Lesson 2 of 13 · 6 min
Ligands and denticity
NCERT §5.2
EDTA, the reagent in Farhan's burette, is a long molecule with arms. His teacher says it 'wraps around' a calcium ion. Wrapping needs more than one point of attachment.
The lesson in notes
In short
A coordination entity is one metal atom or ion with a set number of ions or molecules attached to it. Examples: [CoCl₃(NH₃)₃], [Ni(CO)₄], [PtCl₂(NH₃)₂], [Fe(CN)₆]⁴⁻ and [Co(NH₃)₆]³⁺.
The central atom or ion holds the ligands in a definite geometry and accepts their electron pairs, so it acts as a Lewis acid: Ni²⁺ in [NiCl₂(H₂O)₄], Co³⁺ in [CoCl(NH₃)₅]²⁺ and Fe³⁺ in [Fe(CN)₆]³⁻.
Ligands are the ions or molecules bonded to the central atom. They range from simple ions (Cl⁻) and small molecules (H₂O, NH₃) to larger molecules and even proteins.
Unidentate ligands bind through one donor atom (Cl⁻, H₂O, NH₃). Didentate ligands bind through two: ethane-1,2-diamine (en, H₂NCH₂CH₂NH₂) and oxalate (C₂O₄²⁻). Polydentate ligands, such as N(CH₂CH₂NH₂)₃, have several.
EDTA⁴⁻ (ethylenediaminetetraacetate) is hexadentate: it binds one metal ion through two nitrogen and four oxygen atoms.
A chelate ligand uses two or more donor atoms on the same metal ion at once, and the number of such donor groups is its denticity. Chelate complexes are generally more stable than similar complexes of unidentate ligands.
An ambidentate ligand has two different donor atoms and binds through either one: NO₂⁻ through N or O, and SCN⁻ through S or N.