Surface-metal-ligand complexes

Molecular simulation methods can be a complement to surface complexation modeling on metal-bacteria adsorption reactions, which provides a more detailed and atomistic information of how metal cations interact with specific functional groups within bacterial cell wall. Johnson et al., (2006) applied molecular dynamics (MD) simulations to analyze equilibrium structures, coordination bond distances of metal-ligand complexes. 

The scheme of Fig. 5.5a corresponds to steady state conditions (Table 5.1). We can now apply the general rate law (Eqs. 5.7, 5.8), the rate of the ligand-promoted dissolution, Rl, is proportional to the concentration of surface sites occupied by L (metal-ligand complex, >ML) or to the surface concentration of ligands, C (mol nr2) ... 

In systems containing two or more adsorbates, either competitive or synergistic effects may operate. The commonest synergistic effect is that of ternary adsorption (11.5.4). Competitive behaviour may involve competition for the same surface sites, indirect effects due to the change in the electrostatic properties of the oxide/water interface and in some cases, formation of non sorbing, metal-ligand complexes in solution. 

Figure 7.9. In a ligand-catalyzed reaction, the surface coraplexation with a bidentate ligand is followed by the rate-determining detachment of the metal ligand complex. [From Stumm (1986), with permission.]...

In this paper we examine the assumptions of our previous modeling approach and present new model calculations which consider alternative assumptions. In addition, we discuss the physicochemical factors which affect the formation of surface complexes at the oxide/water interface, in particular the effect of decreasing dielectric strength of the solvent. Finally, to demonstrate the general applicability of the model we present modeling results for a complex electrolyte system, where adsorption of a metal-ligand complex must be considered. 

The results of a newly proposed model for adsorption at the oxide/water interface are discussed. The modeling approach is similar to other surface complexation schemes, but mass-law equations are corrected for the effect of the electrostatic field. In this respect, this model bridges the gap between those models that emphasize physical interactions. The general applicability of the model is demonstrated with comparisons of calculations and experimental data for adsorption of metal ions, anions, and metal-ligand complexes. Intrinsic ionization and surface complexation constants can be determined with an improved double extrapolation technique. 

Metal-ligand complex stability at surfaces is correlated to metal-ligand complex stability in solution. 

Rate Expressions. Far from equilibrium, the overall dissociation of an oligomer or hydrolytic complex on a mineral surface is resolvable into pathways corresponding to different sets of elementary reactions. Usually, the pathways are assumed to be independent, and therefore additive, and the rate-controlling step reaction is the disruption of bonds at, or near, the metal-ligand complex. The rate-controlling step is not movement of the reactive solutes through the bulk aqueous phase to the site(s) of reaction. 

One important result of Ludwig et al. (11,12) is that the rate coefficients correlate with the equilibrium constant in solution for these carefully chosen metal-ligand complexes. This LFER relates the rate coefficient for ligand-promoted dissolution of NiO(s), which is difficult to measure, to the equilibrium constant for forming the Ni(II)-ligand species in solution, which is already known. This correlation works only because there is some structural similarity between the metal-ligand complexes in solution and the surface complexes that are important to dissolution. It will not be observed for all adsorbates. 

An abundant scheme for the preparation of surface metal-organic complexes is ligand exchange, where one of the ligands of a metal complex in solution is substituted with a surface-confined ligand, leading to immobilization of the metal ion in the functional SAM. Examples of this process include Co and Os terpyri-dine complexes [22] Ru tetraamine [23] and dinuclear Ru [24] complexes salen complexes with Fe, Co, and Mn [25] and Zr acetylacetonate [26]. 

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