Transition Metals

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Transition Metals
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Definitions

• Complex A metal ion surrounded by ligands.
• Ligand an electron pair donor i.e. a molecule
  or ion joined onto the metal ion by a dative
  covalent bond to the metal.
• Coordination number The number of atoms
  directly joined to a transition metal by a
  coordinate (dative covalent) bond.

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Typical ligands

• Unidentate H2O NH3 Cl-
• Bidentate C2O42- H2NCH2CH2NH2
• Multidentate EDTA4-
• Watch out for the chelate effect.

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Shapes

• Six-coordinate complexes are octahedral.eg
  Cu(H2O)62 Co(NH3)63
• Four-coordinate complexes are usually tetrahedral
  eg CoCl42- but a few are square planar eg
  Ni(CN)42-.
• Two-coordinate complexes are linear eg AgCl2-.

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Colour

• Most transition metal colours are due to d-d
  electron transitions. The energy gap between the
  split d-orbitals corresponds to visible light
  (Ehf).
• Some of the really strong colours are due to
  charge transfer (such as in MnO4-).

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Changing colour

• Anything that changes the energy difference
  between the d-orbitals causes a change in colour
• Oxidation state
• Ligand
• Coordination number

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Measuring concentration by colour

• A measurement of colour intensity (using a
  colorimeter) can measure the concentration of a
  solution.
• Sometimes a special complex is formed so that the
  colour is more intense.
• This is known as UV/visible spectroscopy.
• eg Fe(H2O)63 SCN- Fe(H2O)5(SCN)2
  H2O

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Reaction types

• The reactions of the transition metals come under
  three headings (but sometimes more than one
  occurs at the same time!).
• Ligand substitution
• Hydrolysis (the acidity reaction)
• Redox

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Ligand substitution

• One kind of ligand is replaced by another.
• For example,
• Cu(H2O)62 4Cl- ? CuCl42- 6H2O

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Hydrolysis (the Acidity reaction)

• One or more hydrogen ions is removed.
• For example,
• Cu(H2O)62(aq) 2OH-(aq) ?
• Cu(H2O)4(OH)2(s) 2H2O(l)

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Redox

• The oxidation state of the metal is changed by
  adding or removing one or more electrons
• For example,
• Co(NH3)62 ? Co(NH3)63 e-

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Titanium

• Mainly known for its chloride, TiCl4, as part of
  the Kroll process for extraction of titanium from
  its ores.
• The chloride is covalently bonded.
• Titanium oxide, TiO2 , is used as a white pigment.

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Vanadium

• Most stable form is V(IV) eg as VO2.
• V(V) can be reduced in acidic conditions (eg Zn /
  HCl)
• VO2 ? VO2 ? V(H2O)63 ? V(H2O)62
• V(H2O)62 is unstable in air since it is
  oxidised by oxygen.

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Chromium

• Cr(H2O)63 Cr(H2O)3(OH)3 Cr(OH)63-
• gtgtgtgtIncreasingly alkaline conditionsgtgt

Cr2O72- CrO42- ltltltltIncreasingly alkaline
conditionsltltltlt
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Manganese

• Mn(H2O)62 MnO2 MnO42- MnO4-
• II IV VI
  VII
• v. pale pink brown deep
  purple

MnO4- is a very strong oxidising agent. Use
acidic conditions. For titrations, make sure you
add sufficient sulfuric acid to take Mn all the
way to Mn2. Otherwise you get incomplete
reduction (the brown solid, MnO2 is formed,
rather than Mn2).
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Iron

• Fe(H2O)62 is non-acidic in water (green).
• Pure Fe(H2O)63 is a lilac colour but on contact
  with water goes rusty brown.
• Fe(H2O)62 forms Fe(OH)2 (a green solid) with
  NaOH but it goes brown (forming Fe(OH)3 on
  standing in air.

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Cobalt

• add ammonia solution
  add hydrogen peroxide (or air)
• Co(H2O)62 ? Co(NH3)62 ? Co(NH3)63
• pink
  brown / yellow orange
• CoCl42- Co(OH)2
• blue
  blue-grey

Add conc HCl
Add NaOH or NH3 (aq)
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Nickel

• add
  ammonia solution
• Ni(H2O)62 Ni(NH3)62
• green
  blue
  
• NiCl42- Ni(OH)2
  Ni(CN)42-
• blue pale
  green a square planar complex!

Add NaOH or NH3 (aq)
Add NaCN (aq)
Add conc HCl
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Copper

• add ammonia solution
  
• Cu(H2O)62 Co(NH3)4(H2O)22
• blue
  deep blue / violet
• CuCl42- Cu(OH)2
• yellow-green pale
  blue

Add conc HCl
Add NaOH or NH3 (aq)
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Silver

• Small quantity of NaOH, NH3(aq)
  excess NH3 (aq)
• Ag(aq) Ag2O(s)
  Ag(NH3)2
• Cl-
• AgCl(s)
• excess Cl-
• AgCl2- Ag(S2O3)23-
  Ag(CN)2-

NaCN(aq)
Na2S2O3(aq)
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Uses of transition metalsand their complexes

• V2O5 is used as a catalyst in the Contact
  Process (manufacture of sulfuric acid).
• Cis-platin (a platinum complex) is a very
  effective anti-cancer drug.
• Fe2 is an important part of haemoglobin.
• Ag(NH3)2 is Tollens reagent (test for
  aldehydes).
• Ag(S2O3)23- is formed during photographic
  processing.
• Ag(CN)2- is used in electroplating.