Common Structures

Why do we want to know the structure for a coordination compound? The compound's structure (i.e., how the ligands are arranged around the metal atom) determines its physical and chemical properties. For example, a tetrahedral compound will behave differently than a square planar compound that contains the same metal atom and the same ligands.

Consider the complex ion, [CoCl4]2-. The coordination number of [CoCl4]2- is equal to 4; thus, the structure of [CoCl4]2- might be:

  1. tetrahedral - ligands attached at the corners of a tetrahedron
  2. square planar - ligands attached at the corners of a square
  3. something else?
Coordination compounds with a coordination number (CN) of 4 are generally either tetrahedral or square planar. The [CoCl4]2- complex ion is experimentally known to have a tetrahedral structure.

A Comparison of Tetrahedral and Square Planar Structures
example sketch model
[CoCl4]2-
tetrahedral
CN = 4
ligands attached at the corners of a tetrahedron
[Ni(CN)4]2-
square planar
CN = 4
ligands attached at the corners of a square

Coordination numbers range from 1 to 12, with 2, 4 and 6 being the most common. Linear and octahedral are the most common structures for coordination numbers 2 and 6, respectively.

A Comparison of Linear and Octahedral Structures
example sketch model
[Ag(NH3)2]+
linear
CN = 2
ligands and metal atom connected in a straight line
[PtCl6]2-
octahedral
CN = 6
ligands attached at the corners of an octahedron