Coordination number

The coordination number in chemistry is mainly related to complex compounds and organometallic compounds and is defined in three different ways:

Coordinate number is the size that determines whether a given complex is able to accept any additional ligands or if its coordination sphere is already full. Nowadays, however, because of the difficulty in fixing this number for many complex complexes, it is a departure from the notion of accurately calculating the number of electrons that make up the bonding system of a given compound.

From the definitions given above, the first two are officially recognized by the IUPAC terminology committee, and the third, although formally unrecognized, is often used especially in organometallic chemistry. The first of these definitions ignores the fact that multiple bonds may occur between atomic metal and ligand atoms. When a ligand connects to a metal, it actually "blocks" more real co-ordinates than is apparent from this definition. The second definition is slightly better, but ignores the fact that π ligands exist that bind to metal only by the π bond and thus do not "block" any coordination sites. In fact, however, once the ligand is attached, coordinates are blocked, as is apparent from the electron calculus. The last definition best translates the number of electrons involved in the bonding of metal and ligands to the coordination number, but for various reasons is not universally accepted.

In the case of many complex compounds, different definitions apply to different results. Eg for the cyclopentadienyl complex shown in the figure below (M = any metal atom):

According to the first definition, the coordination number is: 8 (5 carbon atoms directly attached to the cyclopentadienyl ligand + 3 chlorine atoms); According to the other: 3 (chlorine atoms are attached with σ bonds, and all carbon atoms of cyclopentadienyl are bonded with π); Finally, according to the third, it is for M = eg Manganese 9, because in the formation of the total bonding system there are 18 electrons in total (from manganese 7 + 5 from cyclopentadienyl ligand + 2 × 3 from chloride ligands).

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