Phosphines cone angle concept

Cone angle concepts, 2,1012-1028 chelate effects, 2,1012 definitions, 2,1015 models, 2,1015 phosphines chiral, 2,1014 polydentate, 2,1013 solubility effects, 2,1014 use, 2,1028... 

C. The graph indicates that no reaction occurs for d > 141°. This is borne out very well, for no reaction is found for phosphines PPh3, PPr 3, PCy3 and PBu3, all for which 6 145°. Whatever the merits of Tolman s cone angle concept, his ideas have stimulated much research, particularly in the field of bulky ligands. 

Therefore, it was Tolman rather than Chatt who was able to show how steric effects associated with phosphine ligands could be put on a more quantitative basis using the cone angle concept.A mathematically defined cone was projected from the metal atom to the surface of the ligand as defined by the van der Waals radii of the atoms on the surface of the ligand. I suspect that Chatt, Wilkinson and Malatesta all probably had an appreciation of the importance of steric effects, but in my opinion they lacked the mathematical background to articulate it in a way which could be readily appreciated by other coordination chemists. 

The cone angle concept has been further developed by Mingos when applied to metal clusters. Since the function of the phosphine ligand is to stabilize the cluster by covering the surface, the important phosphine parameter is the plane of coverage . Figure 6. In the idealized case shown this would be a circle of radius d/2. In practice the projection would be cog-like, as demonstrated by Ferguson and Alyea inter alia in the context of mononuclear complexes. 

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In addition to pioneering the cone angle concept, C. A. Tohnan proposed a parameter X (chi) as a measure of the electronic effect of phosphine and related ligands, based on infrared spectra of complexes containing these ligands (C. A. Tohnan, J. Am. Chem. Soc., 1970, 92, 2953). 

Tolman introduced the concept of the cone angle in 1970 to separate and quantify steric and electronic factors in the reactions of phosphines with transition metals. The original measurements were made from simple physical models, and this approach has been extended to mathematical and computer modeling. Other workers have used X-ray diffraction data to observe and measure cone angles directly. These various approaches are outlined below. 

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