Ligand cone angle

The ligand cone angle (a common example being the Tolman cone angle or θ) is a measure of the size of a ligand. It is defined as the solid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone (see figure). Tertiary phosphine ligands are commonly classified using this parameter, but the method can be applied to any ligand. The term cone angle was introduced by Chadwick A. Tolman, a research chemist at Dupont. Originally applied to phosphines, the cone angles were originally determined by taking measurements from accurate physical models of them.