Competition Experiments Between Different Ligands

Useful biological information can sometimes be obtained from studies of the competition between halide ions and other ions or small molecules for the same macromolecular binding sites. Formally, the halide NMR excess relaxation rate in a system of two classes of sites upon addition of a second ion or molecule Y competing for the X binding sites may be described by the equation  

When only one class of binding sites is present or all classes of binding sites except one is completely blocked by a strongly binding molecule (in practice such a molecule might be a surfactant like sodium dodecylsulfate - anionic amphiphilic molecules are known to 

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To measure the relative rates of various ligands to replace CO, chemists must turn to competition experiments (see Section 8.8.3). Competition experiments using the coordi-nately unsaturated 16-electron species Ni(CO)a, Fe(CO)4, and Mo(CO)s, and alkenes, phosphines, and amines as ligands show only small differences in product ratios (between 1 and 10). Because these three 16-electron coordinately unsaturated species are not very selective in their reactions, they must all be very reactive. They react with almost any nucleophile with a very low energy of activation. In support of this view, the rate constants for the addition of the new ligands to such unsaturated systems have been estimated to be near 10 M s that is, they are approaching diffusion control. 

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