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

The use of pressure to perturb the electronic and physical structure of coo rdination compounds provides an important insight into the nature of the in teraction between a transition-metal center and the surrounding ligands as well as intermolecular interactions. This perturbation of the electronic st ructure has become known as 'pressure-tuning' - a technique which has found many applications in science. These types of measurements can reveal inter esting phenomena such as changes in electronic spectra due to structural di stortions (piezochromism), spin-crossovers (changing the ground electronic state of the coordination compound), changes in the ligand-field causing sh ifts in electronic transitions, and changes in radiative processes and emit ting states. When the conventional theoretical parameters for analyzing ele ctronic 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, th ere are several types of DACs that are used to study perturbations of elect ronic energy levels. In this review, some examples of the effects of pressu re-tuning on the electronic energy levels of coordination compounds will be described. These examples will be grouped according to the particular elec tronic phenomenon exhibited upon application of pressure or the class of co ordination compounds studied. Also, the most common DACs currently availabl e for high-pressure research will be illustrated and the techniques for the ir use will be discussed. (C) 2001 Elsevier Science B.V. All rights reserve d.