Iodide complexes, thermodynamic

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]. 

I wish to point out that these reactions were studied in either neutral or acidic solutions where the cyanide cobalt system is really unstable thermodynamically. I raise the question about oxidation-reduction in the iodo complex. This wasn t mentioned in the paper. It seems to me it would provide an alternate path which might increase the reaction rates in the case of the iodide complex. 

If this interpretation is valid, the formation of complexes in aqueous solution by other hard donors should occur according to a pattern very similar to that found for the fluoride complexes, while, conversely, complexes of other soft donors should be formed according to a pattern similar to that found for cyanide, or iodide, complexes. Moreover, donors of intermediate softness, or hardness, should show a transitional behaviour between the extreme types also in the matter of thermodynamics of reaction. It has been a chief aim of this treatise to investigate whether this is in fact true. 

In the field of metal halides there has been a particularly large increase of our knowledge of gaseous metal iodide homo- and hetero-complexes. Thermodynamic data of gaseous species and condensed phases of quasi binary systems with mixed anions have been determined. 

Bist, H.D. and Person, W.B. (1967) Thermodynamic properties and ultraviolet spectra of cyanogen iodide complexes with some n donors. J. Phys. Chem., 71, 3288-3293. 

These are thermodynamically relatively weak oxidants (Table 18) and their action is relatively restricted, for example, to inorganic ions of moderate reducing power such as iodide, to polyfunctional organic compounds such as hydroxy-acids, and, in the cases of Ag(I) and Cu(II), to CO and H2. Fe(III) is particularly affected by hydrolysis and all these oxidants form complexes with suitable ligands. Cyanide ion and 1,10-phenanthroline form strong complexes with Fe(III) which greatly affect its behaviour. Tris-l,10-phenanthrolineiron(III) (ferriin) displays... 

Binary Compounds. The thermodynamics of the formation of HfCl2, of HfCl4, fused sodium and potassium chlorides have been described. The reduction of ZrXj (X = Cl, Br, or I) with metallic Zr or A1 in molten AICI3 has been studied at temperatures from 250 to 360 °C, depending on the halide. The electronic spectra of the initial reaction products were consistent with either a solvated Zr complex or an intervalence Zr "-Zr" species. Further reduction resulted in the precipitation of reduction products which were identified by analysis and i.r., electronic, and X-ray powder diffraction spectra. The stability of the trihalides with respect to disproportionation was observed to increase from chloride to iodide thus ZrC and ZrCl2,0.4AlCl3 were precipitated, whereas only Zrlj was formed. ... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

The stage is therefore set for the competition experiment in which the bromo complex is put in and all the various ligands which react rapidly with this complex at their characteristic second-order rates, and a little hydroxide to see if any aquo complex is generated during the reaction. The results are best demonstrated in the system, Pt(dien)Br+, plus iodide, plus hydroxide. This works nicely because hydroxide is the thermodynamic product thus hydroxide will finally prevail. 

Relative thermodynamic stabilities of crown ether complexes with several cations can also be evaluated by means of Ti competition experiments. (20) Addition of 18C6 to a solution of caesium iodide affects... 

Fe (CN)6] , when included in 17, is involved in an electron transfer reaction with iodide which cannot take place in the absence of the macrocyclic species. In fact, the E°(Fe 7Fe ) value for the hexacyano complex is considerably lower than E°(l2/I ), so that the electron transfer from I to Fe center is thermodynamically disfavored. However, in the presence of 17, in an acidic aqueous solution (e.g., pH = 4.4), the redox potential of Fe /Fe couple inside the supercomplex is increased to such an extent that the reaction (see Eq. 1), which is essentially iso-ergonic, proceeds to an equilibrium situation K = 1 M , at 25 °C). 

The acetyl complex, [Rh(CO)l3(COMe)], exists as an iodide-bridged dimer [ Rh(CO)l2(g-I)(COMe) 2]2 in the solid state [23] and in non-coordinating solvents, but it is readily cleaved to monomeric species ([Rh(CO)(sol) I3(COMe)r or [Rh(CO)(sol)2l2(COMe)]) in coordinating solvents [24-26]. Coordination of CO to [Rh(CO)l3(COMe)] rapidly gives the frans-dicarbo-nyl acetyl species, [Rh(CO)2l3(COMe)], for which low-temperature 13C NMR spectra reveal restricted rotation of the acetyl ligand [24]. Stoichiometric addition of acetyl iodide to [Rh(CO)2l2] initially generates czs,/flc-[Rh(CO)2l3(COMe)], which subsequently isomerizes to the thermodynamically preferred frans-dicarbonyl isomer [27]. The observation that acetyl iodide readily adds to [Rh(CO)2l2] shows that, for the overall catalytic reaction to be driven forward, the acetyl iodide must be scavenged by hydrolysis. 

Silver iodide (K p = 8.51 x 10 ) is less soluble than AgCl in aqueous solution, and so reduction of Ag(I) in the form of solid Agl is thermodynamically less favourable than reduction of AgCl (see problem 7.11 at the end of the chapter). However, Agl is much more soluble in aqueous KI than AgCl is in aqueous KCl solution. The species present in the iodide solution is the complex [Agl3] , the overall stability constant (see Section 6.12) for which is sslO " (equation 7.32). Following a similar procedure to that detailed above, we can use this value to determine that the... 

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