General Properties of Transition Metals (Oxford AQA International A Level Chemistry)

General Properties of Transition Metals

• Transition metals are elements with an incomplete d-subshell or that can form at least one stable cation 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

• Therefore, the elements of the first transition series are 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

• There are two exceptions to the Aufbau Principle in the first row of d-block:

  • Chromium

  • 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 4s¹ not [Ar] 3d⁹ 4s²

• This is because the [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹ configurations are energetically more stable and are preferred configurations

  • When forming cations, remove the 4s electrons first

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 state

• 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

Complex formation

• 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

• A molecule or ion surrounding the central metal atom or ion is called a ligand

An example metal complex

• 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 iron(III) ion can form complexes of [Fe(NH₃)₆]³⁺, [Fe(OH)₆]^3- and [Fe(H₂O)₆]³⁺ 

Formation of coloured ions

• Another characteristic property of transition elements is that their compounds are often coloured

  • For example, a solution of [Fe(H₂O)₆]²⁺ (Fe oxidation state is +2) is green

  • Whereas, a solution of [Fe(H₂O)₆]³⁺ (Fe oxidation state is +3) is orange / brown

Catalytic activity

• 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 from or donating electrons to reagents within the reaction

• Substances can also be adsorbed onto their surface and activated in the process

Ligands & Complex Ions

• Transition element ions can form complexes which consist of a central metal ion surrounded by ligands

• A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons

  • This is the definition of a Lewis base - electron pair donor

  • Since ligands have a lone pair of electrons available to donate, they can also be described as nucleophiles

• 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

  • 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

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

Examples of complexes with monodentate ligands

• Tetrahedral complexes, [CuCl₄]^2-, and square planar complexes, [NiCN₄]^2-, have a co-ordination number of 4

• Octahedral complexes, [Fe(H₂O)₆]²⁺ and [Co(NH₃)₆]²⁺, have a co-ordination number of 6

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 (C₂O₄^2- ) which is sometimes written as ‘ox’

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 EDTA^4-, which is a hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion

Example of a polydentate ligand complex