Rh(acac)(CO)(PR3) and Rh(oxinate)(CO)(PR3) complexes—substitution chemistry and structural aspects

The substitution of CO in Rh(acac)(CO)2 by the phosphorus ligands P(OPh)3, P(NC4H4)3, and PPh2(NC4H4) has been studied kinetically by stopped-flow spectrophotometry as a function of temperature. With P(OPh)3 and P(NC4H4)3, both CO ligands are replaced in a stepwise fashion via the intermediate Rh(acac)(CO)(PR3). However, the disubstituted complexes Rh(acac)(PR3)2 are thermodynamically unstable. Judged from the activation parameters, the individual steps are associative processes. In the case of PPh2(NC4H4) only the monosubstituted complex is formed. The differences in the substitution rates as well as the stability of the various products are largely dominated by electronic (e.g. basicity) effects. X-ray structures of some of the mono-substituted complexes are given. In addition, also the reaction of Rh(oxinate)(CO)2 with P(OPh)3 has been studied kinetically showing that oxinate has a labilizing effect relative to acetylacetonate