Chemistry

9.8 Chemistry of Art: 5. Coloured complex ions

Syllabus reference (October 2002 version)
5. The formation of complex ions by transition metal ions increases the variety of coloured compounds that can be produced	Students learn to: explain what is meant by a hydrated ion in solution describe hydrated ions as examples of coordination complex or a complex ion and identify examples describe molecules or ions attached to a metal ion in a complex ion as ligands explain that ligands have at least one atom with a lone pair of electrons identify examples of chelated ligands discuss the importance of models in developing an understanding of the nature of ligands and chelated ligands using specific examples	Students: use available evidence and process information from secondary sources to draw or model Lewis structures and analyse this information to indicate the bonding mechanisms in selected complex ions involving the first transition series process information from secondary sources to give an example of the range of colours that can be obtained from one metal such as Cr in different ion complexes

Extract from Chemistry Stage 6 Syllabus (Amended October 2002), © Board of Studies, NSW.
[Edit: 9 Jul 09]

Prior Learning: HSC module 9.3.4

Background: Transition metals are renowned for their ability to form complex ions. A complex ion is composed usually of a central metal cation, while the number and types of molecules (such as water) or anions attached to it define the ion.

explain what is meant by a hydrated ion in solution

• A hydrated ion is an ion in which a specific number of water molecules is associated with each formula unit, for example colourless Cu²⁺ forms blue Cu(OH₂)₄²⁺ in aqueous solution.
  
  
• Why is Cu(OH₂)₄²⁺ considered a better representation of the structure of the hydrated ion Cu(H₂O)₄²⁺ ?

  Answer: In this situation water is acting as a ligand. The atom or atoms of a ligand which coordinate to a metal atom are listed first, for example water as a ligand is (OH₂ ) but as a solvent it is H₂O.

describe hydrated ions as examples of coordination complex or a complex ion and identify examples

• Coordination compounds or complexes, typically contain at least one complex ion. The complex ion consists of a central metal cation bonded to molecules and anions called ligands. To maintain electrical neutrality the complex ion is associated with simple ions called counter ions.
  
  
• Examples include [Co(NH₃)₆]Cl₃  and [Pt(NH₃)₄]Br₂ where NH₃ is the ligand, and Al(OH₂)₆ ³⁺ where H₂O is the ligand.

describe molecules or ions attached to a metal ion in a complex ion as ligands

• The molecules or anions attached to the central metal cation are called ligands. The number of ligand atoms bonded to the central metal atom is the coordination number.

process information from secondary sources to give an example of the range of colours that can be obtained from one metal such as Cr in different ion complexes

• The Internet or chemistry text books can be used to find information on ion complexes of chromium and their colours. The table below will be useful. See if the results you obtained in your first-hand investigation support the given information.

Chromium compounds	Colour
potassium dichromate (Cr2O72-)	orange
potassium chromate (CrO42-)	yellow
chromium (III) oxide(Cr2O3)	green
potassium chromium (III) sulfate (KCr(SO4)2.12H2O)	purple

explain that ligands have at least one atom with a lone pair of electrons

• Ligands have at least one atom with a lone pair of unbonded electrons.
  
  
• Examples of ligands with an atom with a lone pair of electrons are: NH₃, H₂O, Cl^-, CN^-.

Examples of ligands with more than one atom with a lone pair of electrons are: oxalate ion ^-OOC-COO^- and the triphosphate ion [P₃O₁₀]^5-.  These are able to attach to a suitable metal ion using more than one atom.

use available evidence and process information from secondary sources to draw or model Lewis structures and analyse this information to indicate the bonding mechanisms in selected complex ions involving the first transition series

• Look in Chemistry text books or on the Internet to find information about Lewis structures. AUS-e-TUTE, Australia.
  Also General Chemistry Online by Fred Senese, USA.
  
  
• You need information on drawing or modelling Lewis structures of ions.
  For example Cu⁺  +  4CN^-               [Cu(CN) ₄]^3-
  In this equation the Cu⁺ links with four cyanide ions CN^- to form four covalent bonds around the central copper ion.
  
  
• When you have organised the information analyse it to gain a better understanding of bonding mechanisms in the ions you have chosen .

identify examples of chelated ligands

• Chelated ligands are ligands that have more than one donor atom. Ligands with two or more donor atoms tend to form rings in the complex ion. Chelate comes from the Greek word meaning crab's claw, and these ligands surround the metal ion like a crab.
  
  
• Examples of ligands that can chelate include the oxalate ion ^-OOC-COO^- and the triphosphate ion [P₃O₁₀]^5-

discuss the importance of models in developing an understanding of the nature of ligands and chelated ligands using specific examples

• Models are important for explaining:
  • how metal ligands form
  • why certain geometrical shapes are preferred
  • why these compounds are brightly coloured and often magnetic
  • why some ligands (such as oxalate and triphosphate ions) can chelate.
     
• Models suggest that the geometry of a complex ion depends on the number and type of metal ion orbitals occupied by ligand lone pairs.
  
  
• Lewis (electron dot) diagram models of ligands show electron pairs able to fill  metal ion orbitals while covalently bonding the ligand to the metal ion.
  
  
• The model suggests that the geometry of the complex ion depends on the number and type of metal ion orbitals occupied by ligand lone pairs.

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