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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.
==Asymmetric cases==
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" as well as diphosphines. In such asymmetric cases the substituent angles half angles, ''θi''/2, are averaged and then doubled to find the total cone angle, θ. In the case of diphosphines the ''θi''/2 of the backbone is approximated as half the chelate bite angle assuming a bite angle of 74, 85, 90 deg. for diphosphines with methylene, ethylene, and propylene back bones respectively. The Manz cone angle is often easier to compute than the Tolman cone angle.
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抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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