AQA A Level Chemistry

Revision Notes

6.2.1 General Properties of Transition Metals

General Properties of Transition Metals

  • Transition metals are elements with an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell
  • This definition distinguishes them from d-block elements, because scandium and zinc do not fit the definition
    • Scandium only forms the ion Sc3+, configuration [Ar] 3d0
    • Zinc only forms the ion Zn2+, configuration [Ar] 3d10
  • The elements of the first transition series are therefore titanium to copper

 

 

 

 

 

 

The transition elements and the d-block elements

Electron Configuration

  • The full electronic configuration of the first d-series transition metals is shown in the table below
  • Following the Aufbau Principle electrons occupy the lowest energy subshells first
  • The 4s overlaps with the 3d subshell so the 4s is filled first
  • Remember that you can abbreviate the first five subshells, 1s-3p, as [Ar] representing the configuration of argon( known as the argon core)

Table showing the electronic configuration of the first d-series transition elements

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

  • From AS Chemistry you should recall two exceptions to the Aufbau Principle, chromium and copper
  • In both cases an electron is promoted from the 4s to the 3d to achieve a half full and full d-subshell, respectively
  • Chromium and copper have the following electron configurations, which are different to what you may expect:
    • Cr is [Ar] 3d5 4s1 not [Ar] 3d4 4s2
    • Cu is [Ar] 3d10 4snot [Ar] 3d9 4s2
  • This is because the [Ar] 3d5 4s1 and [Ar] 3d10 4sconfigurations are energetically more stable

Worked Example

Writing electronic configuration of transition element ions

State the full electronic configuration of the manganese(III) ion

Answer

Step 1: Write out the electron configuration of the atom first:

Mn atomic number = 25

1s22s22p63s23p64s23d5

2 + 2 + 6 + 2 + 6 + 2 + 5 = 25 electrons

Step 2: Subtract the appropriate number of electrons starting from the 4s subshell

Mn(III) = 22 electrons

1s22s22p63s23p63d4

General properties

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

Variable Oxidation States

  • 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

Forming 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

Forming coloured compounds

  • 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

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

  • Transition element ions can form complexes which consist of a central metal ion and ligands
  • A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons to the bond
  • This means ligands have a negative charge or a lone pair of electrons capable of being donated
    • This definition may seem familiar: a ligand is the same as a nucleophile
  • Different ligands can form different numbers of dative bonds to the central metal ion in a complex
    • Some ligands can form one dative bond to the central metal ion
    • Other ligands can form two dative bonds, and some can form multiple dative bonds
  • Co-ordination number is number of co-ordinate bonds to the central metal atom or ion

Examples of ligands Table

Chemistry of Transition Elements - Examples of ligands table, downloadable AS & A Level Chemistry revision notes

Monodentate Ligands

  • Monodentate ligands can form only one dative bond to the central metal ion
  • Examples of monodentate ligands are:
    • Water (H2O) molecules
    • Ammonia (NH3) molecules
    • Chloride (Cl) ions
    • Cyanide (CN) ions

Monodentate Ligands, downloadable AS & A Level Biology revision notes

Examples of complexes with monodentate ligands

Bidentate Ligands

  • Bidentate ligands can each form two dative bonds to the central metal ion
  • This is because each ligand contains two atoms with lone pairs of electrons
  • Examples of bidentate ligands are:
    • 1,2-diaminoethane (H2NCH2CH2NH2) which is also written as ‘en’
    • Ethanedioate ion (C2O42- ) which is sometimes written as ‘ox’

Chemistry of Transition Elements - Bidentate Ligands, downloadable AS & A Level Chemistry revision notes

Examples of complexes with bidentate ligands

Multidentate Ligands

  • Some ligands contain more than two atoms with lone pairs of electrons
  • These ligands can form more than two dative bonds to the and are said to be multidentate ligands
  • An example of a multidentate ligand is EDTA4-, which is a hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion

Chemistry of Transition Elements - Polydentate Ligands_1, downloadable AS & A Level Chemistry revision notes

Example of a polydentate ligand complex

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 usually fit around a central metal ion, each donating a lone pair of electrons, forming 6 dative bonds
    • Since there are 6 dative bonds, the coordination number for the complex is 6
  • 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 chromium(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

Ammonia and Water Complexes, downloadable AS & A Level Biology revision notes

Cobalt(II) and chromium(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
  • 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-

Chloride Complexes, downloadable AS & A Level Biology revision notes

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

  • A complex with chromium(III) as a central metal ion and hydroxide ions as ligands, will have an overall charge of 3-
    • The central metal ion has a charge of 3+
    • Each hydroxide ligand has a charge of 1-
    • There are 6 hydroxide ligands in the complex, so the overall negative charge is 6-
    • The overall positive charge is 3+
    • Therefore, the overall charge on the complex is -3Chromiun(III) complex with hydroxide ions, downloadable AS & A Level Biology revision notes

Chromium(III) ions form a complex ion with hydroxide ions

Exam Tip

The word dentate should remind you of dentistry. It comes from the French word dents meaning teeth and indicates the number of ‘teeth’ that the ligand bites onto the transition metal ion with.

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