Pearson suggested a simple rule for predicting the stability of co-ordination compounds formed between acids and bases and is often termed as the HSAB Principle. “ It states that hard acids prefer to bond to hard bases and soft acids prefer to combine with soft bases”.

In other words, hared acids will show the following order of preference for binding the donor atoms r ions within a group:
                                                   F^- >> CI^- > Br^- > I^-
                                                   R₂O >> R₂S
                                                   R₃N > > R₃P
Soft acids will show the opposite trend and the following order of perforce for binding the donor atoms or ions within a group is observed.

                                                   I^- >> Br > CI^- > F^-
                                                   R₂S >> R₂O
                                                   R₃P >> R₃N
Hard acids and hared bases are not easily polarizable and their interactions are predominantly ionic. On the other hand soft acids and soft bases are usually polarizable and hence their interaction are predominant covalent.

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