Metal—HS complexation

The stability constant is probably the most important quantitative parameter for the characterization of a metal-ligand complex in that it provides a numerical index of the affinity of the metal cation for the ligand and allows the development of quantitative models able to predict the speciation of metal ions in the system studied. Several different theoretical and experimental approaches have been attempted for the determination of stability constants of metal—HS complexes and modeling metal-HS complexation reactions. Data analysis and interpretation is, however, still controversial, due to the intrinsically complex and ill-defined nature of HSs. The multiligand, polyelectrolitic nature of HS macromolecules results in the inability to describe quantitatively the types, concentrations, and strengths of the several nonidentical binding sites in HSs and in the impossibihty to ascertain and measure the stoichiometry of metal-HS complexation (MacCarthy and Perdue, 1991). 

Because of the extreme complexity and wide controversy existing on the subject, only a general overview of the theoretical and experimental approaches is provided in this chapter. For a more detailed information, the reader is referred to the several reviews and book chapters available on the topics of stability constants and conceptual and mathematical models of metal-HS complexation (e.g.. Perdue and Lytle, 1983 Sposito, 1986 Stevenson and Fitch, 1986 Perdue, 1989 MacCarthy and Perdue, 1991 Tipping, 1998 Perdue, 2001). 

Comprehensive reviews on the application of UV-Vis spectroscopy to the study of HS and metal-HS complexes include those of Bloom and Leenheer (1989) and Stevenson (1994). 

Titration curves of HS fluorescence quenching versus concentration of added metal quencher have been used to obtain the CC values of HS ligands and the stability constants of HS-metal complexes (Saar and Weber, 1980, 1982 Underdown et al., 1981 Ryan et al., 1983 Weber, 1983 Dobbs et al., 1989 Grimm et al., 1991 Hernandez et al., 2006 Plaza et al., 2005, 2006). Two fluorescence techniques, lanthanide ion probe spectroscopy (LIPS) and fluorescence quenching of HSs by Cu-+, have been used in conjunction with a continuous distribution model to study metal-HS complexation (Susetyo et al., 1991). In the LIPS technique, the HS samples are titrated by Eu-+ ions, and the titration plot of the ratio of the intensities of two emission lines of Eu + is used to estimate the amount of bound and free species of the probe ion. In the other technique, titration curves of fluorescence intensity quenched by Cu versus the logarithm of total added Cu2+ are used. 

Fluorescence spectroscopy has, however, several advantages over most other methods for studying metal-HS complexation in aqueous media. The method is relatively rapid since no separation is required between bound and free metal ion thus, errors associated with the separation step in most speciation methods are avoided. Unlike most other methods, it allows direct measurement of the CC of the ligand through a determination of the concentration of free ligands, thus... 

ESR-related spectroscopies that hold the potential to overcome some resolution limitations and yield more information than the classical ESR approach about the chemical environment of paramagnetic metal ions are the electron—nuclear double resonance (ENDOR) (Kevan and Kispert, 1976) and electron-spin echo envelope modulation (ESEEM) (Kevan and Schwartz, 1979) spectroscopies. Either ENDOR or ESEEM represents by principle a useful tool in extending resolution of the ESR experiment. However, the sensitivity of ENDOR and ESEEM is much lower than that of ESR, and interpretation of ENDOR and ESEEM spectra is not a simple matter, especially if ligands are not well characterized, as is the case for HSs. Both ENDOR and ESEEM techniques have not yet been applied to strictly metal-HS complexes, but the sensitivity and ease of carrying out experiments are improving rapidly, so major scientific activity may be anticipated to occur in this area of ESR spectroscopy. 

Nuclear magnetic resonance (NMR) spectroscopy has been applied to elucidate metal-binding mechanisms to organic ligands mainly by two approaches by measuring the effects of metal complexation on either the relaxation times of H of water molecules solvating the metal cation or on the chemical shifts of NMR-active metal ions (e.g.. Cd, Al, and Pb) (e.g., Connors, 1987 Wilson, 1989 Macomber, 1998). Relatively few and sparse studies have been performed by NMR on metal-HS complexes. A comprehensive and updated review has been provided by Kmgery et al. (2001) on the various applications of NMR spectroscopy to the study of metal-HS interactions. 

To date, few, if any, of the NMR studies on metal-HS complexation have attempted an examination of metal-N spin couplings, due primarily to the low sensitivity and low natural abundance of nuclei. However, with high-resolution instrumentation, natural abundance N-NMR may provide an additional useful technique for examining metal-HS complexation. 

A variety of separation and nonseparation methods have been used to speciate metal ions in the presence of HS, assess complexing capacity of HS, and calculate conditional stability constants and quotients of metal-HS complexes. Because of the extreme amplitude of the topic, only a brief overview of the methods most commonly applied to soil HS is provided here. A number of reviews and book chapters have been published on the topic, to which the reader can refer for details (Saar and Weber, 1982 Stevenson and Fitch, 1986 Weber, 1988 Dabek-Zlotorzynska et al., 1998 Nifant eva et al., 1999). 

