Coordination entity, ligands and coordination number
A ligand donates an electron pair to the central metal. The coordination number counts the donor atoms bonded to it, not the number of ligands.
The coordination entity and the central atom
A coordination entity is a metal atom or ion at the centre with a fixed number of ions or molecules bound to it. Examples: [CoCl₃(NH₃)₃], [Ni(CO)₄] and [Fe(CN)₆]⁴⁻. The central atom or ion is the one to which these groups are attached in a definite shape: Ni²⁺ in [NiCl₂(H₂O)₄], Co³⁺ in [CoCl(NH₃)₅]²⁺ and Fe³⁺ in [Fe(CN)₆]³⁻. It accepts electron pairs, so it acts as a Lewis acid. The ligands give the pairs, so they act as Lewis bases.
Ligands grouped by denticity
| Type | Donor atoms used | Examples |
|---|---|---|
| Unidentate | 1 | Cl⁻, H₂O, NH₃, CN⁻, CO |
| Didentate | 2 | Ethane-1,2-diamine (en), H₂NCH₂CH₂NH₂, through two N; oxalate (ox), C₂O₄²⁻, through two O |
| Polydentate | More than 2 | N(CH₂CH₂NH₂)₃ through four N; EDTA⁴⁻ (ethylenediaminetetraacetate), hexadentate, through 2 N and 4 O |
| Ambidentate | 1, but it has two possible donor atoms | NO₂⁻ (through N or O); SCN⁻ (through S or N) |
Denticity, chelates and ambidentate ligands
- Denticity is the number of donor atoms a ligand uses to bind the metal.
- When a di- or polydentate ligand holds one metal through two or more donor atoms, it forms a ring called a chelate. Chelate complexes are more stable than similar complexes of unidentate ligands.
- In [Co(en)₃]³⁺, each en forms a five-membered ring: Co, N, C, C and N.
- An ambidentate ligand has two different donor atoms but uses only one at a time. NO₂⁻ can bind through N (nitrito-N) or through O (nitrito-O). This gives linkage isomers.
Counting the coordination number
Find the coordination number of the metal in (a) [CoCl₂(en)₂]⁺ (b) K₃[Fe(C₂O₄)₃] (c) [Ca(EDTA)]²⁻ (d) [Ni(CO)₄].
- Rule: coordination number = sum of (number of each ligand × its denticity).
- (a) 2 Cl⁻ × 1 + 2 en × 2 = 2 + 4 = 6.
- (b) 3 C₂O₄²⁻ × 2 = 6.
- (c) 1 EDTA⁴⁻ × 6 = 6.
- (d) 4 CO × 1 = 4. The extra π bonding between Ni and CO is not counted.
6, 6, 6 and 4.
Number of ligands vs coordination number
The two numbers are equal only when every ligand is unidentate.
EDTA is a hexadentate chelating agent. Doctors use it to treat lead poisoning, and water-testing labs titrate with it to measure the hardness of borewell water.
2 marks: “What is an ambidentate ligand? Give two examples.” A unidentate ligand with two different donor atoms, either of which can bind the metal (one at a time): NO₂⁻ (N or O) and SCN⁻ (S or N).
2 marks: “What is a chelate? Why is [Co(en)₃]³⁺ more stable than [Co(NH₃)₆]³⁺?” A ring formed when a di- or polydentate ligand binds one metal through two or more donor atoms. Each en forms a chelate ring, and chelate complexes are more stable than complexes of similar unidentate ligands.
1 marks: “What is the coordination number of Cr in [Cr(C₂O₄)₃]³⁻?” 6, because each of the three oxalate ions is didentate.
Count the atoms, not the ligands. Uni = 1, di = 2, EDTA = 6. Ambidentate = two doors, but only one is used.