Academic literature on the topic 'Metal-metal bond multinuclear NMR'

The kinetic and mechanistic aspects of this general area tend to be strongly dependent on the particular system. This makes general treatments and explanations impossible, at least at the current stage of understanding. Various aspects of this area have been summarized in some general reviews. Ligands bonded to a metal can undergo a number of structural changes that do not involve complete breaking of the metal-ligand bond(s). Such processes are the subject of the following sections. Many chelate ligands have conformers that can interconvert. For example, the conformers of ethylenediamine interchange by rotation about the carbon-carbon bond, as shown in the following structures: The Ha and Ha' protons are magnetically different from the Hb and Hb' protons, so their interconversion can, in principle, be studied by NMR. These protons may be referred to as exo and endo, respectively. In simple systems, their interconversion is too rapid (k >106 s-1) for this method. However, if there is some constraint (e.g., CH3 groups) or if the coordinating atoms are part of a larger chelate system, then interconversion is slow enough to be detected by NMR. In nonplanar Fe(III)- tetraphenylporphyrinates, the ring inversion rates vary widely, depending on the axial ligand and the substituents on the porphyrin.

Advances in NMR instrumentation and availability have led to increased application to mineral systems and to environmental problems. The sensitivity of high-field NMR systems is nearly sufficient to work at real environmental concentrations. Even with limited sensitivity, the amount of chemical information obtained through NMR spectroscopy makes it a very valuable technique in many model systems. The application of NMR spectroscopy in mineral systems has been primarily limited to studies of the structural metals aluminum and silicon. However, in recent years there have been several publications on mobile cations in minerals, including work on the exchangeable cations in clays. Our interests lie in understanding the sorption of cations in clays, the structural sites available for that sorption, and the role of water in cation–clay interactions. Our goal is to eventually understand the molecular interactions that determine the adsorption and diffusion of cations in clays and, thus, the role of clays in determining cation transport through the geosphere. This fundamental understanding has applications in the fate of heavy metals, radionuclides, and even the mobility of nutrients for plants. It is well known that there are very strong interactions between metals and humic materials and these are also strong contributors to cation mobility. However, for simplicity, we have chosen to focus on the interactions of mobile metal ions with well-characterized clays. An NMR-based approach to this problem can take two complementary directions: first, studies of the structural components of clays such as 29Si and 27Al NMR as a function of cation or hydration; second, NMR studies of probe molecules—which in this case are the cations themselves. High magnetic field, multinuclear NMR spectrometers make it quite possible to study various “uncommon” nuclei with relative ease. It is our experience that using the cations as probe nuclei for studying sorption phenomena yields more information than studies of structural nuclei. This chapter is basically a report of work in progress on several systems that are starting to yield interesting results, which it is hoped will lead to a general understanding of these complex systems.