Rhodium-alkoxo complex

Finally, reaction of the discrete rhodium-alkoxo complex containing a tethered olefin shown in Equation 9.83 has been shown to undergo insertion of the alkene into the metal-alkoxo bond, followed by p-hydrogen elimination. This mechanism has also been shown to occur by a stereochemical labeling study in which the stereochemistry of the olefinic product reveals the mode of addition. The stereochemistry of the product from reaction of the alkoxo olefin complex containing deuterium in the trans position of the coordinated alkene shows that a syn addition of the rhodium and oxygen occurred across the carbon-carbon bond. ... 

For rhodium and iridium compounds alkoxo ligands take over the role of the basic anion. Using /z-alkoxo complexes of ( -cod)rhodium(I) and iridium(I)— formed in situ by adding the /r-chloro bridged analogues to a solution of sodium alkoxide in the corresponding alcohol and azolium salts—leads to the desired NHC complexes even at room temperature [Eq. (10)]. Using imidazolium ethoxyl-ates with [(r " -cod)RhCl]2 provides an alternative way to the same complexes. By this method, it is also possible to prepare benzimidazolin-2-ylidene complexes of rhodium(I). Furthermore, an extension to triazolium and tetrazolium salts was shown to be possible. ... 

Similarly, reference is made to another more recent review (41) of the author s contributions to the chemistry of rhodium, iridium, rhenium, osmium and ruthenium alkoxo, and hydroxo complexes, following a similar earlier publication (40) on the alkoxo derivatives of platinum metals. Reference may also be made to the first X-ray structural study (265, 266) of a siloxy derivative, [(cod)Rh(/i-OSiMe)3]2. 

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. 

---