Coordination Compounds

1
Coordination Compounds

• Metal ion is Lewis acid (electron-pair acceptor)
• Ligand is Lewis base (electron-pair donor)
• They are joined by a coordinate-covalent bond.
• Alfred Werner coordination theory (1890)
• CoCl3 NH3 ? 5 compounds formed
• Primary valence oxidation state of metal
• Secondary valence number of ligands attached to
  metal (coordination number)
• Postulate coordination number is constant

2

• Postulates (more detail)
• The series of Co(III) compounds has a constant
  coordination number of 6.
• Neutral ligands (H2O, NH3, CO, etc.) must be
  covalently bonded to the metal.
• Ionic ligands (Cl, OH, NO2, CN, etc.) may be
  bonded to the metal or free counterions.
• Thus, the series of compounds formed has the
  general formula CoCl3nNH3, where n3-6.

3

• Formula Ions Formed in
  Solution
• Co(NH3)6Cl3 Co(NH3)63 3
  Cl
• Co(NH3)5ClCl2 Co(NH3)5Cl2
  2 Cl
• Co(NH3)4Cl2Cl Co(NH3)4Cl2
  Cl
• (2 forms)
• Co(NH3)3Cl3 none
• Experimental evidence
• As more ions are formed, the molar conductivity
  increases.
• Only free Cl (or Br or I) counterions may be
  precipitated with Ag

4

• Coordination Number
• Values from 2 12 known
• 6 is by far the most common
• 4 is next most common
• Structures
• 6-coordinate octahedral
• 4-coordinate tetrahedral or square planar

5
Figure 20.5The Ligand Arrangements for
Coordination Numbers 2, 4, and 6
6
Ligands (see Table 20.13)

• Denticity of a ligand the number of electron
  pairs donated
• Monodentate donate 1 pair
• Cl, H2O, NH3, CO, CN, SCN, OH
• Bidentate donate 2 pairs
• ethylenediamine, H2NCH2CH2NH2 (en)
• oxalate ion, C2O42
• Chelating Ligand has gt1 attachment point to
  metal (from Greek chele for claw)

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Nomenclature of Coordination Compounds

• Cation named before anion
• Ligands named before metal (alphabetically)
• Add o to end of anionic ligands (e.g, bromo).
  Names of neutral ligands unchanged except for a
  few special cases.
• Numerical prefixes tell how many of each ligand
  present (di-, tri-, tetra-, etc., or bis-, tris-,
  tetrakis-, etc.)
• Oxidation state follows name of metal in
  parentheses (Roman numeral)
• Add ate to end of metal name if the complex is
  anionic.

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• Special Ligand Names
• H2O aqua
• NH3 ammine
• CO carbonyl
• NO nitrosyl
• Nomenclature Examples
• Co(NH3)5ClCl2
• pentaamminechlorocobalt(III) chloride
• Co(NH3)4Cl2Cl
• tetraamminedichlorocobalt(III) chloride

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• K2CuCl4
• potassium tetrachlorocuprate(II)
• Ni(en)3(NO3)2
• tris(ethylenediamine)nickel(II) nitrate
• Triaquatricyanocobalt(III)
• Co(H2O)3(CN)3
• Ammonium hexahydroxoferrate(II)
• (NH4)4Fe(OH)6