Q.What are Lewis acids and bases ?

A ligand is any ion or molecule directly linked to a central metal atom or ion through a coordinate bond, formed when the ligand alone contributes the shared electron pair while the electron-deficient metal ion accepts it -- so, in Lewis acid-base terms, every ligand is behaving as a Lewis base (electron-pair donor) and the metal ion as the Lewis acid (electron-pair acceptor). Ligands are classified by how many donor atoms they use to bind ONE metal ion. A monodentate ligand has a single donor atom and forms exactly one coordinate bond, e.g. Cl-, OH- or CN- (donor atoms Cl, O and N respectively). A polydentate ligand has two or more donor atoms, all bonding the same metal ion simultaneously (a process called chelation); a two-donor-atom polydentate ligand is specifically called bidentate, e.g. ethylenediamine (en), H2N-CH2-CH2-NH2, which binds through both its terminal nitrogen lone pairs, or the oxalate ion, C2O4(2-), which binds through one oxygen lone pair from each of its two carboxylate ends. A ligand can be polydentate to a much higher degree still: ethylenediaminetetraacetate (EDTA4-) is hexadentate, binding through two nitrogen atoms and four oxygen atoms (six donor atoms total) simultaneously …