Nickel carbonyl, exchange reactions

Nickel carbonyl has been shown to have a very high retention— 98.7%—both in the pure hquid and as 10% solution in n-heptane. It was argued that this represents the results following essentially complete isotopic exchange. Since the exchange of CO with Ni(CO)4 is known to occur quite rapidly by a dissociation mechanism, reformation of nickel carbonyl following the nuclear reaction would proceed rapidly by the reaction... 

A3-Pyrroline formation with carbonyl insertion also occurs during the reaction of /V-sulfinylarylamines with diphenylcyclopropenone in the presence of nickel carbonyl (Scheme 36).64 Phenyl isocyanate does not give a pyrroline product under these reaction conditions, hence the SO-CO exchange probably occurs within an intermediate metallocycle. The reaction... 

Basolo and Wojcicki26 have reported that at 0 °C in toluene solution Ni(CO)4 exchanges with radioactive CO by a first-order reaction with k = 7.5 x 10-4 sec-1. Heck24 reports that under the same conditions nickel carbonyl reacts with triphenyl phosphine with k = 4.3xl0-4 sec-1. The fact that both processes are first-order and have approximately the same rate coefficient was originally in-... 

Knowing all these facts, especially the difficult access to fluorophosphines and the poor donating abilities of phosphorus trifluoride (5, 6), we decided to use another approach, which readily led to a number of coordination compounds with fluorophosphine ligands—namely, the fluorination of chlorophosphines already coordinated to the transition metal, where the 3s electrons of phosphorus are blocked by the complex formation. There was no reaction between elemental nickel and phosphorus trifluoride, even under extreme conditions, whereas the exchange of carbon monoxide in nickel carbonyl upon interaction with phosphorus trifluoride proceeded very slowly and even after 100 hours interaction did not lead to a well defined product (5,6). 

Furthermore, 2,2-difluoro-3-hydroxyesters are readily obtained from ClCFjCOOMe and carbonyl compounds by electrolysis in a one-compartment cell using a sacrificial zinc anode and a nickel complex as catalyst. The catalytic cycle for this reaction is shown in Scheme 9 with nickel zinc exchange being a key step. In this process, the CHjCyDMF solvent (9 1) system leads to suppression of undesired Claisen condensation and an increase in the yield of 2,2-difluoro-3-hydroxyester formation. It is notable that high yields are obtained even with ketones and enolizable aldehydes, which are not good participants in the Reformatsky reaction alternative for producing these substances. 

The substitution of CO in metal carbonyls by olefinic and acetylenic compounds is one of the chief methods for preparing tt complexes of transition metals. Unfortunately this procedure fails almost completely when applied to nickel carbonyl, and this may be one of the reasons why until recently no tt complexes of nickel with olefinic or acetylenic ligands were known. The reasons for this behavior of nickel carbonyl will become clearer, if both its electronic structure and the mechanism of the ligand exchange reactions are considered. 

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. 

Excellent examples of metal exchange reactions are provided by complexes of tetraphenylcyclobutadiene. When either iron pentacarbonyl or nickel carbonyl is reacted with tetraphenylcyclobutadiene palladium bromide, the corresponding metal is exchanged with the release of palladium(O) metal. The reaction proceeds only in aromatic solvents and seems to be generally applicable to a variety of transition metal carbonyls. As illustrated in Fig. 6-8, the first step is proposed to involve the formation of uncomplexed tetraphenylcyclobutadiene, a mixed metal halide carbonyl, carbon monoxide. 

Complex [6-37] also undergoes a similar exchange reaction with nickel carbonyl and cobaltocene, equations (6-71) and (6-72). This type of exchange reaction can be given by the general equation (6-73). 

It should be noted that cyclobutadiene always replaces carbon monoxide in reactions with metal carbonyl derivatives. Yields of product parallel the known rate of exchange of CO in the starting carbonyl 184). Highest yields of ligand transfer products are attained with nickel and cobalt carbonyls which are known to very rapidly exchange their CO groups by a D-type mechanism 185-188). Lowest yields have been reported with Mo and W complexes, the carbonyls of which exchange with CO very slowly 188). 

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