Rate Laws, and Dominant Species

A mechanism is the road map of a chemical reaction. It reflects the sequence of reaction steps and the composition of the intermediates. Mechanisms are postulated based on spectroscopic studies, rate studies, product analyses, and isotopic labeling. Certain types of reactions may be unlikely based on energy considerations (e.g., the Woodward-Hoffman Rules or the 16 and 18 Electron Rule Y There is never enough information to prove one mechanism to the exclusion of all others, nor is there usually enough data to uniquely determine all the rate constants for the elementary steps of a proposed mechanism. Some possible paths can be shown to be unimportant. In a typical case, a few rate or equilibrium 

A rate law describes the dependence of the rate of product formation (or substrate disappearance) at steady state on the concentrations of catalyst, cocatalysts, substrates, and ligands. A fact that is not generally appreciated is that the functional form of the rate law often depends on the range of experimental variables investigated, as will be shown subsequently. 

Spectrophotomeric measurements and nmr spectra show that (C2H4)2PtL2 does indeed form in solution in the presence of added C2H4 and that it provides the intermediate for the relatively rapid exchange of free and coordinated ethylene which occurs in solution at room tem-perature. 

In a stoichiometric reaction, the rate law tells the difference between the composition of the major initial species in solution and the composition of the transition state of the rate-determining step (abbreviated CTSRDS). Thus, under conditions where (C2H4)2PtL2 is the major Pt(0) species. 

CTSRDS = (C2H4)2PtL2 - C2H4 + RAT = (RAD(C2H4)PtL2 

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