Question

Define coordination number.

Hint:

Coordination Number = Number of Ligands $\times$ Denticity of each ligand.

Answer 1

Step 1: Concept
The coordination number in a coordination complex refers to the secondary valency of the central metal atom or ion.

Step 2: Meaning
It is an integer that quantifies how many coordinate bonds are formed directly between the central metal and its surrounding ligands.

Step 3: Analysis
When calculating the coordination number, the total number of donor atoms directly bonded to the central metal ion is counted. Thus, a bidentate ligand contributes two donor atoms, while a hexadentate ligand such as EDTA contributes six donor atoms.

• The coordination number of a complex is defined as the total number of donor atoms directly bonded to the central metal ion.

• It is important to note that the coordination number depends on the number of donor atoms rather than the number of ligand molecules.

• A monodentate ligand donates only one lone pair through one donor atom.

• Therefore, each monodentate ligand contributes: \[ \boxed{1} \] to the coordination number.

• A bidentate ligand such as ethylenediamine (en) contains two donor nitrogen atoms.

• Hence, each ethylenediamine ligand contributes: \[ \boxed{2} \] to the coordination number.

• Ethylenediaminetetraacetate (EDTA) is a hexadentate ligand containing six donor atoms.

• Therefore, a single EDTA molecule contributes: \[ \boxed{6} \] to the coordination number.

• Thus, the coordination number always represents the total number of coordinate bonds formed between the ligands and the central metal ion.

Step 4: Conclusion
The formal definition directly reflects this principle.

Final Answer: The coordination number of a central metal ion in a complex is defined as the total number of ligand donor atoms to which it is directly bonded via coordinate covalent bonds.