/-0.5 cm(3) mol(-1) for malonic acid, and Delta V-1 double dagger=-3.5+/-0.2 cm(3) mol(-1), Delta V(-1)double dagger=-1.9+/-0.4 cm(3) mol(-1) for citric acid, respectively. Existing data so far for Pd(H2O)(4)(2+) complex formation reactions obey a linear correlation between Delta V-1 double dagger and partial molar reaction volumes Delta V-1 double dagger according to Delta V-1 degrees=(0.92+/-0.04)Delta V-1 degrees-(2.2+/-0.2) cm(3) mol(-1), the slope of 0.92 indicating that bond-making between palladium and the entering ligands largely dominates the formation of the transition state. There are no linear correlations between Delta V-1 double dagger and partial molar volumes VL Of the entering ligands, as has been claimed in previous literature for related reactions, or between Delta V-1 double dagger and Delta S-1 double dagger.