Coordination Number 3

Once again, this is not very common for first row transition metal ions. Examples with three different geometries have been identified:

• Trigonal planar Geometry: Well known for main group species like CO₃^2- etc., this geometry has the four atoms in a plane with the bond angles between the ligands at 120 degrees. 

Figure 1: The Trigonal planar [Cu(CN)₃]^2- system

• Trigonal pyramid Geometry: More common with main group ions. 

• T-shaped: The first example of a rare T-shaped molecule was found in 1977.