Question

Compare the 31P NMR chemical shifts of methyldiphenylphosphine, triethyl phosphite and 1,2-bis(diphenylphosphino)ethane with those of their cis- Mo(CO)4(L)2 complexes. What is the effect of coordinating the phosphorus to the metal? Are these changes consistent with the changes in the electron density at the phosphorus? Explain your answer

Answer 1

As phosphorus is coordinated to the metal, so some extra electron density of metal will move to vacant pi * orbtal of the phosphorus due to backdonation. And due to this backdonation we will see a downfield shift of P in ³¹P nmr.

So, in case of methyldiphenylphosphine, triethyl phosphite and 1,2-bis(diphenylphosphino)ethane like ligands we will observe upfield signal in ³¹P nmr whereas in case of complexes with taking these ligands like cis- Mo(CO)₄(L)₂  , then we will observe a downfield shift of signals in ³¹P nmr .

Yes, these changes are consistent with the changes in the electron density at the phosphorus.