Lena Gustafson

Kinetics and Mechanism for Chloride Anation of Some Platinum(IV) Aqua Complexes in the Presence of Platinum(II))

Chloride anations of PtCl5H2O−, trans-PtCl4(H2O)2 and trans-Pt(CN)4ClH2O− in the presence of PtCl42− have been studied at 50°C in 1.00M perchloric acid medium. It is shown that PtCl5H2O− is formed as the primary reaction product in the two last-mentioned anations. This is not compatible with the generally used mechanism for platinum(II)-catalyzed platinum (IV) substitutions, which gives PtCl62− as the direct reaction product for these two reactions. Other examples of formation of aqua complex intermediates in previously studied platinum(II)-catalyzed platinum (IV) substitutions can be found by examination of data from the literature. The chloride anation of PtCl5H2O− follows the rate law: rate = (k′ + k″ [Cl−]) / (1 + k‴[Cl−]) x [PtCl42−][PtCl5H2O−]The usual interpretation of k‴ as the stability constant for a five-coordinate complex PtCl53− is ruled out by other experiments, which indicate a much smaller stability constant than that obtained from the kinetics. This is also supported by a stopped-flow study at 25°C in 0.5 M perchloric acid medium of the substitution of bromide by chloride in trans-Pt(NH3)4Br2 for large concentrations of entering ligand and in the presence of Pt(NH3)42+. A modified reaction mechanism is suggested which can describe all experimental results. The primary step is the formation of a dimer complex from the platinum(IV) substrate complex and the simple platinum(II) complex, hydrated in the axial positions. This dimer might decompose directly to a platinum(II) complex and a platinum(IV) aqua complex. Alternatively, it might react with the incoming ligand to form a new dinuclear complex, which decomposes to the platinum(II) complex and the substituted platinum(IV) complex. (Less)

Platinum(IV) and bromo aqua complexes. Acid hydrolysis equilibria and kinetics of bromide anation

Abstract The stepwise stabililty constants, Kn for the bromo aqua complexes PtBrn−1(H2O)7−n5−n; n=4, 5, 6, formed in the successive acid hydrolyses of PtBr62−, have been determined to be K6=(.19±0.1) X 103 M−1, K5= (1.0±0.2) X 104 M−1 and K4=(1.1±0.2) X 105 M−1. A bromide assisted mechanism is proposed for the bromide anations of PtBr4(H2O)2 and PtBr5H2O-. Both these reactions are found to be first order with respect to platinum complex. The dependence of the observed rate constant on the bromide concentration can be described by the eqn. Kexp=κ′[Br−]+κ″[Br−]2 The last term is probably due to a reaction path via a bromide associated outer sphere complex, which might also be involved in the path described by the first term. A spectrophotometric method has been used. To suppress the protolysis of the aqua complexes, 0.50 M perchloric acid was used as the ionic medium. The temperature was 25.0°C.

Bromide anation kinetics for some platinum(IV) bromo aqua complexes

Abstract Bromide anations of PtBr 5 H 2 O − in the presence of PtBr 2− 4 or bromine, of trans -PtBr 4 (H 2 O) 2 in the presence of PtBr 2− 4 and of cis -PtBr 4 (H 2 O) 2 in the presence of bromine have been studied in 0.50 M perchloric acid medium at 25 °C. The rate law for the bromide anation of PtBr 5 H 2 O − indicates two parallel reaction paths. The term k′Br − ] 2 corresponds to a bromide assisted mechanism. The second term in the rate expression can be accounted for by assuming that a dimeric intermediate is formed from the platinum(IV) substrate and hydrated PtBr 2− 4 . The formation of the reaction product PtBr 2− 6 is preceded by a substitution of water by bromide in this dimer. A bromide assisted mechanism can also describe the bromide anation kinetics of cis -PtBr 4 (H 2 O) 2 having the rate law The bromide anation of trans-PtBr 4 (H 2 O) 2 in the presence of PtBr 2− 4 occurs by the two-term rate law indicating two parallel paths, k′ corresponding to formation of PtBr 5 H 2 O − and k″ to formation of PtBr 2− 6 . Trans-PtBr 4 (H 2 O) 2 reacts so slowly with bromide in the absence of PtBr 2− 4 that no reaction can be observed. The large difference in bromide anation rates for PtBr 5 H 2 O − and cis -PtBr 4 (H 2 O) 2 compared to trans -PtBr 4 (H 2 O) 2 might indicate that halide assisted anations are possible only for complexes having a halide (or some other effective bridging group) in trans -position to the aqua ligand to be replaced.

Rates and mechanisms for halide anation reactions of platinum(IV) complexes

Abstract The bromide anation of tr-PtCl 4 BrH 2 O − has been studied at 25°C and the chloride anations of tr -PtCl 4 BrH 2 O − and PtBr 5 H 2 O − at 50°C. These reactions proceed via halide assisted mechanisms, compatible with the experimental rate laws: The bromide anation of tr-PtBr 4 CIH 2 O − studied at 50°C gives PtBr 5 Cl 2− via a bromide assisted path andPtBr 2− 6 via an REOA-path. The observed rate law is PtBr 5 Cl 2− reacts subsequently with bromide to form PtBr 2− 6 via a rate-determining (bromide assisted or REOA) formation of an intermediate aqua complex according to the mechanism The observed rate law at 50°C is All reactions have been studied in a 0.50 M perchloric acid medium. A general survey of reactivity correlations and mechanisms for platinum(IV) anation reactions is given. Effects of reductants, bridging and entering ligands, and of the four non-labile ligands in the plane around the platinum atom are quantitatively compared.