Ligand cone angle

Ligand cone angle (also known as the Tolman cone angle) 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.

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The concept of cone angle is of practical importance in homogeneous catalysis because the size of the ligand affects the reactivity of the attached metal center. In a famous example, the selectivity of hydroformylation catalysts is strongly influenced by the size of the coligands.

The concept of cone angle is most easily visualized with symmetrical ligands, e.g. PR₃. But the approach has been refined to include less symmetrical ligands of the type PRR'R".

The term cone angle was introduced by Chadwick A. Tolman.^[1]

Cone angles of common phosphine ligands in degrees:

One remarkable features become clear from these data: some ligands occupy more than half of the coordination sphere of a metal center.

References

1. ^ Tolman, C. A. (1977). "Steric Effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis". Chem. Rev. 77: 313-48. doi:10.1021/cr60307a002.