Intermediate Topic 205 of 1102 Bonding & Molecular Structure
Coordination Number
The number of donor atoms directly bonded to the central metal ion.
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In-depth explanation of Coordination Number
The coordination number is the number of donor atoms directly bonded to the central metal ion in a complex. For a polydentate ligand, each donor atom counts separately toward the coordination number. The most common coordination numbers are 4 and 6, corresponding to tetrahedral or square planar geometry and to octahedral geometry, respectively. The value depends on the size and charge of the metal ion and the size of the ligands, since larger metals and smaller ligands allow more donors. Coordination number therefore helps predict geometry and the number of ligands a metal can accommodate.
Going deeper: Coordination Number
Key ideas: Predict from size: small ions with large ligands give 4, larger ions with small ligands give 6. Common cases: Ag+ gives 2 (linear, [Ag(NH3)2]+), Ni2+ gives 6 or 4, Pt2+ gives 4 square planar, Zn2+ gives 4 tetrahedral, lanthanides give 8 to 12. Count donor atoms, not ligands: [Co(en)3]3+ has CN 6.
Common mistakes and connections: The usual error is to count ligands instead of donor atoms: [Fe(EDTA)]- has CN 6 with one ligand. Another is mixing up coordination number with oxidation state or charge. Also the coordination number in a crystal means nearest neighbours of an ion. Links to complex geometry, chelation, isomerism and crystal field theory.
Visualization of Coordination Number
Typical coordination numbers: 2 for Ag+, 4 for Zn2+ and Pt2+, and 6 for Co3+ and Fe3+, which commonly form octahedral complexes. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In the complex [Fe(CN)6]3- the iron has a coordination number of six, bonded to six cyanide ions.
Frequently asked questions about Coordination Number
What is Coordination Number?
The number of donor atoms directly bonded to the central metal ion.
Is Coordination Number a beginner, intermediate or advanced topic?
Coordination Number is a intermediate topic in Bonding & Molecular Structure. It builds on the vocabulary of beginner topics in this area.
What should I learn before Coordination Number?
Study these earlier topics in Bonding & Molecular Structure first: Fullerenes, Coordination Compounds, Ligands.
What comes after Coordination Number?
The next topics in Bonding & Molecular Structure are Chelation and Crystal Field Theory.
Can I practise Coordination Number with quizzes?
Yes. The interactive lesson includes a self-check question on Coordination Number, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Where Coordination Number fits in Bonding & Molecular Structure
Coordination Number is topic 90 of 104 in Bonding & Molecular Structure.