Contents I. Introduction 81 A. General Aspects 81 B. Brief Comparison of the Methods for me 82 Characterization of Unstable Intermediates 11. Methods and Devices 82 A. General Remarks 82 B. Main Elements of Gas Stream X-ray 83 Diffraction Cooling Devices 1. Basic Principles 83 2. Gas Supply UnR 83 3. Delivery Nozzle 84 4. Transfer Line 84 5. Refill System 84 8. Automatic Temperature Control 85 85 Crystals 1. Crystal Growth in a Capillary 85 2. Crystal Growth in One Place and 86 Transfer to the Diffraction Device under Protective Conditions Elsewhere 111. Use of Three-Dimensional Structures of 87 Unstable Inorganic Molecules and Solids in the Evaluation of Their Reactivity C. Growih and Handling of Thermolabile Single A. Correlation of Instability and Molecular 87 B. Reaction Intermediates 89 C. Unstable Phases in Solid-state Chemistry 90 IV. Conclusion and Outlook 90 V. Acknowledgments 90 VI. References 90 Structure I. Introductlon A. General Aspects The chemist is currently confronted with compounds that are unstable for either kinetic or thermodynamic reasons. I f these compounds are produced during t h e course of a reaction sequence, t h e y are designated as reaction intermediates. As well as chemical procedures, a variety of physicochemical methods has been used (see n e x t chapter) to elucidate t h e composition and three-dimensional structure of t h e intermediate. T h e most complete and exact results are obtained by diffraction methods, followed by three-dimensional structure determinations. With X-ray diffraction on single crystals, the sample has to be cooled because of its instability, and the crystal growth and transfer must b e performed under low-temperature conditions. In this review the special techniques for the synthesis and handling of single crystals of unstable compounds are described. As far as we know, there is only one