Phosphines and phosphites electronic effects

The readers will be aware of the basic principles of bonding in organometallic complexes, especially the bonding of alkenes and carbon monoxide, and this will not be dealt with here. We will say a few words about ligands containing phosphorus as the donor element. 

The IR frequencies represent a reliable yardstick of the electronic properties of a series of phosphorus ligands toward a particular metal. The latter restriction is important, because it is clear that the proton basicity scale (considering a proton as a metal) is not the same as a metal phosphine stability series. 

When mixed phosphorus compounds are considered the sum of the individual contributions of the substituents can be used to calculate the %-value of the ligand as was shown by Tolman. The Rvalue for a single substituent R is simply 1/3 of the %-value of the ligand PR3. In extreme cases the additivity rule may not apply but as a first approximation it remains useful. 

For rhodium complexes the IR stretch frequency of ram-RhClL2(CO) has been used [14], 

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