HSAB, general description

According to the HSAB principle, hard acids react preferentially with hard bases and soft acids react preferentially with soft bases. It is clear from the above description of the various factors leading to the enunciation of the HSAB principle that the general concepts underlying the HSAB principle are not new. To a good approximation, hardness is synonymous with electrovalent bonding while softness is not the same as covalent bonding. 

The original essentially empirical nature of the HSAB principle has perhaps hindered it from becoming accepted as a general principle of nature. In Sect. 4 we describe formal proof for this principle with certain restrictions added [15]. The qualitative description for hard and soft acids and bases as given above of course is not helpful in proving the HSAB principle from basic theoretical principles. Furthermore, it does not easily allow the rank ordering of different acids and bases. 

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