Structural effects in molecular metal halides

Metal halides are a relatively large class of inorganic compounds that participate in many industrial processes, from halogen metallurgy to the production of semiconductors. Because most metal halides are ionic crystals at ambient conditions, the term "molecular metal halides" usually refers to vapor-phase species. These gas-phase molecules have a special place in basic research because they exhibit the widest range of chemical bonding from the purely ionic to mostly covalent bonding through to weakly interacting systems. Although our focus is basic research, knowledge of the structural and thermodynamic properties of gas-phase metal halides is also important in industrial processes. In this Account, we review our most recent work on metal halide molecular structures. Our studies are based on electron diffraction and vibrational spectroscopy, and increasingly, we have augmented our experimental work with quantum chemical computations. Using both experimental and computational techniques has enabled us to determine intriguing structural effects with better accuracy than using either technique alone. We loosely group our discussion based on structural effects including "floppiness", relativistic effects, vibronic interactions, and finally, undiscovered molecules with computational thermodynamic stability. Floppiness, or serious "nonrigidity", is a typical characteristic of metal halides and makes their study challenging for both experimentalists and theoreticians. Relativistic effects are mostly responsible for the unique structure of gold and mercury halides. These molecules have shorter-than-expected bonds and often have unusual geometrical configurations. The gold monohalide and mercury dihalide dimers and the molecular-type crystal structure of HgCl(2) are examples. We also examined spin-orbit coupling and the possible effect of the 4f electrons on the structure of lanthanide trihalides. Unexpectedly, we found that the geometry of their dimers depends on the f electron configuration. Metal halides are unique in exhibiting strong vibronic interactions such as the Jahn-Teller effect and the related Renner-Teller effect. Some metal trihalide molecules have an almost T-shape due to static Jahn-Teller distortions. The nonlinear structure with a 150 degree bond angle of the chromium dichloride molecule demonstrates the Renner-Teller effect. Finally, we present a few examples of unknown structures that appear to be thermodynamically stable, including gold and silver triiodides and all silver subhalides. The combination of experimental and computational techniques has brought new insights to the structural chemistry of metal halides. We expect that the continuing progress in computational chemistry will shed further light on the intricate details of these and other molecular structures.