Thermodynamics of Late-Metal-Alkoxo Bonds

The relative M-X bond strengths in late metal amides and alkoxides have been measured by the studies on reversible M-X/H-X exchange (Equations 4.60 and 4.61). These essentially thermoneutral processes suggested a roughly 1 1 correlation of H-X and L M-X bond strengths, which provide an estimation of relative L,M-X homolytic bond strengths 

Measurements of the strengths of late-metal-alkoxo bonds by theoretical methods - and reaction calorimetry have provided a few benchmarks. These results provide some clearer comparisons between flie strengths of M-0 bonds in early and late metal alkoxides. For example, the Co-0 bond strength in Co(CO) (OH) was calculated to be 55 kcal/mol, and the Rh-0 bond dissociation energy (BDE) in several octaethylporph)rrin rhodium alkox-ide complexes was estimated to be -50-55 kcal/mol by thermodynamic measurements and reactivity studies. These values are much lower than the M-0 bond strengths noted in the previous section on early metal alkoxides. 

CHAPTER 4 COVALENT (X-TYPE) LIGANDS BOUND THROUGH METAL-HETEROATOM BONDS 

Many of the typical reactions that form transition metal-heteroatom bonds can be used to prepare late transition metal alkoxides. - For example, metathetical exchange between late metal halides and alkali metal salts of the corresponding alcohols often forms late metal alkoxides. Complexes that are more reactive than metal halides, such as metal acetates or triflates, are sometimes used when exchanges with halides are slow or reversible (Equation 4.66). Late metal fluorides can also be used for these exchanges, as shown in Equation 4.67.  

In chemistry related to the reactions of alkali metal alkoxides with late metal halide complexes, late metal alkoxides have been formed by the reaction of alcohols with metal halides in the presence of a base. This process has been used to form arylpalladium alkoxides during the coupling of aryl halides with alcohols discussed in Chapter 19 (cross-coupling). In addition, Milstein used an intramolecular version of this process to prepare the metallaoxetane in Equation 4.68.  

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