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 Just Accepted

This article has been peer reviewed and accepted for publication. It is in production and has not been edited, so may differ from the final published form.


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

Bill Casey

Abstract

It is via ligand-exchange kinetics that environmental geochemists can establish an understanding of molecular processes, particularly for insulating oxides where there are not explicit electron exchanges. The subject is enormous and it is useful to organize reactivity trends by separately treating terminal- and bridging functional groups on small molecules, and then examine large nanometer-sized aqueous oxide ions that more closely resemble minerals. The substitution of ligands for terminal water molecules and hydroxyl functional groups, which are the essence of solute adsorption, are robust and relatively insensitive to small changes in structure. They are, of course, sensitive to bond strengths and acid-base chemistry, which means they often scale with the metal-charge densities and can vary enormously for different metals. Predicting rates is a conspicuous success of rare-events methods of simulation. 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 to finish the chelate. In contrast, substitutions at bridging oxygens are exquisitely sensitive to details of the ion structure, and of course molecular size. Included in this class are oxygen-isotope-exchange and mineral-dissolution reactions. There are common features of the reactions that have been inferred from experiments on the nanometer-sized ions. In these large ions, metastable structures form as intermediates by detachment of a surface metal atom from an underlying, highly coordinated oxygen, such as μ4-oxo or μ6-oxo, via solvation forces. A metastable equilibrium is established via concerted motion of many atoms, and because of this heteroatom substitutions have an averaging effect on isotope-exchange reactions. The newly undercoordinated metal in the intemediate 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 and thus are probably not worth speculating about without detailed structural information, such as would be available in a cluster of defined stoichiometry but not usually at mineral interfaces.

EN14043  Accepted 27 May 2014
 
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