Ligands can be bidentate (e.g. NH2CH2CH2NH2 and ethanedioate ion C2O4 2- ) which have two atoms with lone pairs and can form two coordinate bonds per ligandCu(H2O)6 2+ + EDTA4- [Cu(EDTA)]2- + 6H2O Equations to show formation of mutidentate complexes N CH2 CH2 N CH2 CH2 CH2 CH2 C C C C O O O -O -O OOO The EDTA4- anion has the formula with six donor sites(4O and 2N) and forms a 1:1 complex with metal(II) ions 3- C C O O C C O O C C O O Cr O O O O O O 3+ H2C NH2 NH2 CH2 H2C NH2 NH2 CH2 CH2 NH2 NH2 Cr CH2 A complex with bidentate ethanedioate ligands e.g. [Cr(C2O4 )3 ]3- Learn the two bidentate ligands mentioned above but it is not necessary to remember the structure of EDTA. There are 3 bidentate ligands in this complex each bonding in twice to the metal ion. N Goalby chemrevise.org 2 Ligands can be multidentate (e.g. EDTA4- which can form six coordinate bonds per ligand). Bidentate Ligands Cu(H2O)6 2+ + 3NH2CH2CH2NH2 [Cu(NH2CH2CH2NH2 )3 ]2+ + 6H2O Cu(H2O)6 2+ + 3C2O4 2- [Cu(C2O4 )3 ]4- + 6H2O Equations to show formation of bidentate complexes Octahedral shape with 90o bond angles Ethane-1-2-diamine is a common bidentate ligand. Ethane-1-2-diamine Ethanedioate Partial substitution of ethanedioate ions may occur when a dilute aqueous solution containing ethanedioate ions is added to a solution containing aqueous copper(II) ions. In this reaction four water molecules are replaced and a new complex is formed. 2- OH2 O O C C O O Cu O O C C O O OH2 Cu(H2O)6 2+ + 2C2O4 2- [Cu(C2O4 )2 (H2O)2 ]2- + 4H2O Multidentate Ligands Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood. CO is toxic to humans as CO can from a strong coordinate bond with haemoglobin. This is a stronger bond than that made with oxygen and so it replaces the oxygen attaching to the haemoglobin.
188.8.131.52 Substitution reactions (A-level only)
Ligands can be bidentate (eg H2 NCH2 CH2 NH2 and C2 O4 2–).
Ligands can be multidentate (eg EDTA4–).
Haem is an iron(II) complex with a multidentate ligand.
Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood. Carbon monoxide is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin.