Nickel complexes stabilization

Quite a number of silylnickel and silylenenickel compounds have been reported, and only a few can be cited here2305-2309 Bis(silyl)nickel complexes have been implicated as important intermediates in Ni-catalyzed double silylation of various organic substrates. Using an o-bis(dimethylsilyl)carborane ligand, complex (930) could be isolated and structurally characterized 2310 Its unusual thermal stability is attributed to the advantageous steric and electronic properties of the carboranyl unit. (930) undergoes... 

According to Scheme 1, the resulting phosphine-stabilized silylene nickel complex 11 is transformed to the planar Ni(II) complex 12, releasing unstable silylenes (other fragments than Me2Si also can be obtained similarity). Thus complexes like 11 may be regarded as a depot form for silylenes. 

Other template cyclizations. In another Schiff-base template reaction, 1,3-diaminopropane monohydrochloride was reacted with biacetyl in methanol in the presence of Ni(n) to yield the nickel complex of the corresponding cyclic tetraimine - see [2.16] (Jackels et al., 1972). The success of the procedure illustrated is quite dependent on the reaction conditions employed. Attempts to isolate the metal-free macrocycle were unsuccessful - this once again emphasizes the stabilizing role of the metal... 

Table 6.1 summarizes the thermodynamic parameters relating to the macrocyclic effect for the high-spin Ni(n) complexes of four tetraaza-macrocyclic ligands and their open-chain analogues (the open-chain derivative which yields the most stable nickel complex was used in each case) (Micheloni, Paoletti Sabatini, 1983). Clearly, the enthalpy and entropy terms make substantially different contributions to complex stability along the series. Thus, the small macrocyclic effect which occurs for the first complex results from a favourable entropy term which overrides an unfavourable enthalpy term. Similar trends are apparent for the next two systems but, for these, entropy terms are larger and a more pronounced macrocyclic effect is evident. For the fourth (cyclam) system, the considerable macrocyclic effect is a reflection of both a favourable entropy term and a favourable enthalpy term. 

Recent developments are ring-cleavage reactions of the heterocycles [R2M-SbR2]w with 4-(dimethylamino)pyridine leading to base-stabilized monomers, [L — R M-SbR, (R = Me, SiMe3 R = Me, Et, i-Bu M = A1, Ga).83,84 Reaction of [L-Al(Me2)-Sb(SiMe3)2] [L = 4-(dimethylamino)pyr-idine] with [Ni(CO)4] leads to the corresponding tricarbonyl nickel complex (Equation 4).85... 

A. Cyclo-Oligomerization with Zerovalent PR3/P(0R)3-Stabilized Nickel Complexes as the Catalyst... 

Scheme 6. Interplay of the C8- and C -production channels for the cyclo-oligomerization of 1,3-butadiene with zero valent PR3/P(OR)3-stabilized nickel complexes as the catalyst. Free energies (AG, AGJ in kcalmol-1) are given relative to the favorable rf-synrfiC A-cis isomer of 2a for catalysts bearing strong a-donor ligands namely I (L = PMe3), III (L = PPrj), VI (L = PBU3), and -acceptor ligands namely V (L = P(OMe)3), IV...

For PR3/P(OR)3-stabilized nickel complexes, there are two borderline cases known from the experimental investigation of Heimbach et al. 1 which, unlike the usual behavior, redirect the cyclo-oligomerization reaction into the Ci2-cyclo-oligomer production channel. Catalysts bearing either strong a-donor ligands that must also introduce severe steric pressure (e.g., PBu Pr2) or sterically compact n-acceptors (like P(OMe)3) are known to yield CDT as the predominant product. From a statistical analysis it was concluded,8a,8c that the C8 Ci2-cyclo-oligomer product ratio is mainly determined by steric factors (75%) with electronic factors are less important. 

The following conclusions can be drawn (a) ir-Allylnickel compounds are probably not involved in the catalytic dimerization of cyclooctene, because the highest reaction rate occurs when only traces of these compounds can be detected further, the concentration of the new 7r-allyl-nickel compound (19) becomes significant only after the catalytic reaction has ceased, (b) The complex formed between the original 7r-allylnickel compound (11) and the Lewis acid is transformed immediately upon addition of cyclooctene to the catalytically active nickel complex or complexes. In contrast to 7r-allylnickel compounds, this species decomposes to give metallic nickel on treatment of the catalyst solution with ammonia, (c) The transformation of the catalytically active nickel complex to the more stable 7r-allylnickel complex occurs parallel with the catalytic dimerization reaction. This process is obviously of importance in stabilizing the catalyst system in the absence of reactive olefins. In... 

The purpose of this review is to provide the reader with a scenario of what can be accomplished with nickel complexes. It is useful to do this via a series of broad schemes, hypothetical in some cases, which show the main reaction patterns, without considering mechanistic details. Examples will illustrate the various processes, but extension to other substrates or to different conditions often requires use of alternative ligands or solvents, or a change from a neutral complex to a cationic or anionic species, as indicated above. For references to syntheses with nickel see Baker et al. (10). For criteria for the synthesis of coordination compounds and stability of organotransition metal complexes in general, see references (11. 12). Organometallic literature has been collected periodically by Bruce (13). 

While apparently no metallasilsesquioxane complexes of nickel and palladium have yet been prepared, several literature reports have appeared on platinum compounds containing silsesquioxane ligands. Abbenhuis reported the synthesis and characterization of three platinum(II) complexes stabilized by the chelating diphosphine ligand dppe (= 1,2-bis(diphenylphosphino)ethane) as outlined in Scheme 63. ... 

Zinc, nickel, lead, and cadmium salts Ba-Cd-Zn stabilizers Organo-nickel complexes CdS, CdSe, lead stearate Cadmium laurate Stabilizers Pigments Lubricants... 

Macrocyclic Fi-donor ligands and vitamin Bj, analogues. The free amine [(97) tet] can be prepared from the previously reported nickel complex. Cobalt-fin) complexes have been prepared with both planar (bcde octahedral) and folded (abed octahedral) co-ordination. Derivatives of the three ligand configurations arising from restricted inversion at the four chiral co-ordinated secondary amino-groups have been prepared (see Scheme 2) and their stabilities and configurations discussed. ... 

There is a difference between the thermodynamic terms stable and unstable and the kinetic terms labile and inert. Furthermore, the differences between the terms stable and unstable, and labile and inert are relative. Thus, Ni(CN)4 and Cr(CN)6 are both thermodynamically stable in aqueous solution, yet kinetically the rate of exchange of radiocarbon-labeled cyanide is quite different. The half-life for exchange is about 30 sec for the nickel complex and 1 month for the chromium complex. Taube has suggested that those complexes that react completely within about 60 sec at 25°C be considered labile, while those that take a longer time be called inert. This rule of thumb is often given in texts, but is not in general use in the literature. Actual rates and conditions are superior tools for the evaluation of the kinetic and thermodynamic stability of complexes. 

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