Aqueous Aluminum Polynuclear Complexes and Nanoclusters: A Review

Abstract

Most undergraduate students of aqueous geochemistry are told that polynuclear aqueous complexes can largely be ignored because they form only from concentrated metal solutions that are rare at the Earth’s surface. However, these polynuclear complexes can serve as models for more-complicated surface structures and are the precursors to nanometric and colloidal solids and solutes. There are many reasons why polynuclear complexes should be foremost in the minds of geochemists, and particularly those geochemists who are interested in molecular information and reaction pathways: 1. Polynuclear complexes contain many of the structural features that are present at mineral surfaces, including a shell of structured water molecules. Because aqueous nanoclusters tumble rapidly in an aqueous solution, one can use solution NMR spectroscopy to determine the structure and the atomic dynamics in these clusters in ways that are impossible for mineral surfaces. 2. Some polynuclear complexes are metastable for long periods of time and may represent an important vector for the dispersal of metal contaminants from hazardous waste. The chemical conditions found in many polluted soils: high metal concentrations, elevated temperatures, and either highly acidic or highly alkaline solutions with a large pH-gradient, are needed to synthesize many polynuclear complexes. It is easy to make a solution that is 5 M in dissolved aluminum at 4 < pH < 6, composed of nanometer-sized clusters that are stable for months or years. 3. Polynuclear complexes lie at the core of many biomolecules, including metalloproteins such as ferritin and enzymes such as nitrogenase. Recent work has suggested that they are present in natural waters (e.g., Rozan et al. 2000) and serve as nuclei for crystal growth. 4. Aqueous clusters are sufficiently small that they can serve as experimental models for ab initio computer simulations that relate bonding to reactivity. In this chapter we discuss polynuclear complexes of aluminum. There is …