General Properties of Transition Metals (AQA A Level Chemistry)

• 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

• 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 4s¹ not [Ar] 3d⁹ 4s²

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

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
• A molecule or ion surrounding the central metal atom or ion is called a ligand
• 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
• Substances can also be adsorbed onto their surface and activated in the process

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

• 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

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

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

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

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-

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 -3

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