Oxidative addition anion effects

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

The mono(diphosphine) complexes, [Rh(dppp)]BF4 and RhCl-(dppp), are less effective than [Rh(dppp)2] + but are still more active than RhCl(PPh3)3. The mono(diphosphine) catalysts also decompose slowly under the reaction conditions, which renders them less useful than the bis(diphosphine) catalysts. The slower rate of decarbonyla-tion observed with the mono(diphosphine) catalysts compared with the bis(diphosphine) catalysts presumably is due to the lower basicity of the former which retards the rate of oxidative addition (vide infra). Consistent with this is the observation that [Rh(COD)(dppp)]BF4 (COD = 1,5-cyclooctadiene) shows a higher rate for catalytic de-carbonylation of benzaldehyde than does [Rh(dppp)]BF4 (22). An additional observation is that the type of anion, Cl or BF4 , has no apparent effect on decarbonylation rates for the bis(diphosphine) catalysts however, for the mono(diphosphine) complexes the chloride salts show slightly lower rates than their tetrafluoroborate analogues. 

The effect of hydrogen bonding on allylic alkylation was studied in the base-free reaction of phenylallyl carbonate with dimethyl malonate.1 31,1321 While the reaction proceeds rapidly in THF in the presence of four equivalents of PPh3, it is very sluggish in [C4Ciim][BF4], The reaction in THF was significantly inhibited when small quantities of the ionic liquid were added. Only with excess external base did the reaction proceed in a comparable rate in the ionic liquid. It was shown that the oxidative addition of allylic acetate to Pd(0) is reversible and in THF the resulting acetate anion... 

Prior to these investigations by HCC the promotional effect of iodide on the oxidative addition of Mel was investigated by others [9, 39, 40]. Foster demonstrated that the rate enhancement of this reaction in anhydrous medium was attributable to increased nucleophilicity of the rhodium catalyst with added iodide. The rationale for this observation was the generation of an anionic rhodium carbonyl complex, [Rh(CO)2l(L)]. Generation of this species was observed only with iodide added to certain neutral Rh species. No rate enhancement occurred with iodide added to the anionic complex, [Rh(CO)2l2] [39]. Similarly, in solvents with a high water concentration, iodide salts exhibited no rate enhancement in the presence of [Rh(CO)2l2] [11]. Maitlis and co-workers, in more recent investigations, reported a promotional effect of iodide in aprotic solvents on the oxidative addition of CH3I on [Rh(CO)2l2] [9a, 9c]. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements [Fe(III) and Mn(IV)] form very insoluble oxides and hydroxides, while the reduced forms [Fe(II) and Mn(II)] are orders of magnitude more soluble [in the absence of... 

Oxidative-addition reactions have their origin in the inverse dependence of coordination number on d-electron population that characterizes low-spin transition metal complexes, notably those with nearly filled (d -d ) d-subshells, e.g., [Co(III)(CN)6] (d ) [Co(II)(CN)5]3- (d ) [Ni(II)(CN)4]2- (d ) [Ag(I)(CN)2] (d ) etc. One of the consequences of this trend is the oxidations of such complexes, which tend to be accompanied by increases in the preferred coordination numbers of the metal atoms and hence by the incorporation of additional ligands into their coordination shells. The ligands required to complete the coordination shells may in certain cases be derived from the oxidant itself and, indeed, such complexes are especially effective as reductants for molecular oxidants, which typically undergo dissociative reduction to yield anionic ligands,... 

In this chapter, the effect of a series of transition metal stearates on the thermal oxidation of polypropylene in homogeneous solution is examined, and the results obtained are compared with that in bulk reported previously (16). In addition, the effects of the anion of copper compounds, the concentration of copper, the solvent, and the additives on the copper compound-catalyzed thermal oxidation of polypropylene are studied, and the mechanism of the copper catalysis in solution is discussed. 

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