Kinetics and Mechanism of Ligand Interchange in Pentacyano-L-osmate(II) Complexes (L = H2O, NH3, N-Heterocyclic Ligands)
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- 作者：Leonardo D. Slep ; Pablo Alboré ; s ; Luis M. Baraldo ; and José ; A. Olabe
- 刊名：Inorganic Chemistry
- 出版年：2002
- 出版时间：January 14, 2002
- 年：2002
- 卷：41
- 期：1
- 页码：114 - 120
- 全文大小：87K
- 年卷期：v.41,no.1(January 14, 2002)
- ISSN：1520-510X
文摘
The new complex, K3[Os(CN)5NH3]·2H2O, a convenient precursor for the pentacyano-L-osmate(II) series, was preparedand characterized by chemical analysis, cyclic voltammetry, and IR and UV-vis spectroscopies. By controlledaquation in weakly acidic medium, the [Os(CN)5H2O]3- ion was generated. Weak absorptions in the UV region forL = 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. Thekinetics 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 pzand 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 entropieswere 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 Cwas 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 valuesmeasured in the range 60-95 C). These small values are associated with high activation enthalpies (range 30-35kcal mol-1) and positive activation entropies (range 10-20 cal K-1 mol-1). The evidence for both the formationand dissociation processes shows that dissociative mechanisms are operative, as for the iron and rutheniumanalogues.
C, the formation rate constants for the neutral ligands pzand 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 entropieswere 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 Cwas 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 valuesmeasured in the range 60-95 C). These small values are associated with high activation enthalpies (range 30-35kcal mol-1) and positive activation entropies (range 10-20 cal K-1 mol-1). The evidence for both the formationand dissociation processes shows that dissociative mechanisms are operative, as for the iron and rutheniumanalogues.