What is Coordination Number?
Coordination number of the central atom in a crystal or molecule is the number of ions, atoms, or molecules bonded to it. This term was originally defined by Alfred Werner in the year 1893. Coordination number is also called ligancy.
The lowest coordination number is 2 whereas the highest coordination number is 16. Its value depends on the relative sizes ligand, central atom, and charge got from the electronic configuration of the ion. In coordination complexes, the bond that counts is only the first sigma bond between the ligands and the central atom. Pi bonds are not included during the calculation.
How to determine coordination number?
The steps given below will help you calculate the coordination number of coordination compounds.
Step 1: The first step is to spot the central atom in the given chemical formula. Generally, it is a transition metal.
Step 2: Discover the exact position of the molecule, an ion or the atom closest to the central metal atom. To do this easily, identify the ion or molecule which is directly beside the metal symbol of the coordination compound.
Step 3: Count the number of atoms in the nearest molecule or ion or atom. Even if the central atom is bonded to one different element, note the number of atoms of that element. If in case the central atom occurs in the middle of the formula, add up the atoms of the whole molecule.
Step 4: Identify the number of nearest atoms in total. If the metal has two bonded atoms, directly combine together and add both numbers.
Coordination number Example
● Consider complexions [Ni(NH3)4]2+ and [PtCl6 ]2–. The coordination number of Ni and Pt are 4 and 6.
● Consider complexions, [Co(en)3 ]3+ and [Fe(C2O4 )3 ]3–. The coordination number of Co, as well as Fe, is 6 because en (ethane-1,2-diamine) and C2O42– are didentate ligands.
Coordination number geometry
| Coordination no. | Geometry |
| 2 | Linear |
| 3 | Trigonal planar, T-shaped, trigonal pyramid |
| 4 | Square planar, tetrahedral |
| 5 | Square pyramid, trigonalbipyramid |
| 6 | Trigonal prism, hexagonal planar, octahedral |
| 7 | Capped octahedron, pentagonal bipyramid, capped trigonal prism |
| 8 | Dodecahedron, hexagonal bipyramid, cube, square antiprism |
| 9 | 3 face centredtrigonal prism |
| 10 | Bicapped square antiprism |
| 11 | All-faced capped trigonal prism |
| 12 | Cuboctahedron |
To get in-depth knowledge about various other chemistry concepts such as Werner’s Theory of Coordination Compound, Nomenclature of coordinate compounds, Dobereiner triads, subscribe to BYJU’S YouTube channel
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