Variation of Substitution Rates with Metal Ion

The variation in rates of substitution with different metal ions has been an area of interest for many years. This information on the qualitative level is useful for synthetic purposes and for studies on equilibrium properties. It gives an indication of the conditions required for a particular preparative procedure or of how long one must wait for a system to reach equilibrium. The wide range of rates for the bansition-metal ions has been of considerable interest, both to rationalize the relative rates and to apply these rationalizations to the probable reaction mechanism. This topic has been the subject of several recent reviews.  

Since the pioneering work of Taube and co-workers, shortly followed by Plane and Hunt, and Swift and Connick, one of the most widely studied reactions for bansition-metal complexes has been that of water solvent exchange, using either OHj or 

The results of such studies are summarized in Table 3.13. The NMR studies prior to about 1970 should be viewed with cautirm, and most of the early work has been repeated with modem instrumentation and methods of analysis. For the M(OH2)5 OH) ions, these studies determine kK, where is the acid dissociation constant for the parent M(OH2)g ion. The AH° and AV° of often must be approximated, and this produces some uncertain in tte activation paramet for the M(OH j(OE ions. 

A noteworthy feature of the data in Table 3.13 is tte wide range of rate constants for the 2+ ions, k varies ftom 3.9x10 s for Pl(II) to 4.4x10 s for Cu(II) and for the 3+ ions, k varies from l.lxKT s for Ir(III) to 1.8x10 s for Ti(III). The following sectitms present attempts to explain these variations and their mechanistic implications. 

Taube was the first to offer an explanation for the variable lability of these metal ions by classifying them qualitatively on the basis of their reactivity. 

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