Coordination compound is formed when a central metallic cation (M) is surrounded by charged or non-charged (L) to form a stabilised entity.
MLn is known as a coordination compound or a complex. The term entity is used here for MLn to make known that it is not a covalent molecule nor an ionic molecule. The ligands are just associated to M under chemically significant conditions.
L is known as the ligand and n is the coordination number, the total number of charged or non-charged species that will associate directly to M.
As an example the reaction;
will take place under certain conditions and SiF6‾² is a complex or coordination compound.
Complex compounds are more common with transition elements since there are many vacant d-orbitals present. For example: [Co(NH3)6]Cl3, K[PtCl3(NH3)], and [Co(NH3)6]Cl3.
When the reagents involed are neutral, the neutral complex compound produced is given the special name of adduct. Example:
The more stable complex compounds are formed by ligand molecules which can hold onto the metal cation with more than one point of association. Example EDTA. Such ligands are known as chelating ligands, and the complex formed is known as a chelate complex. [Greek khele = claw; ate = able to]
Complex compounds do take definite structures. The popular structures being;
- tetrahedral four coordination;
- planar four coordination;
- trigonal bipyramid five coordination;
- square pyramid five coordination; and
- octahedral six coordination.
