Lewis acid?

Lewis Acids are the chemical species which have empty orbitals and are able to accept electron pairs from Lewis bases. This term was classically used to describe chemical species with a trigonal planar structure and an empty p-orbital. An example of such a Lewis acid would be BR₃ (where R can be a halide or an organic substituent).

Some common examples of Lewis acids which can accept electron pairs include:

• H⁺ ions (or protons) can be considered as Lewis acids along with onium ions like H₃O⁺.
• The cations of d block elements which display high oxidation states can act as electron pair acceptors. An example of such a cation is Fe³⁺.
• Cations of metals such as Mg²⁺ and Li⁺ can form coordination compounds with water acting as the ligand. These aquo complexes can accept electron pairs and behave as Lewis acids.