Q.What are bidentate ligands ? Give one example.

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, powerful enough to wrap around and effectively encapsulate a single metal ion. A special, separate case is the ambidentate ligand, which genuinely HAS two different donor atoms available but uses only ONE of them at a time in any given complex, giving rise to a pair of linkage isomers -- the nitrite ion, NO2-, can bind through nitrogen ('nitro', M-NO2) or through oxygen ('nitrito', M-O-N=O), and the thiocyanate ion, SCN-, can bind through sulfur ('thiocyanato', M<-SCN) or through nitrogen ('isothiocyanato', M<-NCS).