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Kinetics and Mechanism of Ligand Interchange in Pentacyano-L-osmate(II) Complexes (L = H2O, NH3, N-Heterocyclic Ligands)

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journal contribution
posted on 15.12.2001, 00:00 by Leonardo D. Slep, Pablo Alborés, Luis M. Baraldo, José A. Olabe
The new complex, K3[Os(CN)5NH3]·2H2O, a convenient precursor for the pentacyano-L-osmate(II) series, was prepared and characterized by chemical analysis, cyclic voltammetry, and IR and UV−vis spectroscopies. By controlled aquation in weakly acidic medium, the [Os(CN)5H2O]3- ion was generated. Weak absorptions in the UV region for L = H2O, NH3, and CN- were found at 287, 272, and 240 nm, respectively, and were assigned to d−d transitions, in terms of a model for tetragonally distorted ions also valid for the members of the iron and ruthenium series. The kinetics of the formation and dissociation reactions of the [Os(CN)5L]n- ions, L = pyridine (py), pyrazine (pz), N-methylpyrazinium (mpz+), etc., were studied. At 25.0 °C, the formation rate constants for the neutral ligands pz and isonicotinamide were ca. 0.13 M-1 s-1 and slightly increased for L = mpz+ and decreased for isonicotinate. The enthalpies of activation were ca. 22.0 kcal mol-1, independently of the entering L, and the activation entropies were all positive, ca. 11−13 cal K-1 mol-1. The dissociation reactions showed a saturation rate behavior of kobs (s-1) as a function of the concentration of the scavenger ligand. The specific dissociation rate constant at 25.0 °C was 1.06 × 10-7 s-1 for L = NH3 and around 10-9 s-1 for py, pz, and mpz+ (extrapolated to 25.0 °C from values measured in the range 60−95 °C). These small values are associated with high activation enthalpies (range 30−35 kcal mol-1) and positive activation entropies (range 10−20 cal K-1 mol-1). The evidence for both the formation and dissociation processes shows that dissociative mechanisms are operative, as for the iron and ruthenium analogues.