3.5.4 Transition Metals - Shapes of complex ions
Shapes of complex ions
There are several basic shapes that complex ions can adopt.
- 1 Octahedral
- 2 Tetrahedral
- 3 Square planar
- 4 Linear
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:
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).
Two coordinate complexes are linear:
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).