AQA A Level Chemistry复习笔记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

6.2.1-Transition-elements-and-d-block-elements

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

6.2-Chemistry-of-Transition-Elements-Electronic-configuration-of-transition-elements-table

  • 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

 

Worked Example

Writing electronic configuration of transition element ionsState 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
  • 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

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 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

 

6.2-Chemistry-of-Transition-Elements-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

     

Monodentate-Ligands

 

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’

     

6.2-Chemistry-of-Transition-Elements-Bidentate-Ligands

 

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

 

6.2-Chemistry-of-Transition-Elements-Polydentate-Ligands_2

 

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

 

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

 

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 -3ChromiunIII-complex-with-hydroxide-ions

     

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

 

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