Reaction ligand exchange

Ligand exchange reactions with labeled carbon monoxide performed by Basolo and Wojcicki (32) show that the carbonyls V(CO)ft, Cr(CO)6, Mn2(CO)io, and Fe(CO)s exchange CO groups only slowly, whereas Ni(CO)4 and Co2(CO)8 exchange rapidly. The kinetic lability of nickel carbonyl can in part be attributed to the thermodynamic weakness of the Ni—C bonds. The essential point, however, is that the exchange rate is independent of carbon monoxide concentration which supports a dissociative mechanism. 

The Arrhenius activation energy for this rate-determining dissociation is 13 kcal/mole (in toluene solution). This value agrees well with the activation energy of the thermal and photochemical decomposition of nickel carbonyl in which the above dissociation (Eq. 16) is also rate determining (33,34,35). 

As a consequence of enhanced w-bond stabilization of the remaining Ni—C bonds, the exchange rate for carbon monoxide and additional phosphines decreases as x in Ni(CO)4 xL increases (37, 32). This explains why it is frequently observed that not all CO groups can be substituted by phosphines. A total replacement should only be possible if the ligands have comparable o- and Tr-bonding abilities with carbon monoxide (e.g., the halides of phosphorus or some isonitriles). The first steps of the ligand exchange may be represented by Eqs. (16-19)  

If TT-bond stabilization is accepted as being the driving force for CO replacement, then one would predict that simple olefins should not react with nickel carbonyl to yield stable substitution products. Successful exchange 

REACTIONS OF NICKEL CARBONYL WITH OLEFINIC COMPOUNDS YIELDING ISOLABLE COMPLEXES 

The outer ligands in M3Q4L9 (type-I) and M3Q7L6 (type-II) clusters can be easily replaced through nucleophilic substitution reactions. These reactions are discussed separately for each cluster type. 

Main-group element chemistry is replete with ligand exchange reactions or metatheses. We will encounter a fair number of such reactions in this book, a few examples being as follows  

For a mechanistic discussion, we will choose the last reaction. A reaction pathway may not be obvious at this point, based on the mechanistic paradigms we have discussed so far. Clearly, a series of simple D and A reactions, whereby oxide and fluoride ligands detach from one central atom and reattach to another one, are unreasonable, given the covalent nature of the molecules involved. An A reaction, however, is still a promising starting point  

The oxo-bridged P+-0-P intermediate may now react in a number of different ways. A fluoride could depart from the anionic iodine and reattach to the cationic phosphorus center, as shown below  

A second fluoride could then attack the neutral phosphorus center, leading to the final products PF5 and IOF5  

Other pathways are also conceivable for the initially formed oxo-bridged pathway. For example, a 1,3-shift of a fluoride provides quick access to the neutral intermediate F4P-O-IF6  

Organic reactions mediated by organometallic complexes often begin with coordination to a metal or metals, where subsequent reactions ensue. This binding leads to activation, reminiscent of the activation steps we examined when catalysis was discussed in Chapter 10. Therefore our first discussion is the manner in which ligands coordinate to a metal and exchange between metals. 

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The ligand exchange reaction of tin halides with bis(trifliioroinethyl)iner-cury has been used to prepare trifluoromethyltin halides [6 7] (equation 5)... 

Ligand exchange reactions can be used to prepare perfluoroalkylzinc compounds Solvated trifluoromethylzinc compounds can be synthesized via the reaction of dialkylzincs with bis(trifluoromethyl)mercury [36] (equation 27) A similar exchange process with bis(trif]uorometliyl)cadinium and diraethylzinc gives a mixture of tnfluoromethylcadmium and zinc compounds [77]... 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

Trends in reactivity for ligand-exchange reactions of octahedral metal carbonyls. G. R. Dobson, Acc. Chem. Res., 1976, 9, 300-306 (48). 

Anomalies in ligand exchange reactions for platinum(II) complexes. D. S. Martin, Inorg. Chim. Acta, Rev., 1967,1, 87-97 (40). 

Zirconium complexes, 3, 363-440 acetylacetone, 2,372,377 ligand exchange reactions, 2, 381 rearrangement, 2, 383 alkyl... 

On the other hand, however, using such mixtures can lead to ligand-exchange reactions 1S0) during the initiation or previous reactions ... 

Casey, W. H. and Ludwig, C. (1995). Silicate mineral dissolution as a ligand-exchange reaction. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. Brantley, eds), Mineralogical Society of America Washington, DC, Reviews in Mineralogy 31, 87-117. 

This simple ligand exchange reaction is less likely for [Ni ( 11)3. As Figure 18-12 shows, when one... 

A discussion of ligand exchange reactions of organometallic compounds associated with oxidation-reduction processes leading to free-radical formation will be found in Volume 14 (Free-radical polymerization). 

Thiolysis by NaSR under the same conditions gave N3P3F5SR (R = Me or Ph). Ligand-exchange reactions between a series of organo-substi-tuted cyclotriphosphazatrienes have been studied and their synthetic potential demonstrated. Typical of these reactions is ... 

Razi, M.T., Otiko, G. and Sadler, P.J. (1983) Ligand exchange reactions of gold drugs in model systems and in red cells. ACS Symposium Series, 209, 371-384. 

Snyder, R.M., Mirabelli, C. and Crooke, S.J. (1986) Cellular association, intracellular distribution, and efflux of auranofin via sequential ligand exchange reactions. Biochemical Pharmacology, 35, 923-932. 

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