STERIC AND ELECTRONIC EFFECTS ON THE RH-103 NMR SHIFTS OF (COD)RH(PHOSPHINE) COMPLEXES

Rh-103 NMR data of several series of (COD)Rh(phosphine) complexes, (COD)Rh(PR(3))Me, (COD)Rh(PR(3))Cl, {(COD)Rh(PR(3))(2))}Y, {(COD)Rh(PPh(3))(py)}PF6, (COD)Rh(PR(3))(2)Me, and of one NBD complex, {(NBD)Rh-(PPh(3))(2)}PF6, have been measured by inverse two-dimensional H-1, Rh-103- or (31)p, Rh-103{H-1} NMR. Within the series, delta(Rh-103) extends from -332 for (COD)Rh(PMe(3))(2)Me to +557 ppm for (COD)Rh(P(NMe(2))3)Cl. Upon variations in the first coordination sphere, deshielding increases in the order PR(3) < CH3 < py similar to Cl. Going from P(OMe)(3) to trialkylphosphines to P(NMe(2))(3) the same trend for delta(Rh-103) is observed within different series of compounds, which allows an interpretation of the Rh shift which is based on the properties of the phosphorus ligand. A systematic variation of PR(3) reveals that paramagnetic shielding of the metal nucleus is caused by an increase in the basicity as well as the cone angle of the ligand. Within a subseries, delta(Rh-103) may quantitatively be described by invoking parameters that are a measure of the basicity and the cone angle of the phosphine ligand. There is no general correlation between delta(Rh-103) and P-31 NMR data. Explanations of the observed trends in delta(Rh-103) in terms of the paramagnetic shift are given.