Coordination Number

Coordination Number

Coordination number, also known as ligancy, is the total number of points of attachment to the central atom. This article states some important information related to ligancy of a molecular compound.
Coordination number is used to determine the number of neighbors an atom has. It is used in different contexts in the field of chemistry and crystallography. In chemistry, it was first defined by Alferd Werner, as the total number of neighbors of a central atom in a molecule or ion. In terms of crystallography, it is defined as the number of identical atoms or ions in the interior of a crystal.

How to Find a Coordination Number?
If you have been told to find the coordination number of molecular compounds, or you are just interested in finding out one, refer to the steps mentioned below. There are some things that you should have with you. You will need correct chemical formula, periodic table of elements, a calculator, a pen, and a sheet of paper.
  1. Check the chemical formula of the compound. It has been generally observed that the central metal atom is a transition element. You can identify this central metal atom by looking at the periodic table of elements.
  2. The next thing you need to do is to identify the atom, molecule, or the ion, which is nearest to the central metal atom. Check the chemical formula to find the atom, molecule, or ion, which is besides the symbol of central metal atom. In case the central metal atom is exactly in the middle of the formula, note down all the nearest atoms, molecules, or ions.
  3. Now, you need to note down what the central metal atom is connected to. If the central metal atom is connected to just one element, calculate number of atoms of that element, but if the central metal atom is connected to a molecule or an ion, you need to add the number of atoms in the whole molecule.
  4. Finally, calculate the number of nearest neighboring atoms and in case of directly bonded atoms, add both numbers together.
The coordination numbers of transition metals vary from 1 to 12, but usually most of them have ligancy of 2, 4 and 6. The ligancy of NaCl is 6, whereas that of gold is 3. The ligancy of fcc, that is face centered cubic shape is 12. The compounds which have number 4, generally have tetrahedral or square planar shapes, the most common example being that of carbon.

Radius Ratio
Radius ratio is defined as the radius of the positive ion (+) divided by the negative ion (-).

Coordination NumberRadius Ratio
30.155 - 0.225
40.225 - 0.414
60.414 - 0.732
80.732 - 1.000

In the year 2010, researchers in US were able to find a compound which broke the previous record for the highest coordination number. The compound is thorium aminodiboranate Th(H3BNMe2BH3)4 and the ligancy of this compound is 15. Researchers have found out the reason behind this as thorium is extremely large and hydrogen atoms are small. According to Keith Izod an expert at Newcastle University in the UK, 'Although the determination of ligancy can be somewhat subjective, the reported 15-coordinate thorium compound certainly pushes the limit of numbers observed so far.'

The study of coordination numbers is one of the basics of bonding in molecular compounds. A too high ligancy doesn't always mean that the molecule is going to be stable. A ligancy of 4 to 6 implies a stable molecule.