Nickel hydrolytic reactions

The reactions between some metallic salts, ammonium salts, and ammonia will be taken up briefly. Many of the bivalent metals such as nickel, magnesium, etc., form hydroxides insoluble in water but soluble in solutions of ammonium salts. The generally accepted explanation for the solubility in solutions of ammonium salts or for the non-precipitation by ammonia, if ammonium salts are present, is that the ammonium ion of the ammonium salts drives back or represses the electrolytic dissociation of the ammonium hydroxide so that the hydroxide ion is not present in sufficient concentration to exceed with the metal ion the solubility product of the metal hydroxide. The new explanation depends upon hydrolytic reactions and equilibria as outlined. 

Ordinarily, a catalyst is not limited to a single reaction. Thus finely divided platinum catalyzes the hydrogenating reactions just as well as the oxidizing reactions HCl catalyzes hydrolytic reactions just as well as polymerizations as to nickel, the experiments of Sabatier and Senderens have shown that it... 

Although more hydrolytically sensitive than the phosphine boranes, diorganochlorophosphines can be more accessible than diorganophosphines and are not pyrophoric. Thus, the reaction of a chlorophosphine with an aryl halide or aryl triflate in the presence of zinc as a reducing agent and (DPPE)NiCl2 as catalyst provides a convenient procedure for P—C coupling (Equation (49)).150 A related nickel-catalyzed process driven by electrochemical reduction has also been reported 151... 

A3-Pyrrolinones have also been obtained from metal-mediated cyclooligomerization processes in which concomitant hydrolytic or carbonyl insertion occurs. For example, tert-butyl isocyanide is converted in aqueous methanol by zerovalent nickel compounds e.g., Ni(t-BuNC)4, Ni(CO)4, into a di(alkylamino)-A3-pyrrolinone in moderate yield (Scheme 34). The reaction takes a different course in anhydrous methanol in which a di-tert-butylamino)ethylene derivative is formed, albeit in poor yield (Scheme 34).62... 

Hydride transfer from organic substrates to olefins (219) or halides (220), catalyzed by halogeno(triphenylphosphine)nickel complexes, and halide replacement reactions (example 13, Table VIII) by hydrolytic cleavage of nickel complexes have also been described. 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

The ion-exchange reaction of the synthetic zeolites NaX and NaY with cobalt, zinc and nickel ions is shown to be non-stoichiometric at low bivalent-ion occupancy, the hydrolytic sodium loss being about twice as large for NaX ( 5 ions/unit cell) as for NaY. The effect is more pronounced at high temperatures and disappears at high occupancies. Reversibility tests in NaX toward zinc and cobalt ions, as studied by a temperature-variation method, show the temperature history to be an important factor in the irreversibility characteristics. The low-temperature partial irreversibility, induced by a high-temperature treatment (45°C) is interpreted in terms of a temperature-dependent occupancy of the small-cage sites by divalent cations, which become irreversibly blocked at low temperature (5°C). 

However, details of this process including the mode of urea binding, the protonation state of individual surround protein residues, and the exact identity of the nucleophile are still under debate. Cyanate also was proposed as a possible intermediate in the urease mechanism (33). Recent quantum chemical calculations and molecular dynamics simulations indicated that hydrolytic and ehmination mechanisms might indeed compete, and that both are viable reaction channels for urease (34—37). Finally, an important issue is Why does urease require nickel as the metal of choice, whereas most other metallohydrolases use zinc While it was speculated that, inter alia, the relatively rigid and stable coordination environment around the Ni(II) ions as opposed to the higher kinetic lability and lower thermodynamic stability of Zn(II) complexes might play a role (31), this fundamental question has not yet been answered. 

Among the mononuclear hydrolytic species of nickel, only the stability of NiOH is well documented. Baes and Mesmer (14) have compiled the following values for the hydrolysis reactions ... 

Insertion of carbon monoxide followed by reductive coupling is an old reaction that continues to be studied and developed. Examples of this reaction are the carbonylation of nickel dialkyl complexes to give ketones, " alkyl-alkoxides to yield esters, and alkyl-amides to give organic amides (Equation (79)). The carbonylation of the binuclear complex 145 leads to the formation of an isoquinolone ring with the elimination of one of the two Ni atoms, but no GO insertion occurs at the second metal moiety. Hydrolytic cleavage of the remaining Ni-G bond provides the free heterocyclic base (Scheme 45). ... 

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