Types of Coordination Complexes

Coordination complexes contain metal in neutral and different oxidation states and are surrounded by the ligands through covalent coordinate bonds. The types of coordination complexes depend on various factors:

1. Charge on the coordination sphere
2. Type of the ligands in coordination compounds
3. Number of the metal in the coordination sphere

Ligands in coordination compounds

Ligands in the coordination compounds are atoms, molecules, and ions that coordinate with the central metal. They generally act as Lewis bases and donate electrons to metal ions through a dative or coordinate bond between metal and ligands.

Numerous ligands are present in the coordination chemistry, with different and unique properties. The type of ligands depends on the donating sites of the ligands.

Ligands can be classified based on their charges during the coordinate bond formation between metals and ligands as follows:

1. Cationic ligands:  Ligands that have an overall positive charge are called cationic ligands. E.g., NO+, N2H5+.
2. Anionic Ligands: Ligands that have an overall negative charge are called anionic ligands. E.g., CN-, Cl-
3. Neutral Ligands: Ligands that don’t have any charge on the coordination sphere are called neutral ligands. E.g., CO, PPh3, NH3.

Further classification of ligands

Ligands can be classified based on denticity. The number of atoms that connect the ligand to the central metal is referred to as the denticity of the ligand. Here are its types: 

1. Unidentate Ligands: The ligands with only one coordination site and a donation site to metal are called unidentate ligands. E.g., Ammonia (NH3), water (H2O), Chloride (Cl-), etc.
2. Bidentate ligands: The ligands that have two coordination sites and form two coordinate bonds with the metal through two different coordinating sites are called bidentate. E.g., NH2CH2CH2NH2 (en), Bipyridyl (bpy), (S)-BINAP, (R)-BINAP
3. Polydentate ligands: The ligands with more than two coordinating sites to form coordinate bonds with the metal are called polydentate ligands. These include tridentate, tetradentate, pentadentate, and hexadentate ligands.

EDTA is a Multidentate ligand with a maximum of six coordinating sites, and it can change its denticity. It can bind with two Nitrogen atoms and four oxygen atoms. These kinds of ligands generate different stereoisomerism.

1. Chelate Ligand: These are the polydentate ligands. When bound to the metal centre, they form a chelating ring. Common examples include EDTA and ethylenediamine.
2. Ambidentate ligands:  Ligands that have the ability to coordinate with the metal from two different elements present in the same ligand but one at a time are called ambidentate ligands. Ambidentate ligands are responsible for the Linkage isomerism present in the coordination compounds.

E.g., SCN-, NO2-. 

Types of coordination complexes

Here are the different types of coordination complexes:

• Cationic complexes: The complexes in which the positive charge carried by the coordination sphere (the central metal ion and the ligands as one common entity) are called cationic complexes.

Example: [Co(NH3)6]Cl3 is a cobalt complex that contains an overall charge of +3, which is neutralized by three chloride ions outside the coordination sphere. 

• Anionic complexes: The charge carried by the coordination sphere (the central metal ion and the ligands as one common entity) is negative in anionic complexes.

Example: K4[Fe(CN)6] is an iron complex with an overall charge of -4, which is neutralized by four potassium ions outside the coordination sphere. 

• Neutral complexes: The coordination sphere (the central metal ion and the ligands as one common entity) contain no net charge in neutral complexes. This could be due to the participation of neutral ligands themselves or a balance of charge caused by an equal number of positive and negative charges in the complex. 

Example: [Ni(CO)4] is a nickel complex with no net charge as the ligands involved in the coordination complex (carbon monoxide) are themselves neutral.

• Heteroleptic complexes: This coordination complex involves more than one type of ligand directly attached to the central metal ion. 

Example: [Co(NH3)5Cl]SO4 is a cobalt-containing complex that contains two types of ligands in the coordination sphere. Out of the two types of ligands involved, ammonia is a neutral ligand, and chloride is a negatively charged ligand.

• Homoleptic complexes: The coordination complexes that involve only a single type of ligand in the coordination sphere are called homoleptic complexes. 

Example:  K4[Fe(CN)6] is an iron complex that contains only cyanide ligands in the coordination sphere, thus making it homoleptic.

• Mononuclear complex: The involvement of a single central metal cation (usually a transition metal) in the coordination sphere makes the complex mononuclear.

Example: K4[Fe(CN)6] is a simple complex that contains only one type of transition metal, i.e., Iron. 

• Polynuclear complex: 

The involvement of multiple central metal cations (the cations can be of the same transition metal or different metals) in the coordination sphere makes the complex polynuclear. 

Example: [Re2Cl8]2- is a complex that has two central metal ions, and both of them are of the same type. This complex has one sigma, two pi, and one delta bond.

Conclusion

Coordination compounds are categorized based on ligands, the charge on the coordination sphere, and the number of metals in coordination spheres. An understanding of various types of coordination compounds can help understand them better and classify them under the correct sub-heads.