ON PEARSON HSAB PRINCIPLE

Some 35 examples of the exchange reaction AB + CD = AC + BD (type 1), for which Pearson's HSAB (hard-soft acid-base) principle was originally devised, are examined in terms of the experimental or calculated hardness (eta) values of the various species involved. The calculation of the eta-values has been done at the MNDO level by using eta = (epsilon(LUMO) - epsilon(HOMO))/2, where epsilon is energy, LUMO indicates the lowest unoccupied molecular orbital, and HOMO is the highest occupied molecular orbital. In the case of some 75 molecules it is found that the eta-values calculated at the MNDO level match quite well with the experimental eta-values. The examples of the exchange reaction are so chosen that the experimental hardnesses of at least three of the four species involved in a particular example are known. From the present study it is concluded that an exchange reaction proceeds in a direction so as to produce the hardest possible species and the average value of the hardnesses of the products is greater than that of the reactants in a reaction of type 1. The results are explained in terms of the chemical reactivity of the four molecules involved in a particular example. Of the 35 examples studied there are only 5 exceptions.