Inner-sphere complexes 88 stabilities

Considering these thermodynamic values, we suggest that fluoride forms inner-sphere complexes like the acetate and sulfate rather than ion pairs like chloride, nitrate, and thiocyanate. Similar to the other inner-sphere complexes, stability is attributed to the positive entropies which have been interpreted in the earlier studies as reflecting the... 

Manning, B.A. Goldberg, S. (1996) Modeling competitive adsorption of arsenate with phosphate and molybdate on oxide minerals. Soil Sci. Soc. Am. J. 60 121-131 Manning, B.A. Fendorf S.E. Goldberg, S. (1998) Surface structures and stability of ar-senic(lll) on goethite spectroscopic evidence for inner-sphere complexes. Environ. Sci. Techn. 34 2383-2388... 

The data for bromo complexes were obtained in aqueous methanolic solutions and outer-sphere bromo complexes with K = 1.3-1.9 were obtained for Pr, Nd, Sm, which are larger than the values of Ho (0.97) and Er (0.70). Chloro and bromo complex formation in dimethyl formamide studied by titration calorimetry [122] showed the evidence for MC12+, MCC. MCI3, and MCI4 species in solutions. In the case of bromide, monobromo and dibromo inner sphere complexes have been detected. The stepwise formation constants could not be determined for iodo complexes due to the small value of enthalpies of reaction. The stability constants data obtained in DMF are given in Table 4.8. 

Exceptions to the two Irving-Williams orders for inner sphere complexes occur particularly when complexes with more than four ligands or large, assymetric ligands are compared (12). Stability constant sequences for weak outer sphere complexes, such as are formed with chloride ( 1) and fluoride (16, 40), do not obey the Irving-Williams orders. 2+... 

Direct initiation by lower valence states (M" ] of metals proceeds through formation of activated complexes with O2 (23, 45)—mostly via inner sphere complexes. As free reduced metals react rapidly with oxygen (Reaction 6a), this mechanism is active primarily when chelators specifically stabilize the reduced metals. These reactions also proceed mostly facilely in nonpolar solvent (46), e.g., in hydrophobic lipid phases of membranes or in oils. 

Manning, B. A., Fendorf, S. E., and Goldberg, S., 1998, Surface stmctures and stability of arsenic(lll) on goethite Spectroscopic evidence for inner-sphere complexes Environmental Science Technology, v. 32, p. 2383-2388. 

It is difficult to estimate the equilibrium constant for Eq. (1) because this is a substitutional step and involves specific chemical interactions. As a consequence, the stability constants of inner-sphere complexes cannot be obtained readily from first principles. The problem can be circumvented if the inner-sphere complex is relatively stable, because its formation constant can then be obtained from the rate law if it is very stable then the electron transfer becomes intramolecular, and the determination of becomes unnecessary. A similar situation exists also for outer-sphere complexes and often is exploited by using oppositely charged reactants. Under these conditions and, more importantly, k., can be determined directly. 

Sec. 3.5 Electronegativity and the Stabilities of Inner-Sphere Complexes TABLE 3.3 Species formed by hydrolysis of cations in water at 25 C and pH 2 to 12... 

The stabilities (/ aswe values) of inner-sphere complexes formed between metal cations and a given ligand increase as the difference in the electronegativities of the metals and the ligand decrease. Explain this statement with examples. 

Schwarzenbach described cations as being of Class A, B, or C. Pearson propo.sed the concept of hard and soft acids and bases. Pearson s approach is generally more useful than Schwarzenbach s for systematizing and predicting the stabilities of inner-sphere complexes. Explain with examples. 

Explain why the stability of inner-sphere complexes is usually favored by a positive entropy of formation of the complex. 

Cumulative constants of some ferrous and ferric complexes involving the same ligand are compared below in the order of increasing stability of the ferric ion complex. Strong (largely inner-sphere) complexes have been marked with an asterisk. 

Probably some sets of surface reactions reported in literature (outer sphere complex + inner sphere complex) can be replaced by single reaction formulated in terms of the CD model. A few publications report two sets of surface species along with their stability constants that produced equally good simulation of experimental data. These sets are separated by or in Tables in Chapter 4. 

The thermodynamic data for complexation of trivalent lanthanide and actinide cations with halate and haloacetate anions are reported. These data are analyzed for estimates of the relative amounts of inner (contact) and outer (solvent separated) sphere complexation. The halate data reflected increasing inner sphere character as the halic acid pKa increased. Use of a Born-type equation with the haloacetic acid pKa values allowed estimation of the effective charge of the carboxylate group. These values were, in turn, used to calculate the inner sphere stability constants with the M(III) ions. This analysis indicates increasing the inner sphere complexation with increasing pKa but relatively constant outer sphere complexation. 

We assume that the effective charge remains the same upon complexation by Ln(III) and An(III). Based on the success of the fluoride and oxycarbon calculations, we use De = 57. We also use di2 = 2.38 X (r+3 = 1.0 X, r0- - 1.83 A) and with equation (1) calculate inner sphere stability constants. The numbers we obtained are listed in Table III along with the values of 30 (based on Eu(III) complexing) and the per cent inner sphere complexation. The latter data is obtained from the relation 3T = 0 + i ... 

The main advantage of the above experimental procedure is that in Ni(C104)2 media the medium effect can be eliminated. In the case of nickel, the results are interpreted in terms of the existence of [Ni(H20)5]Cr and [NiCl(H20)5] complexes. The overall stability constant of the NiCl species (jS, = 0.37 at / = 0) is due mainly to the stability of the outer-sphere ion pair. The jS value for the inner-sphere complex was reported to be around 0.01 M . 

---