HSAB theory
HSAB concept is an initialism for "hard and soft (Lewis) acids and bases". Also known as the Pearson acid-base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. 'Hard' applies to species which are small, have high charge states, and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable.
Read article on Wikipedia