Studying Coordination at High Temperatures

Of the several methods available for studying the structure of coordination compounds as room-temperature solids or solutions, very few have obvious simple application to the liquid and vapor phases at high temperatures. X-ray and neutron diffraction studies did reveal the average coordination numbers in single alkali halides, for example, and we might anticipate some future success with electron diffraction of simple vapors. 

The method of vibrational spectroscopy has found useful application to groups such as the NOg ion but studies of such entities as NiClf or PtCll by the laser Raman technique are still awaited. While NMR, ESR, and magnetic susceptibility studies of molten salt systems have been made, only the last have been of much use in coordination assignment. The techniques used to measure stability constants of complexes, such as polarography, potentiometry, and chromatography, are beginning to be applied to molten salt systems but the results to date require caution in interpretation. 

Phase rule studies,vapor pressure measurements,and mass spectrometry are useful in so far as they give indications of stoichiometry but we must not assume that the formation of the compound CsgNiCls, for example, is evidence for 5-coordinate nickel. 

To date the most useful method for studying coordination of transition metal ions in melts and a few vapors has been electronic spectroscopy and this will form the kernel of the succeeding sections. 

A thorough review of the coordination chemistry of Group VIII would, by this time, fill a book of several volumes. We would draw the attention of our readers to three texts and to three serial publications. 

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