Hard-soft acid base , energetics

Perhaps the traditional way of looking at the energetics of such reactions uses the concept of hard and soft acids and bases and suggests an alternative expression for A//r (equation 11)... 

Finally, let us not forget that there is a single, simple frame of understanding which correctly, albeit qualitatively, predicts the energetic order of mercury carbodiimide and mercury cyanamide, and it is based on chemical ideas, namely Pearson s concept of hard and soft acids and bases soft prefers soft and hard prefers hard. The absolute softness of Hg + (7.7 eV, see Table 2.3) lies between those of Pb and Ag+ such that a cyanamide anion and not a carbodiimide anion will be the preferred bonding partner for Hg +, just as for Pb + and Ag+. I cannot refrain from noting that I find this quite remarkable. 

Koch, E. Acid-base interactions in energetic materials I. The hard and soft acids and bases (HSAB) principle—insights to reactivity and sensitivity of energetic materials. Propellants, Explosives, Pyrotechnics 2005, 30,164. 

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