Polarizabilities of Metals and Ligands

The binding of a metal ion to a ligand can be considered in terms of Lewis acid-base theory (Lewis, 1923 Allred and Rochow, 1958 Brown and Skowron, 1990) because, in accepting an electronic pair, the metal ion acts as a Lewis acid. When a metal ion coordinates a ligand, it can affect the electron distribution of the ligand and therefore its reactivity. 

The interaction of an acid with a base is one of the best-known chemical reactions  

This reaction can be generalized by replacing the acid by an electron-pair acceptor [Lewis acid (A), e.g., metal ion] and the base by an electron-pair donor [Lewis base (B), e.g., ligand) to give  

Results of measurements of polarizabilities lead to the concept of hard 

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