Structures With Monodentate Ligands
Monodentate ligands can be either ions (usually anions) or neutral molecules.
In many three-dimensional representations spheres are used for simplicity and clarity. Stick structures are similarly used when it is desired to show all atoms and preserve the three-dimensional structure of the ligand. For the coordination complex, [Ni(NH3)6]2+, each ammonia ligand is represented as a sphere, an abbreviated structure, in the figure below left, the sphere represents the nitrogen donor atom of the ligand and in the stick representation for the same complex shown in the figure below right the nitrogen donor atoms are shown with attached hydrogens, a complete structure.
Common Applications of Monodentate Ligands
The cation tetraamminecopper (II), [Cu(NH3)4]2+ is common in waterbed conditioners, responsible for preventing the growth of fungi and bacteria. The cation is dark-blue in color and has a square planar geometry. The dicarbonyldiiodoruthenium (II) cation, [RhI2(CO)2]-, is used as a catalyst in the "Monsanto Process" for making acetic acid, the component in vinegar which gives it its acidity and subsequent tang.
Structures With Bidentate Ligands
Bidentate ligands have two donor atoms donate two pairs ("bi") of electrons to a metal atom. The neutral ligand ethylenediammine, abbreviated as en, contains two N atoms that each donate a pair of electrons to a metal atom. A stick figure representation for ethylenediamine is shown below left. As the two amine groups are joined by an ethylene group a five membered ring is formed after the two nitrogen atoms have coordinately bonded to the central metal atom as shown in the figure below right. This five-membered ring is a very stable structure just as five-membered rings are for organic compounds. Ethylenediamine is therefore a chelating ligand.
ethylenediammine (en) cis-[Ni(en)2Cl2]
The oxalate anion contains four O atoms, two carbonyls and two deprotonated alcohol groups, each capable of making a coordinate bond. Resonance structures suggest that the negative charge is delocalized between two sp2 hybridized oxygens bridged by a sp2 hybridized carbon. The formation of the two coordinate bonds localizes the electrons by electrostatic attraction to the metal ion changing the hybridization of the bonded oxygens to sp3. In the complex [Ni(ox)2]2-, below right, two oxalate ions are bonded to a Ni2+ ion. The CN of 4 for the 3d8 electronic configuration Ni2+ is square planar.