Question

Define coordination number.

Hint:

Coordination number counts only sigma bonds between ligands and metal. Pi interactions, if present, are not included in the coordination number.

Answer 1

Concept: In a coordination complex, a central metal atom or ion is surrounded by ligands that donate lone pairs to form coordinate covalent bonds. The coordination number depends on the number of donor atoms directly bonded to the metal, not the number of ligands.
Step 1: Define coordination number. The coordination number (C.N.) of a metal ion is the total number of ligand donor atoms directly bonded to the central metal ion in a coordination sphere.
Step 2: Understand the role of ligand denticity. Coordination number depends on how many donor atoms each ligand provides:

• Monodentate ligands donate one donor atom each (e.g., NH$_3$, Cl$^-$).
• Bidentate ligands donate two donor atoms (e.g., ethylenediamine, en).

Step 3: Illustrative examples.

• Six monodentate ligands $\Rightarrow$ C.N. = 6
• Three bidentate ligands $\Rightarrow$ C.N. = $3 \times 2 = 6$

Step 4: Final conclusion. Coordination number is the number of donor atoms from ligands directly attached to the central metal ion.