Phosphines and phosphites steric effects

Ligands never form a perfect cone either and in some instances steric interactions nearby the metal may be important, while for other properties interactions more distant from the metal may dominate. This can be taken into account when the cone angle is determined. This point has been addressed in the procedure for determining the so-called solid angle, which utilises crystal structure data. The procedure involves the projection of all atoms of the ligand on the metal surface and the solid angle tells us how much of the surface is covered by this projection [17], 

Thermochemistry has been used to determine heats of formation of metal-phosphine adducts. When electronic effects are small, the heats measured are a measure of the steric hindrance in the complexes heats of formation decrease with increasing steric bulk of the ligand. Recent thermochemistry concerning ligand effects can be found in reference [18], 

Tolman s equation includes steric effects and the quantifications of steric effects in LFER have also been useful. The equation reads  

The values for % and 0 are as defined above and used as dimensionless numbers. Property as in Hammett relations should be the logarithm of a rate constant, equilibrium constant, etc. An important refinement for the steric contribution was made after the recognition that below a certain threshold of cone angles the property no longer changed and one should use the value 0 for c [29], Above the threshold a linear relation was found. Thus, for each property a steric threshold 0 was defined and the equation now reads  

When one of the substituents at phosphorus is hydrogen another parameter was needed for PZ jHj, to account for the number i of hydrogen substituents giving the equation  

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