Topic 17 · page 5 of 20

Bidentate and hexadentate ligands

Some ligands have more than one atom that can donate a lone pair. They wrap round the metal ion and form two, or six, dative bonds. Count bonds, not ligands.

10 min read17.16

Read first: Ligands and complex ions, Shapes of complex ions

Counting donor atoms 17.16

Water, ammonia and hydroxide each have one atom that donates a lone pair, so each forms one bond. Many other ligands have two or more such atoms, placed so that they can reach the same metal ion.

The word comes from "tooth": the ligand bites the metal ion in more than one place.

1,2-diaminoethane: a bidentate ligand 17.16

1,2-diaminoethane, NH₂CH₂CH₂NH₂ (often written en), has a nitrogen atom with a lone pair at each end of a short chain. Both nitrogen atoms can bond to one metal ion, forming a ring.

It is neutral, so it does not change the charge of the complex. Three of them give six bonds:

[Ni(NHX2CHX2CHX2NHX2)X3]X2+ \ce{[Ni(NH2CH2CH2NH2)3]^2+}

This ion has 3 ligands but a coordination number of 6, and it is octahedral.

EDTA: a hexadentate ligand 17.16

The EDTA⁴⁻ ion has two nitrogen atoms and four oxygen atoms (one in each of four carboxylate groups, –COO⁻) that carry lone pairs. All six bond to the same metal ion, so it wraps right round it.

Left: 1,2-diaminoethane, H2N-CH2-CH2-NH2, with a lone pair on each nitrogen forming two bonds to a metal ion M. Right: the displayed formula of the EDTA 4- ion, two nitrogen atoms joined by a CH2CH2 chain, each nitrogen carrying two CH2COO minus arms, with the six donor atoms (two N and four O minus) highlighted.
A bidentate ligand (left) and EDTA4- (right). The highlighted atoms donate lone pairs.

One EDTA⁴⁻ ion is enough to give coordination number 6:

[Cu(EDTA)]X2− \ce{[Cu(EDTA)]^2-}

The charge works out as +2 + (−4) = 2−.

Complex Ligands Bonds per ligand Coordination number
[Cu(H₂O)₆]²⁺ 6 H₂O 1 6
[Ni(NH₂CH₂CH₂NH₂)₃]²⁺ 3 en 2 6
[Cu(EDTA)]²⁻ 1 EDTA⁴⁻ 6 6
[Cr(NH₂CH₂CH₂NH₂)₃]³⁺ 3 en 2 6

Another bidentate ligand is the ethanedioate ion, ⁻OOC–COO⁻, with one oxygen atom on each carboxylate group able to bond to the metal ion.

MethodIdentifying a ligand's denticity

  1. Draw the displayed formula.
  2. Find the atoms with a lone pair that can donate (usually N or O).
  3. Count only those that can reach the same metal ion: separated by a short chain, not on opposite sides of a big molecule.
  4. The number is the number of bonds the ligand forms.

Check yourself

  1. Define the terms bidentate and hexadentate. Give one example of each.
  2. State the coordination number of [Cr(NH₂CH₂CH₂NH₂)₃]³⁺ and explain how you worked it out.
  3. State the coordination number of [Co(EDTA)]²⁻ and the shape you would expect.
  4. Is CH₃CH₂NH₂ a bidentate ligand? Explain.
  5. Work out the overall charge of a complex of Fe³⁺ with one EDTA⁴⁻ ion.
  6. Identify all the atoms in EDTA⁴⁻ that donate lone pairs.

Answers cover these until you have tried the questions

  1. A bidentate ligand forms two dative bonds to the metal ion, for example NH₂CH₂CH₂NH₂. A hexadentate ligand forms six, for example EDTA⁴⁻.
  2. 6. Each of the 3 ligands is bidentate and forms 2 dative bonds: 3 × 2 = 6.
  3. EDTA⁴⁻ forms six dative bonds, so the coordination number is 6. Six bonding pairs give an octahedral shape.
  4. No. It has only one nitrogen atom, so only one lone pair to donate: it is monodentate.
  5. +3 + (−4) = 1−, [Fe(EDTA)]⁻.
  6. Two nitrogen atoms and four oxygen atoms (the O⁻ of each of the four carboxylate groups): six donor atoms in all.