Everything about Ligand Cone Angle totally explained
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.
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, for example PR
3. But the approach has been refined to include less symmetrical ligands of the type PRR'R" as well as diphosphines. In the case of diphopshines the cone angles are given for half the chelate assuming a
bite angle of 74, 85, 90 deg. for diphosphines with methylene, ethylene, and propylene back bones respectively.
The term cone angle was introduced by Chadwick A. Tolman.
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.
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