Hard-soft classification

It is established that the comparative strength of donors relies on the nature of the acceptor and vice versa. The hard and soft acid-base (HSAB) classification is frequently employed to rationalize some of the variations. While two acids (A₁  and A₂) are in competition for two bases (B₁  and B₂) the equilibrium

Fig. 1. Molecular orbital interaction between a donor (:D) and an acceptor (A).

will lie down in the direction in which the harder of the two acids is in combination  with the harder base and the softer acid with the softer base. Like a standard for comparison the prototype hard acid H⁺  and soft acid [(CH₃)Hg]⁺  are frequently used. So the equilibrium will lie to the left side or right as per degree of hardness of the base B.

Instances of hard acids are H⁺, cations of very electropositive metals like Mg²⁺ and nonmetal fluorides such as BF₃. Soft acids that includes cations of late transition and post-transition metals like Cu⁺, Pd2⁺  and Hg2⁺. The hardness of bases get increases with  the group  number  of the donor  atom  (example  NH₃<H₂O<F^-) and get decreases down  any group (example  NH₃>PH₃, and F^->Cl^->Br^->I^-).

Even though the hard-soft classification provides a helpful systematization  of several trends it does not by itself provide an description of the different behavior. Usually it is referred as that hard-hard interactions have a greater electrostatic component and soft-soft ones rely more  on orbital  interactions  but several  other issues may be involved.  Soft donor and acceptor atoms are frequently large and van der Waals' forces might contribute to the bonding; some soft bases like CO also demonstrate π-acceptor behaviour.  It is also significant to remember that hard and soft behaviour is defined in a competitive situation. While reactions are studied in solution some competition  with solvation is all the time present.