Shapes of Complex Ions (Oxford AQA International A Level Chemistry)

Shapes of Complex Ions

• The shape / geometry of a transition metal complex depends on:

  • The size of the ligands

  • The number of co-ordinate bonds

Linear complexes

• Central metal atoms or ions with two co-ordinate bonds form linear complexes

• Linear complexes have:

  • A 180^o bond angle

  • A co-ordination number of 2

• The most common examples are:

  • Copper(I) ion, Cu⁺, with two ammonia ligands

  • Silver(I) ion, Ag⁺, with two ammonia ligands - this is know as Tollens' reagent

Examples of a linear complex

Tetrahedral complexes

• Central metal atoms or ions with four co-ordinate bonds often form tetrahedral complexes

  • Complexes with four chloride ions most commonly adopt this shape / geometry

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

• Tetrahedral complexes have:

  • A 109.5^o bond angle

  • A co-ordination number of 4

Example of a tetrahedral complex

Square planar complexes

• Central metal atoms or ions with four co-ordinate bonds can sometimes form square planar complexes

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

  • An example of a square planar complex is cisplatin

• Tetrahedral complexes have:

  • A 90^o bond angle

  • A co-ordination number of 4

Example of a square planar complex

Octahedral complexes

• Central metal atoms or ions with six co-ordinate bonds form octahedral complexes

• An octahedral complex could be:

  • A central metal ion with six small, monodentate ligands

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

  • A central metal ion 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

  • A central metal ion with one multidentate ligand

  • The polydentate ligand, for example EDTA^4-_, forms all six coordinate bonds

• Tetrahedral complexes have:

  • 90^o and 180^o bond angles

  • A co-ordination number of 6

Examples of octahedral complexes

Types of ligands table

Drawing stereochemical formulae

• Chemists use a convention of wedge drawings to represent three dimensional molecules

• The convention is that:

  • A solid line is a bond in the same plane as the paper

  • A dotted line is a bond receding behind the plane of the paper(this can also be hatched or shaded wedges)

  • A solid wedge is a bond coming out of the paper

Isomerism in Complex Ions

• Transition element complexes can exhibit stereoisomerism

 Geometrical (cis-trans) isomerism

• Even though transition element complexes do not have a double bond, they can still have geometrical isomers

• Square planar and octahedral complexes with two pairs of different ligands exhibit cis-trans isomerism (this is a special case of E-Z isomerism)

• An example of a square planar complex with two pairs of ligands is the anti-cancer drug cis-platin

  • Whereas cis-platin has beneficial medical effects by binding to DNA in cancer cells, trans-platin cannot be used in cancer treatment

Cisplatin and transplatin

Cis-platin (the Z-isomer) and trans-platin (the E-isomer) is an example of a square planar transition element complex that exhibits geometrical isomerism

• As long as a complex ion has two ligands attached to it that are different to the rest, then the complex can display geometric isomerism

• Examples of octahedral complexes that exhibit geometrical isomerism are:

  • [Cu(NH₃)₄(H₂O)₂]²⁺

  • [Ni(H₂NCH₂CH₂NH₂)₂Cl₂]²⁺ also written as [Ni(en)₂Cl₂]²⁺

• If the two ‘different’ ligands are next to each other then that is the ‘cis’ isomer

• If the two ‘different’ ligands are opposite each other then this is the ‘trans’ isomer

• In [Cu(NH₃)₄(H₂O)₂]²⁺, the two water ligands are adjacent to each other in the cis isomer and are opposite each other in the trans isomer

• In [Ni(H₂NCH₂CH₂NH₂)₂Cl₂]²⁺, the two chloride ligands are adjacent to each other in the cis isomer and are opposite each other in the trans isomer

Optical isomerism

• Octahedral complexes with bidentate ligands also have optical isomers

• This means that the two forms are non-superimposable mirror images of each other

  • They have no plane of symmetry, and one image cannot be placed directly on top of the other

• The optical isomers only differ in their ability to rotate the plane of polarised light in opposite directions

• Examples of octahedral complexes that have optical isomers are:

  • [Ni(H₂NCH₂CH₂NH₂)₃]²⁺

  • [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