Complex ions

Transition metals form at least one cation with vacant d orbitals.

Ligands

Vacant d orbitals on transition metal ions can accept electron pairs from anions or molecules with a lone pair.

As they are “tied” to the metal cation these ions and molecules are referred to as ligands.

Ligands form co-ordinate (dative) covalent bonds with transition metal cations.

Eg; The commonest copper ion is Cu2+

Water has a lone pair; H2O:

This means that it can act as a ligand, donating the lone pair to Cu.

Hydrated copper (ii) ions are therefore complex ions;

Cu2+ + 6H2O → [Cu(H2O)6]2+

[ ] are used to represent a complex ion.

Some ligands form only one co-ordinate bond with the metal cation.

They are referred to as monodentate (“one-toothed”).

Eg H2O

CN-

Bidentate (“two toothed”), ligands can make two co-ordinate bonds.

Bidentate ligands

XX

XX

Eg; Ethylene diamine (en).

• Write out the structures of the following ions and show how they can act as bidentate ligands.

• Benzene 1,2 diol

• Ethandioate ion (C2O42-)

EDTAHow many co-ordinate bonds can ethylene diamine tetra acetate form with a transition metal ion?

It is hexadentate.

Complexes with polydentate ligands are called chelates (after the Greek for claw) and are very effective at removing transition metals from solutions.

Co-ordination number

The co-ordination number of a complex ion is the number of co-ordinate bonds it contains.

The most common co-ordinate numbers are 2, 4 and 6.

Eg; [Cu(H2O)6]2+ Each ion involves six co-ordinate bonds, therefore it has a co-ordination number of 6.

Shapes of complex ions.

Electrons in the dative bonds are all negative.

Like charges repel each other.

So the ligands will arrange themselves so that they will be as far apart as possible.

So the shape of a complex ion depends on its co-ordination number.

Co-ordination number of 2 eg; [Ag(NH3)2]+

Ag+3HN: :NH3

The structure will be linear.

With bond angles of 18Oo

A co-ordination number of 4 has two different possible structures.

a) Planar b) Tetrahedral

Eg; [Pt(NH3)2(Cl)2]2-

Four ligands in the same plane.

All bond angles are 90o.

b) Tetrahedral Complex eg [CoCl4]2-

Four ligands form a tetrahedron

Bond angles are all 109.5 o

Complexes with a co-ordination number of 6 are octahedral eg [Fe(H2O)6]3+

Four ligands are in the same plane.

One above,

One belowNB All bond angles are 90o.

• Predict the shape of the following complex ions;

• 1) [Co(NH3)5Cl]2+

• 2) [Cr(H2O)4Cl2]+

• 3) [FeCl4]-

Haemoglobin

Fe2+ ion with a co-ordination number of 6.

Four co-ordinate bonds are made with lone pairs on the nitrogen of a porphyrin ring (haem)

• Other ligands can form stronger bonds with Fe2+ , binding irreversibly.

• Eg; Carbon monoxide.

Oxygen molecules act as ligands to make the sixth co-ordinate bond.

Water can also weakly bind.A fifth bond is made below the plane of the ring with the protein globin.

The Fe2+/ O2 bond is weak hence easily broken.