Iodide, formation constants with

Based upon analogies between surface and molecular coordination chemistry outlined in Table 1, we have recently set forth to investigate the interaction of surface-active and reversibly electroactive moieties with the noble-metal electrocatalysts Ru, Rh, Pd, Ir, Pt and Au. Our interest in this class of compounds is based on the fact that chemisorption-induced changes in their redox properties yield important information concerning the coordination/organometallic chemistry of the electrode surface. For example, alteration of the reversible redox potential brought about by the chemisorption process is a measure of the surface-complex formation constant of the oxidized state relative to the reduced form such behavior is expected to be dependent upon the electrode material. In this paper, we describe results obtained when iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol (DHT), and 3,6-dihydroxypyridazine (DHPz), all reversibly electroactive... 

The ability of Hg(II) to form tri- and tetra-coordinated ions with suitable ligands is well known. With alkylmercuric halides the formation constants are usually too small for such ions to be conventionally demonstrated (Brown et al., 1965a). It is likely, however, that such complexes would cleave more readily than the uncomplexed materials. These expectations have been strongly supported in a study of the cleavage of allylmercuric iodide by acid and iodide ion (Kreevoy et al., 1966a equations (6) to (8)). The rate was of the form shown in equation (23), in which S is the substrate. The terms which are linear in iodide ion... 

Assume that log P4, for chloride, bromide, and iodide with Zn is — 1, — 0.74, and — 1.25 and with Cd" " is 0.9, 2.53, and 6.1. (a) Calculate the masking index for Zn" " and Cd" " in 1.0 Af chloride, 1.0 Af bromide, and 0.1 Af and 1.0 Af iodide. Assume activity coefficients of unity and that the metal ions are present predominately as their highest complexes, (b) Assuming that the conditional formation constants of Zn" " and Cd with EDTA are the same (Figure 11-3), which condition is most favorable for the titration of zinc in the presence of cadmium ... 

The complexing of zirconium and hafnium ions by fluoride ions is quite extensive compared to chloro complexing, while complex ion formation w ith bromide and iodide ions is negligible. Formation constants for fluoride complexing with zirconium(IV) and hafnium(IV) calculated from the data of Connick (126), Buslaev (94), and Hume (574), have been summarized graphically by Goldstein (213). Slightly different values have been published by Bukhsh (92). Noren (15a, 401-403) has redetermined the equilibrium constants for the reaction. 

In keeping with the hard or class a nature of the Fe " ion the most stable complexes are formed with the small non-polarizable F ion. Consecutive formation constants for some fluoro and chloro complexes in aqueous solution may be found in ref. 322. Bromo complexes are even less stable than chloro complexes while with the easily oxidizable 1 ion no stable simple iodides have been prepared although a very few complexes containing Fe "—I bonds are known in combination with other ligands. Consistent with the stability order F > CL > Br is the occurrence of higher coordination number Fe complexes with F than with Br . Thus, while F readily forms hexacoordinate [FeFfi] complex ions and no tetrafluoro complex ions, CL forms both [FeClgf and [FeCl4] while Br apparently does not form a stable hexabromo complex [FeBr ] ". 

The precipitate has to be separated from the original test solution since the ions present here, e.g., the cations of the substance to be examined, can disturb the complexation. The solubility of silver bromides is intermediate compared to that of silver chloride and silver iodide. So the complex formation constant of the diammninoargentate complex overrules the solubility constant of silver chloride easily, and with difficulty the solubility constant of silver bromide, but it is not able to do so in the case of silver iodide. 

As early as the beginning of this century, Abegg and Bodlander pointed out that noble metals (having positive E relative to hydrogen) prefer to form complexes with iodide, sulfide and cyanide, and hence have what Ahrland et al. call B characteristics. For some reason, this is fairly valid, with the striking exception of thallium(I) and (III), and lead(II), where the metallic elements have negative E. Seen from the point of view of complex formation constants (Bjerrum, 1950) and in particular of the influence of substitution of a ligating... 

Stability constants of their iron complexes as presented in Table 7.1 [30,45-47]. The stability constants with one anion are given, which refers to the reaction order one that has usually been found for the dissolution of passive layers under the influence of the aggressive anions. The reaction of Fe + with HP to form FeF " and F1+ yields the more realistic value of log Kj = 2.28 due to the small dissociation constant of FIF (pFC = -log = 2.98). Table 7.1 also contains the constants K- for Ni +- and Cr3+-halide complexes. Their falling values from fluoride to iodide and Fe + to Ni + support the decreasing tendency for enhanced dissolution of the passive layer and localized corrosion. These data can be referred to the situation at the oxide surface. The fluoro complexes are very stable and form in high concentrations at all surface sites. Therefore their much faster transfer to the electrolyte yields enhanced general dissolution, whereas the attack of the other halides is locally restricted and much less pronounced. Besides the thermod5mamically based values, i.e., the stability constants, the kinetics of complex formation and of the complex transfer to the electrolyte are another decisive factor for the attack of the passive layer. In this sense, the situation of Cr is very special and will be discussed separately. 

Discussion. When potassium iodide solution is added to a dilute sulphuric acid solution containing a small amount of bismuth a yellow to orange coloration, due to the formation of an iodobismuthate(III) ion, is produced. The colour intensity increases with iodide concentration up to about 1 per cent potassium iodide and then remains practically constant. 

Schrauzer and co-workers have studied the kinetics of alkylation of Co(I) complexes by organic halides (RX) and have examined the effect of changing R, X, the equatorial, and axial ligands 148, 147). Some of their rate constants are given in Table II. They show that the rates vary with X in the order Cl < Br < I and with R in the order methyl > other primary alkyls > secondary alkyls. Moreover, the rate can be enhanced by substituents such as Ph, CN, and OMe. tert-Butyl chloride will also react slowly with [Co (DMG)2py] to give isobutylene and the Co(II) complex, presumably via the intermediate formation of the unstable (ert-butyl complex. In the case of Co(I) cobalamin, the Co(II) complex is formed in the reaction with isopropyl iodide as well as tert-butyl chloride. Solvent has only a slight effect on the rate, e.g., the rate of reaction of Co(I) cobalamin... 

It was found by DeLury that the overall rate of reduction of chromate is practically unaffected by the concentration of iodide, i.e. the sum of the rates of formation of iodine and aresenic(V) is constant and just equal to the rate at which chromate is reduced in a raction mixture containing no iodide (Fig. 1). The rate of oxidation of arsenite at a sufficiently high concentration of iodide decreases to i of its original value this is in accordance with the value of ci = 2 found. Fig. 1 well illustrates the general feature of coupled reactions, that the reaction of the inductor is always inhibited by the acceptor. The induced oxidation of iodide can... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

From the temperature variation of the equilibrium constant, thermodynamic parameters for the reaction were also obtained. The extent of formation of [Mo(CO)5l]" was found to be cation-dependent, and while equilibrium constants of 39 and 21 atm L moF were obtained for Bu4P and pyH+, none of the anionic iodide complex was observed for Na. Despite this variation, there seemed to be no correlation between the concentration of [Mo(CO)5l]" and the rate of the catalytic carbonylation reaction. It was proposed that [Mo(CO)5] and [Mo(CO)5l] are spectator species, with the catalysis being initiated by [Mo(CO)5]. Based on the in situ spectroscopic results and kinetic data, a catalytic mechanism was suggested, involving radicals formed by inner sphere electron transfer between EtI and [Mo(CO)5]. 

---