The Rate Law for Associative Substitutions

The rate behavior of associative ligand substitutions of square-planar complexes reflects the two pathways that often occur in parallel with and without solvent assistance. Thus, the corresponding rate law provided in Equation 5.8 contains two terms, one of which tjrpically corresponds to the rate constant for associative substitution with solvent assistance and one of which typically corresponds to the rate constant for associative substitution without solvent assistance. Often a plot of versus [Y] appears like the plot 

For example, the rate law for the replacement of chloride in [Pt(dien)Cl] by thiourea, thiocyanate, iodide, azide, pyridine, nitrite, or Cl in aqueous solution (Equation 5.9a) is -d[Pt(dien)Cl]/dt = [Pt(dien)Cl], in which = k + k [Y], The reaction of Hl-labeled 

Dependence of the Rates on the Incoming Ligand, the Departing Ligand, and the Metal Center 

The dependence of the observed rate constant on the identity of the departing ligand tends to be less than the dependence on the property of the incoming nucleophile, but is large enough to be easily detected. This dependence correlates with the strength of the metal-ligand bond. For the reaction of Pt(dien)X with pyridine in Table 5.4, this dependence follows the trend X = NOj H,0 CP Br P Nj SCN NO/ CN. Since these substitutions are associative, this trend reflects the extent to which the M-X 

These relative rates fit the general trend that reactions of first-row metal complexes tend to be faster than reactions of their second-row analogs, which tend to be faster than reactions of their third-row congeners, but the precise origin of this effect in associative ligand substitutions has been rationalized in more than one way. One text has attributed this trend for associative ligand substitution as reflecting the relative propensities of the metals to form five-coordinate, 18-electron complexes. In a few cases, five-coordinate 

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