Exp. 38 Transition Metal Chemistry p. 423

1
Exp. 38 Transition Metal Chemistry
p. 423

• To observe the various colors associated with
  transition metal ions.
• To determine the relative strength of a ligand.
• To compare the stability of complexes
• To synthesize a coordination compound.

2
Exp. 38 Introduction

• The reds, blues, and greens that we
• associate with some chemicals are
• often due to transition metal ions.
• Transition metals are from
• Scandium to Copper.
• Color changes can occur when
• molecules or ions bond to the
• metal ion to form a complex

3
Exp. 38 Introduction

• These molecules or ions are called ligands
• and are Lewis bases (electron pair donors)
• which bond directly to the metal ion,
• producing a change in the electronic
• energy levels of the metal ion. As a result,
• the energy and wavelength of light
• absorbed and transmitted by the electrons
• in the metal change. The solution has a
• new color.

4
Exp. 38 Introduction

• The complex has a number of ligands bonded
• to the transition metal ion which form a
• coordination sphere. The complex, along
• with its neutralizing ion is called a
• coordination compound.
• Ex Fe(CN)64- - complex
• 6 CN- ions ligands
• Fe(CN)6 coordination sphere
• K4Fe(CN)6 coordination compound

5
Exp. 38 Introduction

• Colors are nice, but there are also practical
• uses for transition metal complexes
• The removal of the calcium in water that causes
  hardness is accomplished by adding a
  polyphosphate to form a soluble calcium complex
• Ca2(aq) P3O105-(aq) ? CaP3O103-(aq)
• A treatment for ingested Pb or Hg is to swallow
  mercaptol, C3H8OS2, which forms a complex with
  the Hg or Pb
• Hg2(aq) 2C3H8OS2(aq) ? Hg(C3H8OS2)22(aq)

6
Exp. 38 Introduction

• Monodentate a single ligand forms one
• bond to the metal ion.
• Bidentate a single ligand forms two bonds
• to the metal ion.
• Chelating agents ligands that form 2 or
• more bonds to a transition metal ion
• (polydentate ligand).
• The complex formed between a chelating
• agent and a metal ion is generally more
• stable that that formed by a monodentate
• and a metal ion.

7
Exp. 38 Introduction

• Coordination number the number of bonds
• between a metal ion and its ligands, or, the
• number of positions available for bonding.
• Ex 4 monodentate coord. no. 4
• 2 bidentate coord. no. 4
• 6 H2O, 6 CN- or 3 bidentate 6
• Coordination numbers 2, 4, 6 are the most
• common.

8
Exp. 38 Introduction

• The stability of a complex can be determined
• by mixing it with an anion known to form a
• precipitate with the cation. The anion most
• commonly used is OH-.
• Ex Cu2 4X- ? CuX42-
• If OH- is added to this equilibrium, the Cu2
• now has to choose which anion it will
• combine with. If ligand X- forms the stronger
• bond, the copper remains in solution and no
• change is observed. If OH- forms a stronger
• bond to Cu2, Cu(OH)2 precipitates.

9
Exp. 38 Procedure Notes

• For the purpose of this experiment,
• the coordination number of Cu2 is
• always 4 and Ni2 and Co2 are always 6.
• F. Do first. You will be using both a Bunsen
  burner and 95 ethanol be aware!
• A.1. Caution! You will be using conc. HCl
• in this step.
• B.1. Caution! You will be using conc. NH3.
• Use the hood.
• D., E., G. omit

10
Exp. 38 Report Sheet

• Question 4