Ligands & Complex Ions (OCR A Level Chemistry A): Revision Note

Structure & Bonding of Transition Element Complexes

• 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

Examples shown: [CuCl₄]^2– (four Cl^–), [Fe(H₂O)₆]²⁺ (six H₂O), [Co(NH₃)₆]²⁺ (six NH₃) and [Ni(CN)₄]^2– (four CN^–); each ligand forms one dative bond using its lone pair.

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

Examples shown: [Cu(en)₃]²⁺ and [Co(C₂O₄)₃]^3–; each of the three bidentate ligands forms two dative bonds, giving six in total (octahedral).

Shape & Coordination of Transition Element Complexes

• Depending on the size of the ligands and the number of dative bonds to the central metal ion, transition element complexes have different geometries

  • Dative bonds can also be referred to as coordinate bonds, especially when discussing the geometry of a complex

Six-coordinate complexes

• Octahedral complexes are formed when a central metal atom or ion forms six coordinate bonds

• This could be six coordinate bonds with six small, monodentate ligands

  • Examples of such ligands are water and ammonia molecules and hydroxide and thiocyanate ions

• It could be six coordinate bonds with three bidentate ligands

  • Each bidentate ligand will form two coordinate bonds, meaning six coordinate bonds in total

  • Examples of these ligands are 1,2-diaminoethane and the ethanedioate ion

• The bond angles in an octahedral complex are 90^o

Examples of octahedral complexes

Examples shown: [Fe(H₂O)₆]²⁺, [Co(NH₃)₆]²⁺, [Cu(NH₃)₄(H₂O)₂]²⁺ and [Co(C₂O₄)₂Cl₂]^3–, all with six dative bonds to the central metal ion.

Four-coordinate complexes

Tetrahedral

• When there are four coordinate bonds the complexes often have a tetrahedral shape

  • Complexes with four chloride ions most commonly adopt this geometry

  • Chloride ligands are large, so only four will fit around the central metal ion

• The bond angles in tetrahedral complexes are 109.5^o

Example of a tetrahedral complex

Example shown: [CuCl₄]^2–, with four Cl^– ligands and Cl–Cu–Cl bond angles of 109.5°.

Square planar

• Sometimes, complexes with four coordinate bonds may adopt a square planar geometry instead of a tetrahedral one

  • Cyanide ions (CN^-) are the most common ligands to adopt this geometry

  • An example of a square planar complex is cisplatin

• The bond angles in a square planar complex are 90^o

Cisplatin is an example of a square planar complex