Effects of high external pressures on the electronic spectra of coordination compounds

The use of pressure to perturb the electronic and physical structure of coordination compounds provides an important insight into the nature of the interaction between a transition-metal center and the surrounding ligands as well as intermolecular interactions. This perturbation of the electronic structure has become known as 'pressure-tuning' - a technique which has found many applications in science. These types of measurements can reveal interesting phenomena such as changes in electronic spectra due to structural distortions (piezochromism), spin-crossovers (changing the ground electronic state of the coordination compound), changes in the ligand-field causing shifts in electronic transitions, and changes in radiative processes and emitting states. When the conventional theoretical parameters for analyzing electronic spectra are adapted to accommodate pressure as a variable, changes to the electronic spectra of coordination compounds can be rationalized and explained. The diamond-anvil cell, DAC, is the most widely used device to apply pressure to a sample. For applications in electronic spectroscopy, there are several types of DACs that are used to study perturbations of electronic energy levels. In this review, some examples of the effects of pressure-tuning on the electronic energy levels of coordination compounds will be described. These examples will be grouped according to the particular electronic phenomenon exhibited upon application of pressure or the class of coordination compounds studied. Also, the most common DACs currently available for high-pressure research will be illustrated and the techniques for their use will be discussed. (C) 2001 Elsevier Science B.V. All rights reserved.