• 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

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

• 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 (C2O42-?) 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 EDTA4-, 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