Formation and shapes of complex ions

• What is one of the most important properties of the d-block elements?

  Their ability to form complex ions. For example when solid hydrated copper (II) sulfate is dissolved in water, a blue solution forms containing the complex ion [Cu(H2O)6]2+
• When is a complex ion formed?


  When one or more molecules or negatively charged ions bond to a central metal ion - these molecules or ions are known as ligands
• What is a ligand?

  A molecule or ion that donates a pair of electrons to a central metal ion to form a coordinate bond or dative covalent bond
• What is a dative covalent bond?

  A dative covalent bond or coordinate bond is a special kind of covalent bond, that forms when one of the bonded atoms provides both of the electrons for the shared pair
• What is the coordination number?

  indicated the number of coordinate bonds attached to the central metal ion
• How do we represent complex ions?

  1. square brackets are used to group all of the parts of the complex ion together
  2. the ligand is inside round brackets and the number of ligands is shown outside the round brackets
  3. the overall charge of the complex ion is shown outside square brackets and is the sum of the charges on the central metal ion and any ligands present
• What is a monodentate ligand?

  a ligand that is able to donate one pair of electrons to a central metal ion
• give examples of monodentate ligands and their charges?

  water = neutral

  ammonia = neutral

  chloride = -1

  cyanide = -1

  hydroxide = -1
• What is a bidentate ligand?

  a ligand that can donate two lone pairs of electrons to the central metal ion, forming two coordinate bonds

  e.g. 1,2-diaminoethane (en) and the ethanedioate (oxalate) ion
• How does 1,2-diaminoethane bond?

  each nitrogen atom donates a pair of electrons to the central metal ion forming a coordinate bond
• How does the ethanedioate ion bond?

  each negatively charged oxygen atom donates a lone pair of electrons to the central metal ion
• give an example of a complex ion containing a bidentate ligand?

  [Co(NH2CH2CH2NH2)3]3+

  1. oxidation number = 3+
  2. coordination number = 6 because there are three 1,2-diaminoethane ligands and each ligand forms 2 coordinate bonds
• What is the shape when there are 6-coordinate complexes?

  Octahedral e.g. MnSO4 dissolved in water gives [Mn(H2O)6]2+

  has an octahedral shape - bond angles 90 degrees

  and complex ion [Co(NH2CH2CH2NH2)3]3+ is octahedral as it has 6 coordinate bonds
• What is the shape when there are 4-coordinate complexes?


  Tetrahedral or square planar
• Tetrahedral

  more common

  bond angles 109.5 degrees around central metal ion

  e.g. [CoCl4]2- and [CuCl4]2-
• When does a square planar shape occur?

  In complex ions of transition metals with 8 d-electrons in the highest energy d-sub-shell.

  e.g. platinum (II), palladium (II) and gold (III)

  e.g. [Pt(NH3)4]2+ complex

  ligands are arranged at the corners of a square.
• Colour

  Compound must absorb visible light, the colour seen is the sum of the remaining colours not absorbed
• Compound absorbing all wavelengths of visible light

  • Appears black
• Compound absorbing every colour except blue


  • Appears blue
• Compound absorbing magenta


  • Appears green
• Compound absorbing blue and red light


  • Appears green
• [Cu(H2O)6]2+ ions

  Absorbs red light, forms a pale blue solution
• Chromium (VI) solutions

  Absorb green, cyan, blue and magenta, appear orange