3.5.4 Transition Metals - Shapes of complex ions


Students should:
  • know that transition metal ions commonly form octahedral complexes with small ligands (e.g. H2O and NH3)
  • know that transition metal ions commonly form tetrahedral complexes with larger ligands (e.g. Cl. )
  • know that square planar complexes are also formed, e.g. cisplatin
  • know that Ag+ commonly forms the linear complex [Ag(NH3)2]+ as used in Tollens' reagent

Shapes of complex ions

There are several basic shapes that complex ions can adopt.

Although there are many other possible shapes these are the most common. The small ligands tend to form octahedral complexes as there is room for six around the central metal ion without interference. Larger ligands tend to form tetrahedral complexes.

The hexaaqua complexes are octahedral:

hexaaquacobalt(II) showing the octahedral structure

The six water molecules bond to the central cobalt ion using a lone pair on the oxygen atom.

There are six water molecules, so the coordination number is 6.

The shape of this six-coordinated complex is octahedral. The ligands occupy all of the points of three-coordinate geometric axes.

Four coordinate complexes are usually tetrahedral. The exception to this rule is the compounds with a d8 transition metal ion (nickel, platinum), which occasionally form square planar complexes.

The copper atom has four chloride ligands bonded to the central atom. Each of the chloride ligands is bonded by means of a lone pair. The coordination number is 4.

The shape of a four-coordinate complex is usually tetrahedral, although in some cases square planar complexes form (d8 metals are a typical exception).

tetrachlorocuprate(II) showing the tetrahedral structure

Two coordinate complexes are linear:

diammine silver(I) structure showing the linear structure

The two ammonia molecules bond to the central silver ion using a lone pair on the nitrogen atom.

There are two nitrogen molecules, so the coordination number is 2.

The shape of this two-coordinated complex is linear. The ligands are arranged at an angle of 180º.

This is the complex known as Tollen's reagent, used for testing for aldehydes and reducing sugars (silver mirror test).