The hard-hard or soft-soft factors according to HSAB principle are independent of acidic or basic character of compounds. However sometimes this rule is violated by inherent acid or base strength leading to more stable hard soft systems. For example according to HSAB principle, the following reactions not favored
                                             SO₃^2-          +        HF     →       HSO₃^-       +        F-
                                            (soft-base)          (hard)         (hard-soft)           (hard)

However, it occurs because the stronger, soft-base SO₃^2- displaced the weaker, hared base F- form the hard acid H⁺. Similarly , a very strong hard base OH^- can displace a less strong soft base, SO₃^2- form the soft acid CH₃Hg⁺ .
                                             CH₃H_gSO₃-   +       OH-     →       CH₃HgOH    +       SO₃^2-
                                           (soft-soft)                 (hard)            (soft-hard)               (soft)

For more help in Limitation Of Hsab Principle click the button below to submit your homework assignment