UF has also been applied to the determination of stability constants for trace metal-HS complexes, equilibrium constants for the distribution of organic compounds between water and synthetic micelles, the binding of metal ions and small compounds to proteins and other macromolecules in biochemistry, etc. This type of application is essential to know precisely to what degree the complexing agent, the complexed species, and the free ions and molecules can pass through the membrane. 

Senesi and Loffredo (2005) proposed, on the basis of the available experimental results, a number of possible metal-HS complexation structures, which are shown in Figure 10.25. It should be noted that chelate structures are predominant, and that in some cases, metal bridging of two different molecules is proposed. 

We are investigating the template formation of 2-D and 3-D metal-di-cyanamide anionic networks, for instance of type Mn(dca)3, by use of [M(N,N)3]n+ cations such as [M(2,2 -bipy)3]2+. A hexagonal sheet network was formed in [Fen(2,2 -bipy)3][Fen(dca)3]2 in which the cations fitted beautifully within the hexagonal windows. The cation remained LS between 4-300 K [66], Attempts to make the Con(2,2 -bipy)32+ analogue unfortunately led to dissociation of dca and bipy and formation of a zig-zag chain structure in the weakly-coupled HS complex [Con(dca)2(2,2 -bipy)2]n. The complex [Fen(propyl-tetrazole)6]2+, which has a very sharp SCO transition [42], unfortunately did not yield a network product. 

Electtonic configurations of octahedral complexes for d1 — d10 metals ions. The high-spin (HS) complexes have A0 smaller than pairing energy, while the low-spin (LS) complexes have A0 larger than pairing energy. 

Catalytic constants for iodination of 2-acetylpyridine (HS) complexed by metal ions"- ... 

Sulfur (S) S042", H2S(waq), HS, and metal sulfide complexes, Me(2)S04°, Mca,I IS()4 and further sulfate complexes with uni- or multi-valent metals... 

A mass balance section for the hydrogen sulfide species was added to the anion mass balance calculations when we observed that strong HS complexing of some trace metals sometimes rendered cation mass balance convergence impossible. 

At low pH most Mn-+ is adsorbed by a soil FA in the form of outer-sphere complexes, but at pH > 8, or T > 50°C, Mn"+ can enter inner-sphere multiligand complexation sites (McBride, 1982). These results indicate that the type and stability of Mn2+ -HS complexes and, in turn, their ease of exchangeability and bioavailability in natural systems are strongly dependent on pH and temperature. Further, HSs isolated from various sources exhibit a high residual complexing capacity for added Mn that can be bound in water-stable forms, but unlike Fe + and Cu " ", it may be displaced completely by protons or strongly complexed metal ions (Senesi et al., 1991). 

The tlirough-space nuclear overhauser effect (NOE) can provide information on sites where metals interact even if the metals do not form stable bonds through which spin coupling can be transferred. For instance, metal-HS interactions can be studied by NOE spectroscopy (NOESY) by measuring interactions between the protons within the HS molecules before and after the addition of metals to understand the conformational changes that occur within the molecules (Kingery et al., 2001). An alternative approach is to measure the heteronuclear overhauser effect (HOE) directly between the metal ion and the HS proton in close proximity by HOE spectroscopy (HOES), as has been demonstrated for organo-Li complexes (Bauer, 1995). 

XAS in the XANES and EXAFS regimes is a powerful, nondestructive, and noninvasive tool for studying specifically the chemistry of several elements in a complex matrix such as HS. In particular, these techniques allow the study of trace metal ion complexation by HS without any limitation due to the type of metal species, which needs not to be paramagnetic as in ESR spectroscopy. Further, additional details can be obtained on the dentality of the central ions and bond lengths and distances, which cannot be revealed by ESR that lacks a scattering component. 

HS ) complexes are also invoked to explain metal solubilities in hydrothermal ore-forming solutions (Hayashi et al. 1990). Unfortunately, there is much less data for gas-and aqueous-phase bisulfide complexes of metals. Table 6 gives some comparison between calculated and experimental bond lengths and vibrational frequencies of zinc... 

Other studies of metal-HS interactions include NMR experiments (Kingery et al. 2001 Senesi and Loffredo 2005 and references therein), which have been used to correlate metal-binding structures with metal-binding characteristics, to elucidate binding mechanisms and to determine complexation equilibrium constants. IR experiments are less helpful, but they generally confirm that the carboxylate groups are often the main ligand for complexed metals (Davies et al. 2001). 

HSAB (Hard and Soft, Acids and Bases) principle was proposed by Pearson (1963, 1968). This is summarized as (1) hard acids are in an affinity with hard bases, (2) soft acids are in large volume and have small charge and their electronegativity is large, (3) hard acids react with hard acids to form ionic compounds, while (4) covalent compounds form by the reaction of soft acids with soft bases. Acids and bases are classified according to this principle (Table 1.6). For instance, the order of hardness is F > Cl > Br > I , Cu" > Ag" > Au". HS tends to combine with soft cations such as Au", Hg" and PF to form HS complexes. Base metal cations such as Zn ", Pb ", Cu", and Fe " tend to combine with relatively hard anions such as Cl to form chloro-complexes. 

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