Time-resolved Infrared Spectroscopy

In these days of the availability to coordination chemists of very sophisticated instrumentation, why should we do infrared spectroscopy, one of the classical ‘‘sporting’’ methods? The short answer is when it provides information that would not otherwise be readily obtainable. For example, X-ray crystallography, which is now such a rapid technique, gives very limited information on solutions. IR spectroscopy is often used as a rapid analytical tool for the characterization of coordination complexes. Some of the more common uses of IR spectroscopy in coordination chemistry include: (i) detecting the presence of particular ligands (e.g., CO, CN, etc.) and their mode of binding, e.g., whether they are terminal or bridging; (ii) determining the mode of coordination of some ligands (e.g., NO₂,¹ N₂,² or H₂)³; or some further structural information, using new subtle approaches;^4,5 (iii) providing some information on bonding (electron distribution, oxidation state, etc.); and (iv) monitoring equilibria and following the progress of reactions, for which IR spectroelectrochemistry⁶ is a particularly good example. However, IR becomes a particularly powerful structural probe when it is used to monitor kinetic processes, from the very slow to the extraordinarily fast, in the latter case particularly for electron-transfer processes. As we shall see, this can refer either to direct, time-resolved experiments or to band width/coalescence behavior, which is very useful for mixed valence compounds. All this requires some understanding about the principles of vibrational spectroscopy, force constants, the use of symmetry, and knowledge of the frequencies (and intensities) of a wide range of functional groups.