Metal proton transfer reactions

In Section 8, the material on solubility constants has been doubled to 550 entries. Sections on proton transfer reactions, including some at various temperatures, formation constants of metal complexes with organic and inorganic ligands, buffer solutions of all types, reference electrodes, indicators, and electrode potentials are retained with some revisions. The material on conductances has been revised and expanded, particularly in the table on limiting equivalent ionic conductances. 

It may be instructive to again consider the energetics of a proton transfer reaction of the type involved in the first step of the examples above, in solution. Under the influence of a possible general base as the proton acceptor and a possible metal ion assisting as a catalyst we can write... 

This proton transfer reaction is not fast, and it is suggested that this may be a more complicated reaction than was anticipated, perhaps occurring by initial addition of OH or OR to the metal followed by H2O or ROH expulsion. In support of this is the isolation of a complex Os(CO)-(CNC6H4CH3)(PPh3)2(H)OR from an analogous reaction sequence. (This is the only reference yet to any osmium carbonyl-isocyanide chemistry.)... 

At least three components of the system change their state in the case of proton transfer reaction (1) electrons of the water molecule and the electrode providing the chemical bonding of the proton with a water molecule and the metal surface, (2) the proton itself, and (3) medium polarization. The characteristic times x, Xp, and x for... 

As we have seen earlier in the case of proton transfer reactions such as occurs between HCl(g) and NH3(g), water is not necessary for the acid-base reaction to take place. This is also true of the reactions between the acidic oxides of nonmetals and the basic oxides of metals. In many cases, they react directly as illustrated in the following equations ... 

The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

In conclusion, oxygen-17 NMR line-broadening provides the unique opportunity to study very fast proton transfer reactions on these metal oxocyano complexes by lowering the concentration of the reacting species through pH manipulation. 

As we have seen, the net surface charge of a hydrous oxide surface is established by proton transfer reactions and the surface complexation (specific sorption) of metal ions and ligands. As Fig. 3.5 illustrates, the titration curve for a hydrous oxide dispersion in the presence of a coordinatable cation is shifted towards lower pH values (because protons are released as consequence of metal ion binding, S-OH + Me2+ SOMe+ + H+) in such a way as to lower the pH of zero proton condition at the surface. 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

Figure 10. Proposed mechanism for the proton-transfer reactions of metal-...

Modified Marcus Parameters for Proton-Transfer Reactions with Deprotonated Metal Peptide Complexes... 

Inorganic solution chemistry often involves proton transfers to and from solvated metal ions as well as to and from the acids and bases that complex metal ions. Eight generalizations are presented below that attempt to summarize the insights regarding proton transfer reactions that have emerged in the past quarter century. The masterful reviews by Eigen (1) and Bell (2) provide much more extensive analysis of most of these points. 

Atmospheric pressure chemical ionization APCI is a method closely related to electrospray ionization. It uses ion-molecule reactions to produce ions from analyte molecules. The sample is electrohydrodynamically sprayed into the source (Figure 14.3). The evaporation of the solvent is often supported by a heated gas at temperatures between 80 and 400°C. Within the source, a plasma is created using a corona discharge needle that is placed close to the end of the metal capillary. In this plasma, proton transfer reactions occur, leading to the ionization of the analyte, mainly by the formation of [M+H]+ ions. Compared to ESI MS, APCI MS is very well suited for the analysis of less-polar components and can therefore... 

C at pressures of about 250—400 kPa (36—58 psi). The two types of catalysts, the amorphous silica—alumina (52) and the crystalline aluminosilicates called molecular sieves or zeolites (53), exhibit strong carboniumion activity. Although there are natural zeolites, over 100 synthetic zeolites have been synthesized and characterized (54). Many of these synthetic zeolites have replaced alumina with other metal oxides to vary catalyst acidity to effect different type catalytic reactions, for example, isomerization. Zeolite catalysts strongly promote carboniumion cracking along with isomerization, disproportionation, cyclization, and proton transfer reactions. Because butylene yields depend on the catalyst and process conditions, Table 7 shows only approximations. 

The type of solvent action that fused nonmetallic oxides have on metallic oxides may be likened to the second type of dissolution process, i.e., proton-transfer reactions. The process may be pictured as follows. The oxygens cannot discriminate between the metal ions (of the metallic oxide), with which they have been associated in the lattice of a metal oxide before dissolution, and the oxygen atoms of the Si04 tetrahedra contained in the solvent—fused silica. The oxygen atoms sometimes therefore leave the metal ions and associate with those of the tetrahedra. Dissolution has occurred with a type of oxygen-transfer reaction (see Fig. 5.69). 

Complexes of the type CpCo(PR3)2 are alkylated at the metal with small alkyl halides to give CpCo(PR3)2R (Scheme 25). Bulky halides produce ring-substituted hydrido cations instead, explained by attack of the electrophile from the exo site followed by ring-to-metal proton transfer. This reaction could be electrophilic addition (see Electrophilic Reaction), 5ei, or more probably radical addition initiated by electron transfer similar to the RX reaction of cobaltocene (Section 7.1). Since the oxidation potential of CpCo(P(alkyl)3)2 is more negative than that of cobaltocene, this latter mechanism is very plausible. 

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