Complex Ions 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. A central metal ion surrounded by ligands. 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. LigandNo. coordinate bonds Examples Unidentate1H 2 O, NH 3, Cl -, CN - Bidentate21,2 diaminoethane NH 2 CH 2 CH 2 NH 2 Ethandioate C 2 O 4 2- Hexadentate6EDTA 4- 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.

Unidentate LigandComplexShape & Bond Angle Coordination Number Water[Cu(H 2 0) 6 ] 2+ Octahedral 906 Ammonia[Co(NH 3 ) 4 (H 2 0) 2 ] 2+ Octahedral 906 Chloride ion[Cu(Cl) 4 ] 2- Tetrahedral Ammonia/Chloride[Pt(NH 3 ) 2 (Cl) 2 ]Square Planar 904 Cyanide - [Cu(CN) 4 ] 2- Tetrahedral Cyanide, CN - [Ni(CN) 4 ] 2- Square Planar 904 Examples of Bidentate Ligands Ethanedioate (oxalate) C 2 O 4 2- Ethanedioate is able to form 2 coordinate bonds with the metal ion by donating 2 lone pairs of electrons (from oxygen atoms). !,2-diaminoethane (ethylenediamine) NH 2 CH 2 CH 2 NH 2 Etylenediamine is able to form 2 coordinate bonds with the metal ion by donating 2 lone pairs of electrons (form nitrogen atoms).

Stereoisomerism Stereoisomers have the same structural formula but they have a different spatial arrangement of their atoms. Cis isomer: unidentate ligands are 90° apart. Trans isomer: unidentate ligands are 180 ° apart. This is an example of stereoisomerism in a square planar complex. In the cis-isomer, the two Cl unidentate ligands are separated by 90. In the trans-isomer, the Cl unidentate ligands are separated by 180. Cis Trans Cis Trans Cis platin 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).

Ligand Substitution Reactions [Cu(H 2 0) 6 ] Cl -  [Cu(Cl) 4 ] H 2 O  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(H 2 0) 6 ] NH 3  [Cu(NH 3 ) 4 (H 2 0) 2 ] H 2 O  Change in colour from blue to dark blue  No change in coordination number  No change in charge [Co(H 2 0) 6 ] Cl -  [Co(Cl) 4 ] H 2 O  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.

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. Oxygen molecules bind to the Fe 2+ ion in the vacant 6 th position of the octahedral complex. The binding of the O 2 is weak enough to allow it to be removed easily. 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 O 2. This leads to drowsiness, lack of consciousness and can be fatal. Globin Protein O2O2