Ligand cone angle

Ligand 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.



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 Tolman.

Cone angles of common phosphine ligands in degrees: H 87 F 104 OCH3 107; CH3 118; CH2CH3 132; C6H5 145; cyclo-C6H11 170; t-Bu 182; C6F5 184; 2,4,6-Me3C6H2 212.

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