Coordination Number
Definition and meaning of Coordination Number in chemistry.
Coordination number is the total count of donor atoms bonded directly to a central metal. You will frequently encounter this concept when studying complex coordination compounds. It also defines the number of nearest neighbor ions surrounding a specific ion in a crystal lattice.
In more detail
The size and charge of the central metal ion mainly set the coordination number. The physical size of the surrounding ligands also plays a major role. Large bulky ligands take up more space and force a lower coordination number.
This final number directly decides the 3D geometry of the entire complex. A coordination number of four usually creates a tetrahedral or square planar shape. A coordination number of six reliably builds an octahedral shape in these compounds.
Students often mistakenly think this number simply counts the total ligand molecules. This assumption leads to wrong answers when dealing with complex polydentate ligands. A polydentate molecule can attach to the central metal in several places at once.
For example, a single bidentate ligand creates two separate coordinate bonds to the metal. You must always count the actual bonding atoms touching the metal center. In simple ionic solids, the coordination number works a bit differently.
It relies heavily on the size ratio between the positive and negative ions. A small positive ion can only fit a few large negative ions around it. This radius ratio dictates how efficiently the ions pack together to form the crystal lattice.
Key facts
| Common values | 2, 4, and 6 are most frequent for transition metals |
|---|---|
| Geometry link | Coordination number dictates the 3D shape of the complex |
| Counting rule | Count the donor atoms instead of the total ligand molecules |
| Example ion | [Co(NH3)6]3+ has a coordination number of 6 |
| Ionic solids | NaCl has a coordination number of 6 for both Na+ and Cl- |
| Typical range | Can range from 2 up to 12 in rare cases |
Look at the complex ion [Co(NH3)6]3+ to see a simple case in action. The central cobalt(III) ion bonds directly to six individual nitrogen atoms. Each nitrogen atom comes from a completely separate neutral ammonia ligand. This gives the entire complex a coordination number of exactly 6. Now consider the complex [Co(en)3]3+ which uses complex bidentate ethylenediamine ligands. Each ethylenediamine molecule is bidentate and forms two separate bonds to the metal. Three of these molecules provide six total bonds to the central cobalt ion. This results in the exact same overall coordination number of 6.
Frequently asked questions
Is coordination number just the number of ligand molecules?
Not always. A monodentate ligand makes one bond per molecule. A polydentate ligand can make multiple bonds. You always count the total number of individual bonds touching the metal.
What determines the coordination number in an ionic crystal?
The physical sizes of the positive and negative ions decide this number. It depends on how many large negative ions can fit around a smaller positive ion without overlapping.
Can a central metal have a coordination number of 2?
Yes. Silver(I) often forms complexes with a coordination number of 2. This creates a straight, linear geometry like the one found in the [Ag(NH3)2]+ complex ion.