Ligand- and oxygen-isotope-exchange pathways of geochemical interest

Abstract

Environmental context. Most chemical processes in water are either ligand-or electron-exchange reactions.Here the general reactivity trends for ligand-exchange reactions in aqueous solutions are reviewed and it is shown that simple rules dominate the chemistry. These simple rules shed light on most molecular processes in water, including the uptake and degradation of pesticides, the sequestration of toxic metals and the corrosion of minerals.Abstract. It is through ligand-exchange kinetics that environmental geochemists establish an understanding of molecular processes, particularly for insulating oxides where there are not explicit electron exchanges. The substitution of ligands for terminal functional groups is relatively insensitive to small changes in structure but are sensitive to bond strengths and acid-base chemistry. Ligand exchanges involving chelating organic molecules are separable into two classes: (i) ligand substitutions that are enhanced by the presence of the chelating ligand, called a 'spectator' ligand and (ii) chelation reactions themselves, which are controlled by the Lewis basicity of the attacking functional group and the rates of ring closure. In contrast to this relatively simple chemistry at terminal functional groups, substitutions at bridging oxygens are exquisitely sensitive to details of structure. Included in this class are oxygen-isotope exchange and mineraldissolution reactions. In large nanometer-sized ions, metastable structures form as intermediates by detachment of a surface metal atom, often from a underlying, highly coordinated oxygen, such as m 4 -oxo, by solvation forces. A metastable equilibrium is then established by concerted motion of many atoms in the structure. The newly undercoordinated metal in the intermediate adds a water or ligand from solution, and protons transfer to other oxygens in the metastable structure, giving rise to a characteristic broad amphoteric chemistry. These metastable structures have an appreciable lifetime and require charge separation, which is why counterions affect the rates. The number and character of these intermediate structures reflect the symmetry of the starting structure. Simple reactivity trends for ligand-and oxygen-isotope exchanges FormalismMost metals in aqueous solutions of interest to environmental chemists are coordinatively saturated. Here saturated is meant in the sense of Pauling's First Rule, relating ionic radius to oxygen packing. Most metals in water have the maximum number of coordinated atoms given by their radius ratios -exceptions certainly exist for highly covalent materials (e.g. Pt II , Nb V ), and some are discussed below, but most materials near the Earth's surface are oxides with a metal that has a fixed coordination or slightly variable number. In sharp contrast, the coordination

Predicting 17O NMR chemical shifts of polyoxometalates using density functional theory

“…Polyoxometalates, polyanionic metal oxide clusters of group V and VI elements, 1,2 have generated a lot of interest as catalysts, as bioactive molecules, and as homogeneous models for understanding the behaviour of heterogeneous metal oxides such as minerals at the solvent-solid interface. [3][4][5] The latter application is very attractive as it offers site-specific information about the dynamic behaviour of the molecule, something which may be impossible to achieve when working with heterogeneous materials. As such, 17 O NMR has proven to be a powerful method for providing details of the rates of exchange between different metal oxide oxygen sites and water, and has revealed a broad range of reactivities exhibited by oxygen sites in even small homoleptic isopolyoxometalate clusters.…”

Testing the hypothesis that solvent exchange limits the rates of calcite growth and dissolution

“…Such states are characteristically found in metadynamics simulations of ion attachment and detachment to mineral surfaces 65,72 as well as unexpected shis in the rates of ligand exchange reactions of poly-oxo-metallates with cation substitutions. 2,73…”

Ligand Steric Interactions Modulate Multidentate Ligand Exchange Pathways: Kinetics of Nickel(II) Ion Capture from N-Substituted IDA Complexes by CDTA

“…In aquatic environments, the speciation of solubilized transition-metal ions is predominated by complexes with in situ multidentate ligands (e.g., metallophores, synthetic chelating agent contaminants, , natural organic matter). , Speciation is far from equilibrium due to the slow kinetics of multidentate ligand exchange reactions where M is the metal ion, and both L and Y are multidentate ligands (charges, protonation, and bound water molecules are omitted here for simplicity). ,,, The stepwise dissociation of relatively stable initial complexes, ML, during multidentate ligand exchange can produce very low rates of reaction. Furthermore, time scales of these reactions vary by many orders of magnitude as the rate-determining step shifts due to factors including steric interactions, ligand structure, ligand/complex rigidity, ligand protonation, electrostatic interactions, and electronic effects. − …”