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Rates of Cleavage of Halogen-Bridged Complexes of Platinum (11) Ralph G. Pearson and Marie1 M. Muir' Contribution f r o m the Department of Chemistry, Northwestern University, Evanston, Illinois. Received January 24,1966 Abstract: The rates of cleavage of some halogen-bridged binuclear complexes of Pt(I1) with amines were followed in several nonaqueous solvents. The rate law was the same as that which has been observed for substitution reactions in square-planar complexes, kobsd= kl k z [ Y ] . The mechanism proposed for the cleavage reactions is very
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similar to the mechanism for reactions of square-planar complexes. The rate-determining step is the bimolecular attack by either nucleophile or solvent to cleave the first bridge, followed by rapid cleavage of the second bridge. The rates of cleavage reactions are much faster than the rates of substitution reactions in analogous mononuclear complexes. The strength of the Pt-X bond in the bridge must be considerably weaker than in a mononuclear complex.
T
he complexes of Pt(I1) which have received the most attention have been mononuclear complexes, in which a single platinum atom is surrounded by its ligands. A considerable amount of work has been done on the mechanisms of reactions of such mononucle-ir complexes, in particular, on the mechanism of substitution reactions of the square-planar complexes of Pt(II).2 The kinetics of these reactions obey a twoterm rate law
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rate = (kl kz[Y])[complex] (1) where Y is a nucleophile. This has been interpreted as two simultaneous reactions, one reaction with the solvent as the incoming nucleophile and the other with Y as the attacking n ~ c l e o p h i l e . ~When the kinetics of these reactions are followed at large excess concentrations of Y, first-order behavior is observed. If the rate constants, kobsd,for these pseudo-first-order reactions are plotted as a function of the concentration of Y, a straight line is obtained. The intercept of this line is kl, the rate constant for reaction of the complex with solvent, and the slope of the line is k2, the second-order rate constant for reaction of the complex with Y. The value of kl should remain constant for a given solvent as Y is varied, so that if kobsd is plotted against [Y] for several different nucleophiles, a family of lines with a common intercept and different slopes is obtained. Much less attention has been given to binuclear complexes, although such complexes have been known for some time.5 It seemed of interest to study the rates of cleavage reactions of some binuclear complexes and t o compare the mechanism of cleavage with the mechanism of reaction of the similar mononuclear complexes of Pt(I1). For this purpose the halogen-bridged complexes were selected, since they undergo cleavage more readily than thio- or arsenido-bridged complexes.6 (1) National Science Foundation Predoctoral Fellow, 1962-1965. (2) F. Basolo and R. G. Pearson, "Mechanisms of Inorganic Reactions," John Wiley and Sons, Inc., New York, N. Y., 1958, pp 172-212. (3) F. Basolo, H. B. Gray, and R. G. Pearson, J . A m . Chem. Soc., 82, 4200 (1960). (4) H. B. Gray, ibid., 84, 1548 (1962). (5) (a) I
