Cooperation between bound waters and hydroxyls in controlling isotope-exchange rates

Mineral oxides differ from aqueous ions in that the boundwater molecules are usually attached to different metal centers, or vicinal, and thus separated from one another. In contrast, for most monomeric ions used to establish kinetic reactivity trends, such as octahedral aquo ions (e.g., Al(H2O)63+), the boundwaters are closely packed, or geminal. Because of this structural difference, the existing literature about ligand substitution in monomer ions may be a poor guide to the reactions of geochemical interest. To understand how coordination of the reactive functional groups might affect the rates of simple water-exchange reactions, we synthesized two structurally similar Rh(III) complexes, [Rh(phen)2(H2O)2]3+ [1] and [Rh(phen)2(H2O)Cl]2+ [2] where (phen) = 1,10-phenanthroline. Complex [1] has two adjacent, geminal, boundwaters in the inner-coordination sphere and [2] has a single boundwater adjacent to a bound chloride ion. We employed Rh(III) as a trivalent metal rather than a more geochemically relevant metal like Fe(III) or Al(III) to slow the rate of reaction, which makes possible measurement of the rates of isotopic substitution by simple mass spectrometry. We prepared isotopically pure versions of the molecules, dissolved them into isotopically dissimilar water, and measured the rates of exchange from the extents of 18O and 16O exchange at the boundwaters.