 Complexes containing bidentate or multidentate ligands are called chelates  Chelation is the process of forming a complex with bidentate or multidentate ligands  Bidentate or multidentate ligands substitute unidentate ligands such as water or ammonia ligands  The chelated complex formed is more stable

 A ligand that can donate two lone pairs of electrons is a bidentate ligand  The two bidentate ligands you need to know are:  1,2-diaminoethane, H 2 NCH 2 CH 2 NH 2  Ethanedioate ion, C 2 O 4 2- Show the lone pairs on the correct atom

 Three bidentate ligands can replace six unidentate ligands in complexes  General equation: [M(H 2 O) 6 ] H 2 NCH 2 CH 2 NH 2 ⇌ [M(H 2 NCH 2 CH 2 NH 2 ) 3 ] H 2 O  The same equation applies for [M(H 2 O) 6 ] 3+  The position of equilibrium lies to the right as the chelated complex is very stable The same equation applies for the ethanedioate ion, C 2 O 4 2-

 Example 1: Hexaaquacopper(II) and 1,2-diaminoethane [Cu(H 2 O) 6 ] H 2 NCH 2 CH 2 NH 2 ⇌ [Cu(H 2 NCH 2 CH 2 NH 2 ) 3 ] H 2 O  Example 2: Hexaaquachromium(III) and 1,2-diaminoethane [Cr(H 2 O) 6 ] H 2 NCH 2 CH 2 NH 2 ⇌ [Cr(H 2 NCH 2 CH 2 NH 2 ) 3 ] H 2 O  There is no change in co-ordination number or shape

 The same equation also applies with [M(NH 3 ) 6 ] 2+ [Cu(NH 3 ) 6 ] H 2 NCH 2 CH 2 NH 2 ⇌ [Cu(H 2 NCH 2 CH 2 NH 2 ) 3 ] NH 3  The effect of chelation is that a more stable complex formed and there is an increase in entropy  There are 4 molecules on the left and 7 on the right  The amount of disorder increases  The entropy increases

 A ligand that can donate more than two lone pairs of electrons is a multidentate ligand  EDTA 4- is a multidentate ligand  The ligand EDTA 4- can donate six lone pairs of electrons to the central metal ion There is a lone pair on two nitrogen atoms and four oxygen atoms EDTA stands for ethylenediaminetetraacetic acid

 One EDTA 4- can replace six unidentate ligands in complexes  General equation: [M(H 2 O) 6 ] 2+ + EDTA 4- ⇌ [M(EDTA)] H 2 O  The same equation applies for [M(H 2 O) 6 ] 3+  The position of equilibrium lies very far to the right as the chelated complex is very stable

 Example 1: Hexaaquacopper(II) and EDTA 4- [Cu(H 2 O) 6 ] 2+ + EDTA 4- ⇌ [Cu(EDTA)] H 2 O  Example 2: Hexaaquachromium(III) and EDTA 4- [Cr(H 2 O) 6 ] 2+ + EDTA 4- ⇌ [Cr(EDTA)] - + 6H 2 O  The co-ordination number or shape does not change  The overall charge on the complex changes

[M(H 2 O) 6 ] 2+ + EDTA 4- ⇌ [M(EDTA)] H 2 O  The position of equilibrium lies very far to the right as the chelated complex is very stable  There are 2 molecules on the left and 7 on the right  The amount of disorder increases  The entropy increases