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.

Ligand No. coordinate bonds

Examples

Unidentate 1 H2O, NH3, Cl-, CN-

Bidentate 2 1,2 diaminoethane NH2CH2CH2NH2

Ethandioate C2O42-

Hexadentate 6 EDTA4-

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 Ligand Complex Shape & Bond Angle

Coordination Number

Water [Cu(H20)6]2+ Octahedral 90 6

Ammonia [Co(NH3)4(H20)2]2+ Octahedral 90 6

Chloride ion [Cu(Cl)4]2- Tetrahedral 109.5 4

Ammonia/Chloride [Pt(NH3)2(Cl)2] Square Planar 90 4

Cyanide- [Cu(CN)4]2- Tetrahedral 109.5 4

Cyanide, CN- [Ni(CN)4]2- Square Planar 90 4

Examples of Bidentate Ligands

Ethanedioate (oxalate) C2O4

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) NH2CH2CH2NH2

Etylenediamine is able to form 2 coordinate bonds with the metal ion by donating 2 lone pairs of electrons (form nitrogen atoms).

StereoisomerismStereoisomers 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 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 IsomerismOptical isomers are stereoisomers that are non-superimposable mirror images of each other (also called enantiomers).

Ligand Substitution Reactions [Cu(H20)6]2+ + 4Cl- [Cu(Cl)4]2- + 6H2O

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+ + 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.

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 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.

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

O2