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Abstract Kinetic study of formation of 1:2 complex between chromium (III) and nicotinic acid in weak acid aqueous solution was investigated spectrophotometrically at λ=575 nm. The studies indicated that the reaction between chromium(III) and nicotinic acid in weak acidic aqueous solutions involves an ion pair is the first product formed in a very fast step. The nonlinearity of log (A∞ - At) versus time plots obtained, are indicative of a complex reaction, not a single process. The independence of the rate constants of the reaction on chromium concentration confirmed the assumption of two consecutive steps. In the first (slow) a bond is formed between Cr and the oxygen atom of the carboxylate group (compound A). In the second step, a bond is formed between the second oxygen atom of the carboxylate group and Cr forming a four-membered ring (compound B). The formation of compounds A and B is the slow step followed by forming chromium nicotinate complex [Cr(nic)2(H2O)3OH] in a fast step. This is due to when one molecule of nicotinic acid enters the inner coordination sphere of the chromium ion and compound B is formed, the electron density on Cr(III) is increased, and the remaining water ligands are labilized very easily. Both steps are followed by pH decrease suggesting that proton release is taking place. The different factors affecting the rate of reaction were studied.- The kinetics oxidation of chromium (lII)-dinicotinate complex by periodate were investigated. The reaction was first-order in chromium(III)-nicotinate complex, accelerated with increasing periodate concentration, pH, temperature and ionic strength of the reaction medium. The oxidation of chromium (III)-nicotinate complex by periodate is assigned an inner-sphere mechanism. A common mechanism for the oxidation of ternary chromium(III) complexes by periodate is proposed and is supported by the excellent isokinetic relationship between ΔH* and ΔS* values for this reaction. 3- The kinetics of substitution of aquo-ligand from hydroxopentaaquo Ferric (III) by N-Phthaloylglycine in aqueous medium has been studied spectrophotometrically. The rate law involving the formation of ion-pair has been established in the pH 2.8 and 50ºC. The reaction between [Fe(H2O)5 OH]2+ and Ph-GLy involves outer sphere association between the two reacting species followed by an associative interchange process in which both bond-breaking and bond-making are equally important. The different factors affecting the rate of reaction were studied. |