Unique properties with hard/soft

This review deals with the chemistry and coordination complexes of isoelectronic analogues of common oxo-anions of phosphorus such as PO3, POl", RPOl" and R2POy. The article begins with a discussion of homoleptic systems in which all of the 0x0 ligands are replaced by imido (NR) groups. This is followed by an account of heteroleptic phosphorus-centered anions, including [RN(E)P(/<-NR )2P(E)NR]2-, [EP(NR)3]3-, [RP(E)(NR)2] and [R2P(E)(NR )] (E=0,S, Se, Te). The emphasis is on the wide variety of coordination modes exhibited by these poly-dentate ligands, which have both hard (NR) and soft (S, Se or Te) centers. Possible applications of their metal complexes include new catalytic systems, coordination polymers with unique properties, and novel porous materials. 

Resonant excitation of core levels involves the use of tunable soft- and hard-X-rays. This energy regime has widely been used since decades for a variety of investigations on atomic, molecular, surface and condensed matter physics, where synchrotron radiation is currently the primary X-ray source for experimental work in this field. Earlier work on resonant excitation of core levels has already shown that there are numerous unique properties, such as e.g. element specific excitations. Methods for determining structural properties, as well as site-selective fragmentation, promised important applications with respect to fundamental and applied research on size effects of matter. 

In Chapter 1, we observed that plastics have experienced a phenomenal growth since World War n, when they assumed enhanced commercial importance. This explosive growth in polymer applications derives from their competitive costs and versatile properties. Polymers vary from liquids and soft rubbers to hard and rigid solids. The unique properties of polymers coupled with their light weight make them preferable alternatives to metallic and ceramic materials in many applications. In the selection of a polymer for a specific end use, careful consideration must be given to its mechanical properties. This consideration is important not only in those applications where the mechanical properties play a primary role, but also in other applications where other characteristics of the polymer sueh as eleetrical, optical, or thermal properties are of crucial importance. In the latter cases, mechanical stability and durability of the polymer may be required for the part to perform its function satisfactorily. 

Polyurethane is a unique material that offers the elasticity of rubber combined with the toughness and durability of metal. Because urethane is available in a very broad range of hardness (soft as an eraser to hard as a bowling ball), it allows the engineer to replace rubber, plastic, and metal with the ultimate in abrasion resistance and physical properties. 

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