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Unless otherwise stated, all images in this file have been reproduced from:

Blackman, Bottle, Schmid, Mocerino and Wille,Chemistry, 3rd Edition 2016 (John Wiley & Sons)

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b.socrative.comroom “CHEM1002”

b.socrative.com

CHEM1002

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Lecture 33:• Hydrolysis of Metal Ions• Metal Complexes• Ligands• Blackman Chapter 13, Sections 13.1-13.4

After lecture 32, you should be able to:• Complete its worksheet and practice problems• Apply solubility equilibria (qualitative and quantitative)• Use ionic product to determine solubility• Apply the common ion effect• Answer Review Problems 10.49 - 10.75 in Blackman

Complexes (1)

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• Metal cations act as Lewis acids (electron pair acceptor)• Water is the Lewis base (electron pair donor)

M(OH2)42+(aq)

M2+

H2O(l) adduct

+

Hydrolysis of metal ions

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Acid-base definitions

• Arrhenius: H+(aq) + OH-(aq) H2O(l)• ACID: H+ producer in aqueous solution e.g. HCl• BASE: OH- producer e.g. NaOH

• Brønsted - Lowry: H+ + A- HA• ACID: proton donor (H+) e.g. HCl• BASE: proton acceptor e.g. NH3

• Lewis: A + :B A:B• ACID: electron pair acceptor e.g. BF3

• BASE: electron pair donor e.g. NH3

cf lecture 22

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[Fe (OH2)5(OH)]2+ + H+[Fe(OH2)6]3+

Acidity of aqueous transition metal ions

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Metal ion hydrolysis

Free Ion Hydrated Ion Ka

Fe3+ Fe(OH2)63+(aq) 6 x 10-3

Cr3+ Cr(OH2)63+(aq) 1 x 10-4

Al3+ Al(OH2)63+(aq) 1 x 10-5

Be2+ Be(OH2)42+(aq) 4 x 10-6

Cu2+ Cu(OH2)62+(aq) 3 x 10-8

Pb2+ Pb(OH2)62+(aq) 3 x 10-8

Zn2+ Zn(OH2)62+(aq) 1 x 10-9

Co2+ Co(OH2)62+(aq) 2 x 10-10

Ni2+ Ni(OH2)62+(aq) 1 x 10-10

AC

ID S

TREN

GTH

• The higher the metal charge, the greater the hydrolysis• The smaller the metal ion, the greater the hydrolysis

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Mn+ L

• A ligand donates an electron pair to the metal ion to form a coordinate or dative bond.

L

• All ligands have at least one lone pair• some ligands have more than one lone pair and can bond to two or

more metal ions• Usually more than one ligand binds to the metal ion

• commonly 4 or 6 ligands coordinate to a metal

The coordination bond

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• A coordination compound:

• a complex ion (the metal and its ligands)

• counter ions to balance its charge

• The metal ion has a positive charge (Co3+, Fe2+, Cu2+ etc)

• The ligands can be:

• neutral (NH3, OH2, pyridine, amines etc), giving rise to positively charged complexes (e.g. [Co(NH3)6]3+)

• negatively charged (Cl-, OH-, CN- etc), giving rise to negatively charged complexes (e.g. [Co(CN)6]3-)

• a mixture of these, giving rise to positive, negative or neutral complexes (e.g. [Co(NH3)3(CN)3]

Metal complexes

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• Square brackets indicate the complex ion:

• [Co(NH3)6]3+

• Co3+ with six neutral NH3 ligands

• Three Cl- counter ions to balance the charge

[Co(NH3)6]Cl3

• When dissolved in water, the complex ions and counter ions are separate

[Co(NH3)6]Cl3 [Co(NH3)6]3+(aq) + 3Cl-(aq)

Metal complexes

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Typical coordination number (CN) for some metal ions.

M+ CN M2+ CN M3+ CNCu+2,4 Mn2+ 4,6 Sc3+ 6Ag+ 2 Fe2+ 6 Cr3+ 6Au+2,4 Co2+ 4,6 Co3+ 6

Ni2+ 4,6 Au3+ 4Cu2+ 4,6Zn2+ 4,6

• The number of ligands bonded to the metal ion• varies from 2 - 8 depending on the metal and ligand sizes and

charges• 4 and 6 are the most common

Coordination number

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Complex Geometry

CN Geometry Examples

2 linear[Ag(NH3)2]+

[AuCl2]-

4 square planar[Pd(NH3)4]2+

[PtCl4]2-

4 tetrahedral[Zn(NH3)4]2+

[CuCl4]2-

6 octahedral[Co(NH3)6]3+

[FeCl6]3-

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• Unidentate or monodentate) ligand – forms one bond

• Bidentate ligand – can form two bonds

• Polydentate ligand – can form > two bonds

• Tridentate

• Tetradentate

• Pentadentate

• Hexadentate etc ...

Chelate ligands

• Chelate effect: complexes with chelate ligands are usually more stable than those with monodentate ligands of the same type

Chelate Ligands

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H2N

H2C CH2

NH2Ni2+

H2N

H2C CH2

NH2Ni2+

• Bidendate• ethane-1,2-diamine (en)

H2N

H2C CH2

NH2

Some chelate ligands

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-O N

O-

O

N

O-

O

O-

O

O

• EDTA is used to treat severe lead poisoning and to soften water in shampoo and soaps

[Co(EDTA)]-

• Hexadendate• ethane-1,2-diaminetetraacetic acid (H4EDTA)

-O N

O-

O

N

O-

O

O-

O

O

EDTA4-

Some chelate ligands

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Learning Outcomes - you should now be able to:

• Complete the worksheet• Explain why solutions of metal ions are acidic• Identify the complex ion, coordination number and ligands• Name coordination compounds• Recognise and explain the stability of chelate complexes• Answer the Practice Examples (next slide) and Review Problems 13.51-

13.58 in Blackman

Learning outcomes: complexes (1)

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Practice examples

1. What are the possible geometries of a metal complex complex witha coordination number of 4?

a) square planar or tetrahedral or octahedralb) square planar or tetrahedralc) octahedral onlyd) tetrahedral onlye) square planar only

2. The nickel(II) ion exists as the [Ni(OH2)6]2+ complex ion in aqueous solution. Define the term complex. By writing a balanced equation for the corresponding reaction, explain why such a solution is acidic.