Complex Ions

1. Complex Ions A central metal ion surrounded by ligands. A ligand is a species that can donate one or more lone pairs of electrons to form a coordinate bond with a central metal ion. This is the hexaquacopper(II) complex ion. The water molecules are acting as ligands – specifically unidentate ligands, as each water is donating one lone pair of electrons in order to form a coordinate bond. The coordination number of a complex is the coordinate bonds formed to the central metal cation in the complex ion. So, for hexaquacopper(II), it is 6. The molecule shape above is octahedral, with a bond angle of 90 degrees. Octahedral molecules have a coordination number of 6. A bidentate ligand donates 2 lone pairs of electrons to form 2 coordinate bonds with a central metal ion. A hexadentate ligand donates 6 lone pairs of electrons to form 6 coordinate bonds with a central metal ion.

2. Examples of Bidentate Ligands Ethanedioate is able to form 2 coordinate bonds with the metal ion by donating 2 lone pairs of electrons (from oxygen atoms). Ethanedioate (oxalate) C2O42- Etylenediamine is able to form 2 coordinate bonds with the metal ion by donating 2 lone pairs of electrons (form nitrogen atoms). !,2-diaminoethane (ethylenediamine) NH2CH2CH2NH2

3. Stereoisomerism Stereoisomers have the same structural formula but they have a different spatial arrangement of their atoms. Cisisomer: unidentate ligands are 90° apart. Transisomer: unidentate ligands are 180 ° apart. This is an example of stereoisomerism in a square planar complex. In the cis-isomer, the two Clunidentate ligands are separated by 90. In the trans-isomer, the Cl unidentate ligands are separated by 180. CisTrans Cisplatin is used as an anti-cancer drug. It prevents the division of cancer cells by binding to the DNA, and ultimately causes apoptosis – programmed cell death. Optical Isomerism Optical isomers are stereoisomers that are non-superimposable mirror images of each other (also called enantiomers).

4. Ligand Substitution Reactions • Change in colour from blue to yellow/green • Change in coordination number from 6 to 4: shape from octahedral to tetrahedral • Change in charge as chloride ions are negative. [Cu(H20)6]2+ + 4Cl-  [Cu(Cl)4]2- + 6H2O [Cu(H20)6]2+ + 4NH3 [Cu(NH3)4(H20)2]2+ + 4H2O • Change in colour from blue to dark blue • No change in coordination number • No change in charge [Co(H20)6]2+ + 4Cl-  [Co(Cl)4]2- + 6H2O • Change in colour from pink to blue • Change in coordination number from 6 to 4: shape from octahedral to tetrahedral • Change in charge as chloride ions are negative.

5. Haemoglobin The haem molecule consists of a porphyrin ring that forms four coordinate bonds with an iron (II) ion. The globin protein forms a fifth coordinate bond with the iron (II) ion. Carbon monoxide, CO, bonds to haemoglobin with a higher affinity than oxygen. If a person inhales CO, a ligand exchange reaction occurs and the CO prevents the transport of O2. This leads to drowsiness, lack of consciousness and can be fatal. Globin Protein Oxygen molecules bind to the Fe2+ ion in the vacant 6th position of the octahedral complex. The binding of the O2 is weak enough to allow it to be removed easily. O2