Complexes by wuzhengqin


A few slides to jog memories
What is a complex?

  Transition metal
  Dative coordinate bonds from ligands
  Ligands are lone pair donors (anions or neutral)
  Complexes have more bonds than the oxidation number of the TM

How can we describe the metal ion in terms of acidity?

  Lone pair acceptor – thus it is a Lewis acid.
  The ligands are Lewis bases.

2, di
4, tetra
6, hexa

H2O aqua
NH3 ammine
Cl- chloro
CN- cyano
OH- hydroxo

Transition metal
Positive charge
English name
Negative charge
Latin and “ate”.
                           Diethane-1,2-diaminedichlorocobalt 3+ ion
[Cu(NH3)4]2+ tetraamminecopper II ion
[Fe(CN)6]4- hexacyanoferrate II ion
                     Shapes of complexes

Octahedral complexes with six-fold coordination are the most common

                          hexaaquairon III ion
        Shapes of complexes

Linear complexes are mainly formed by:

              Silver I
              Copper I

          diamminesilver I ion
              Shapes of complexes

      tetrahedral complexes are mainly formed by:

                                         Large bonding ligands that
                                          cannot pack around the
                                            central atom as in a
                                          octahedral complex, like
                                         TMs in 2+ oxidation states
                                               with chlorine, or
                                          all oxidation states with

                                         Questions tend to focus on
                                         the oxidation states of the
                                          metal ion and chlorine or
                                            bromine ligand and
                                              packing issues.

diamminedichlorocopper II ion
                              Shapes of complexes
                      Square planar complexes are much rarer but
                                you should be aware of:

Cis-platin   cis-diamminedichloroplatinum II                Cis-platin can exchange
                                                            the chlorine ligands
                                                            for nitrogen in DNA
                                                            causing it to distort
                                                            and stop working.
                                                            It is an effective anti-
                                                            cancer treatment for
                                                            hormone related
                                                            cancers like breast
                                                            and testicular cancer.

                                                            Remember that
                                                            cyanide ligands
                                                            form square
                                                            planar complexes
                                                  Don’t always assume
                                                  that chlorine is too
                                                  large to be in
                                                  octahedral complexes.
                                                  the central ion size
                                                  also plays a part.

cis and trans tetraamminedichlorocobalt III ion

                      Rotational asymmetry

What ligand property is also shown in this picture?   Chelation
Chelation using EDTA.

How many binding points has EDTA?              6
What is the charge on EDTA?                    4-
What would you call this, as a ligand?         Hexadentate ligand

                                         Remember that EDTA has 6 binding
                                         sites (lone pair donors) but not all the
                                         sites are identical.

                                                EDTA cobaltate III ion

                                        Overall OCTAHEDRAL

                                        Square planar porphyrin
                                        ring around the Fe II ion
                                        (tetradentate and neutral)

                                        Large globin molecules
                                        dominate the size of the
                                        structure binding at one site

                                        Either water or oxygen binds
                                        to the sixth octahedral site
                                        (concentration dependent)

4 haemoglobin complexes
Silver chemistry – what two complexes should you be aware of?

    Tollen’s reagent: diammine silver I ion [Ag(NH3)2]+ reacts with
    warm aldehyde. The aldehyde is oxidised to an acid and the
    silver is reduced to silver metal and forms a silver “mirror”.

    In the test for silver halides. AgCl and AgBr will dissolve
    in dilute ammonia forming the [Ag(NH3)2]+ complex. AgI will not.

    Photography. Silver is deposited from silver bromide in B&W
    photographic paper. This process continues in light until the
    film is “fixed”. The “fixer” is sodium thiosulphate that complexes
    silver and stops further deposition.
             AgBr + 2S2O32-        [Ag(S2O3)2]3- + Br-
                           dithiosulphateargentate I ion
Exam Questions from June 2003

    The ion C2O42- can act as a bidentate ligand.
             What is the meaning of bidentate ligand.
             Sketch the structure of the octahedral complex ion formed by Fe3+
             ions which contains C2O42- as the only ligand. Include the overall
             charge for the complex ion. 5 marks

    Explain the meaning of the “chelate” effect. 2 marks

    The chloride ion can act as a monodentate ligand.
             Deduce the formula of the linear complex formed when an excess
             of concentrated hydrochloric acid is added to silver chloride.
             Explain why metal II ions do not usually form octahedral complexes
             when chloride ions are the only ligands. 2 marks

         This question then went further to consider catalytic uses.
Exam Questions from June 2004

    State what is meant by each of the following terms:
            Ligand; Complex ion; Coordination number      3 marks

    Using complex ions formed by Co2+ with ligands selected from
    H2O, NH3, Cl-, C2O42-, and EDTA4-, give an equation for each of
    the following:
              A ligand substitution reaction which occurs with no change
              in either the coordination number or the charge on the
              central ion.
              A ligand substitution reaction which occurs with both a change
              in the co-ordination number and the charge on the complex ion.
              A ligand substitution reaction which occurs with no change in
              the co-ordination number but a change in the charge on the
              complex ion.
              A ligand substitution reaction in which there is a large change
              in entropy.
              8 marks
Exam Questions from June 2004

         An aqueous solution of iron II sulphate is a pale green colour.
         When aqueous sodium hydroxide is added to this solution a
         green precipitate is formed. On standing in air, the green
         precipitate slowly turns brown.

                 Give the formula of the complex ion responsible for
                 the pale green colour.

                 Give the formula of the green precipitate.

                 Suggest an explanation for the change in the colour of
                 the precipitate.

                 4 marks
  Exam Questions
  from June 2005

Identify and state the
shape of the copper-
containing species
present in solution in A.

Identify the pale blue
precipitate B and write an
equation, or equations, to
show how B is formed
from the copper-containing
species present in A.

In what way does NH3
behave as a Bronsted-
Lowry base?
 Exam Questions
 from June 2005

Identify the copper-containing
species present in solution C.
state the colour of this copper-
containing species and write
an equation for its formation
from precipitate B.

In what way does the NH3
behave as a Lewis base.

Identify the copper-containing
species present in solution D.
state the colour and shape of
this copper-containing species.

The oxidation state of copper in [CuCl4]3- is +1.
        Give the electron arrangement of a Cu+ ion.
        Deduce the role of copper metal in the formation of [CuCl4]3- from
        the copper-containing species in solution D.

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