Firstly a nucleophile is an electron rich species with a lone pair which can attack an electron deficient centre . A hard nucleophile is generally one which is small and highly charged and tends to interact with electrophiles which are also hard . Interactions between hard Lewis Acids and Bases have a lot of ionic character and depend a lot on electrostatic attraction . Classic examples are Fluoride and Hydroxide . They ' ll prefer interactions with things like early Transition metals ions eg TiIII , Group 1 and 2 ions , and Lanthanide ions . Soft Nucleophiles tend to be larger and less highly charged with large electron clouds . Interactions between soft Lewis acids and bases tend to be much more covalent and determined by orbital overlap . Classic examples are Iodide and Thiolate . They prefer interactions with species which have higher degrees of covalency such as later Transition metals and electrostatic attraction is less relevant .