AQA A Level Chemistry

Revision Notes

6.2.1 General Properties of Transition Metals

General Properties of Transition Metals

  • Although the transition elements are metals, they have some properties unlike those of other metals on the periodic table, such as:
    • Variable oxidation states
    • Behave as catalysts
    • Form complex ions
    • Form coloured compounds

Ions of transition metals

  • Like other metals on the periodic table, the transition elements will lose electrons to form positively charged ions
  • However, unlike other metals, transition elements can form more than one positive ion
    • They are said to have variable oxidation states
  • Because of this, Roman numerals are used to indicate the oxidation state on the metal ion
    • For example, the metal sodium (Na) will only form Na+ ions (no Roman numerals are needed, as the ion formed by Na will always have an oxidation state of +1)
    • The transition metal iron (Fe) can form Fe2+ (Fe(II)) and Fe3+ (Fe(III)) ions
  • The table below shows the most common oxidation states of a few transition metals

Oxidation states of transition elements table

Chemistry of Transition Elements - Oxidation states of transition elements table, downloadable AS & A Level Chemistry revision notes

Coloured complex

  • Another characteristic property of transition elements is that their compounds are often coloured
    • For example, the colour of the [Cr(OH)6]3- complex (where oxidation state of Cr is +3) is dark green
    • Whereas the colour of the [Cr(NH3)6]3+ complex (oxidation state of Cr is still +3) is purple

Chemistry of Transition Elements - Coloured Transition Metal Complexes, downloadable AS & A Level Chemistry revision notes

Examples of some transition metal ions and their coloured complexes

Transition elements as catalysts

  • Since transition elements can have variable oxidation states, they make excellent catalysts
  • During catalysis, the transition element can change to various oxidation states by gaining electrons or donating electrons from reagents within the reaction
    • For example, iron (Fe) is commonly used as a catalyst in the Haber Process, switching between the +2 and +3 oxidation states
  • Substances can also be adsorbed onto their surface and activated in the process

Complex ions

  • Another property of transition elements caused by their ability to form variable oxidation states, is their ability to form complex ions
  • A complex ion is a molecule or ion, consisting of a central metal atom or ion, with a number of molecules or ions surrounding it
  • The molecules or ions surrounding the central metal atom or ion are called ligands
  • Due to the different oxidation states of the central metal ions, a different number and wide variety of ligands can form bonds with the transition element
    • For example, the chromium(III) ion can form [Cr(NH3)6]3+, [Cr(OH)6]3- and [Cr(H2O)6]3+ complex ions

Complex Ions

  • Transition element ions can form complexes which consist of a central metal ion and ligands
  • Copper(II) and cobalt(II) ions will be used as examples of the central metal ions, in the complex formation with water (H2O), ammonia (NH3), hydroxide (OH), and chloride (Cl) ligands
    • A copper(II) ion has an electronic configuration of 1s22s22p63s23p63d9
    • A cobalt(II) ion has an electronic configuration of 1s22s22p63s23p63d7

Complexes with water & ammonia molecules

  • Water and ammonia molecules are examples of neutral ligands
  • Both ligands contain a lone pair of electrons which can be used to form a dative covalent bond with the central metal ion
    • In water, this is the lone pair on the oxygen atom
    • In ammonia, it is the lone pair on the nitrogen atom
  • Since water and ammonia are small ligands, 6 of them can fit around a central metal ion, each donating a lone pair of electrons, forming 6 dative bonds
    • The coordination number of a complex is the number of dative bonds formed between the central metal ion and the ligands
    • Since there are 6 dative bonds, the coordination number for the complex is 6
  • Complexes with a coordination number of 6 have an octahedral shape
  • The overall charge of a complex is the sum of the charge on the central metal ion, and the charges on each of the ligands
  • A complex with cobalt(II) or copper(II) as a central metal ion, and water or ammonia molecules as ligands, will have an overall charge of 2+
    • The central metal ion has a 2+ charge and the ligands are neutral

Chemistry of Transition Elements - Ammonia and Water Complexes, downloadable AS & A Level Chemistry revision notes

Cobalt(II) and copper(II) form octahedral complexes with ammonia and water ligands

Complexes with hydroxide & chloride ions

  • Hydroxide and chloride ions are examples of negatively charged ligands
  • Both ligands contain a lone pair of electrons which can be used to form a dative covalent bond with the central metal ion
  • Hydroxide ligands are small, so 6 of them can fit around a central metal ion and the complex formed will have a coordination number of 6
  • Chloride ligands are large ligands, so only 4 of them will fit around a central metal ion
  • Complexes with 4 chloride ligands will have a coordination number of 4
    • Complexes with 4 chloride ligands will form tetrahedral complexes
    • Whereas hydroxide ligands will form octahedral complexes
  • A complex with cobalt(II) or copper(II) as a central metal ion and chloride ions as ligands, will have an overall charge of 2-
    • The central metal ion has a charge of 2+
    • Each chloride ligand has a charge of 1-
    • There are 4 chloride ligands in the complex, so the overall negative charge is 4-
    • The overall positive charge is 2+
    • Therefore, the overall charge of the complex is 2-
  • A complex with cobalt(II) or copper(II) as a central metal ion and hydroxide ions as ligands, will have no overall charge
    • The central metal ion has a charge of 2+
    • Each hydroxide ligand has a charge of 1-
    • There are 2 hydroxide ligands in the complex, so the overall negative charge is 2-
    • The overall positive charge is 2+
    • Therefore, the overall charge on the complex is 0

Chemistry of Transition Elements - Chloride and Hydroxide Complexes, downloadable AS & A Level Chemistry revision notes

Cobalt(II) and copper(II) form tetrahedral complexes with chloride and octahedral complexes with water and hydroxide ligands

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