Complex inner-sphere complexes

Complexes. Most stable entities that result from the formation of largely covalent bonds between a metal ion and an electron-donating ligand— the interacting ligand is immediately adjacent to the metal cation—are called complexes (inner-sphere complexes). 

As with outer-sphere complexes, inner-sphere complexes are an ideal case. Actual complexes tend to have a component of both types of bonding. As noted above, most multivalent cations are aquocomplexes, with four to six ligand water molecules bonded to the cation. Accordingly, aquo-complexes are themselves inner-sphere complexes. For another ligand, L, to form an inner-sphere complex, it must displace one or more coordinating water molecules, forming a bond usually with some covalent character. This process may be written... 

If a strongly adsorbing bivalent metal ion is added to the system described by Eqs. (39) and (40), in which competitive adsorption of protons and ions of basic electrolyte occurs, then according to the triple layer model [103-105] its addition can cause the formation of two kinds of surface complexes inner-sphere complexes SOM formed at the 0-plain of the triple layer and outer-sphere complexes SO M + formed at the, 3-plain. Some recent studies by Hayes and Leckie [142-145] suggest that the formation of the inner-sphere complexes is more probable for divalent cations like Cu, Pb, Cd" ", etc. than the formation of outer-sphere surface complexes. So, in general [142,143] ... 

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. 

For illite and kaolinite with decreasing solution concentration (Figure 5) there are two important changes. The relative intensity for inner sphere complexes increases, and the chemical shifts become substantially less positive or more negative due to the reduced Cs/water ratio, especially for the outer sphere complexes. Washing with DI water removes most of the Cs in outer sphere complexes and causes spectral changes parallel to those caused by decreasing solution concentration (data not shown). 

The surface behavior of Na is similar to that of Cs, except that inner sphere complexes are not observed. Although Na has the same charge as Cs, it has a smaller ionic radius and thus a larger hydration energy. Conseguently, Na retains its shell of hydration waters. For illite (Figure 6), outer sphere complexes resonate between -7.7 and -1.1 ppm and NaCl... 

The species (MS03)" represents an inner-sphere complex between sulphite and the oxidant formed by ligand-displacement. (MS03)" is formed by abstracting an electron from... 

Copper also binds very strongly as an inner-sphere complex with organic matter while other divalent transition metals such as Ni2+ and Co2+... 

The development of more benign alternatives to cyanide for gold-leaching (see Section 9.17.3.1) such as thiourea, thiocyanate, or thiosulfate, which form stable complexes in water has prompted research to identify suitable solvent extractants from these media. Cyanex 301, 302, 272, Ionquest 801, LIX 26, MEHPA, DEHPA, Alamine 300 (Table 5) have been evaluated as extractants for gold or silver from acidic thiourea solutions.347 Whilst the efficacy of Cyanex 301 and 302 was unaffected by the presence of thiourea in the aqueous feed, the loading of the other extractants is severely depressed. Formation of solvated complexes of gold and of an inner-sphere complex of silver has been proposed.347... 

Chloroform solutions of calixarenes (33 38)293,354 show high affinity for Au111 and good selectivity over Fe111 in some cases.293 Spectroscopic data indicate that complex formation with thioethoxy calix[4]arenes involves formation of inner-sphere complexes with Au—S bonds.354... 

Inner sphere complexes are formal with Pd11,... 

An inner-sphere complex (42) is formed when toluene solutions of 3,3-diethylthietane are contacted with solutions of Prv in hydrochloric acid, but extraction is slow 322... 

It is important to distinguish between outer-sphere and inner-sphere complexes. In inner-sphere complexes the surface hydroxyl groups act as o-donor ligands which increase the electron density of the coordinated metal ion. Cu(II) bound inner-... 

Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ("chemical bond"), an outer-sphere complex (ion pair) or be in the diffuse swarm of the electric double layer. (From Sposito, 1989)... 

Fig. b shows a schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups ("s"), inner-sphere complexes ("a"), outer-sphere complexes ("P") and the diffuse ion swarm ("d"). (Modified from Sposito, 1984)... 

Direct evidence for inner-sphere complexes comes from spectroscopic methods unfortunately, spectroscopic methods alone are seldom sufficiently sensitive to reveal the specific structure of surface complexes. Motschi (1987) used electron... 

As we shall see (Chapter 4), the kinetics of surface complex formation is often related to the rate of H20 loss from the aquo cation. This is another (indirect) evidence for inner-sphere complex formation. 

Standard cations used for measuring cation exchange capacity are Na+, NHJ, and Ba2+. NH is often used but it may form inner-sphere complexes with 2 1 layer clays and may substitute for cations in easily weathered primary soil minerals. In other words, one has to adhere to detailed operational laboratory procedures these need to be known to interpret the data and it is difficult to come up with an operationally determined "ion exchange capacity" that can readily be conceptualized unequivocally. 

The reactivity of the surface (Fig. 5.3), i.e., its tendency to dissolve, depends on the type of surface species present e.g., an inner-sphere complex with a ligand such as that shown for oxalate... 

V Specifically adsorbed species are those that are bound by interactions other than electrostatic ones. To what extent SO and Ca2+ can form inner-sphere complexes is not yet well established. SO2 is able to shift the point of zero proton condition of many oxides. 

Rates of ligand exchange depend quite strongly on the coordina-tive environment of the metal center. The water exchange rate of Fe(H2O)5(OH)is almost three orders of magnitude higher than that of Fe(H20)g+, and follows a dissociative, rather than an associative exchange mechanism (20). Fe(1120)5(OH)has also been shown to form inner-sphere complexes with phenols (27), catechols (28), and a-hydroxycarboxylic acids (29) much more quickly than Fe(H20) +. The mechanism for complex formation with phenolate anion (A-) is shown below (27) ... 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

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