Hydridonickel complexes

The beneficial effect of hydrogen halides in nickel-catalyzed carboxylations was also explained as forming a hydridonickel complex as the true catalyst.155... 

Nickel tetracarbonyl is known to dissociate into the more reactive tricarbonyl readily [step (1)] and this species is known to react readily with a variety of halides by oxidative addition presumably as shown in steps (2) and (3). Subsequent loss of CO would give an equilibrium mixture of the four complexes shown in (3). Step (4) is the well-known carbon monoxide insertion reaction. The acylnickel complex formed in this step then may re-ductively eliminate acid halide [step (5)], which then alcoholizes [step (6)] or it may react directly with alcohol to form ester and a hydridonickel complex (7), which then reacts with CO and decomposes to nickel tricarbonyl and HC1 (8) ... 

Coupling of the /(3-crotonyl [1-methylallyl, CH2 = CH—CH(CH3)—.] group and ethylene, followed by a /i-hydrogen abstraction from the resulting alkyl, gives 1,4-hexadiene and regenerates the hydridonickel complex. 

A dinuclear hydridonickel complex [Ni(/x-H)(dippe)]2 [dippe = 1,2-bis(diisopropylphosphino)ethane] has high reactivity toward thiophene, benzoth-iophene, and dibenzothiophene under ambient conditions to give thianickella-cycle complex (Scheme 3.61) [122]. Moreover, smooth desulfurization of 4,6-dimethyldibenzothiophene can be achieved to give 3,3 -dimethylbiphenyl at 90°C under atmospheric hydrogen. 

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

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