CHELATE RING-CLOSURE KINETICS OF CR(CO)(5) (DPPM) STUDIED BY P-31-NMRSPECTROSCOPY

P-31-NMR spectroscopy was used to study the kinetics of the chelate ri ng-closure reaction in Cr(CO)(5) (DPPM) where DPPM is bis (diphenylpho sphino)methane. DPPM is potentially a bidentate ligand, but in this co mplex it is coordinated to the transition metal in a monodentate fashi on and therefore can undergo chelation. The thermal chelate ring-closu re reaction was carried out in an NMR sample tube at a constant temper ature and the course of the reaction was quantitatively followed by ta king P-31-NMR spectra in appropriate time intervals. Kinetic results h ave shown that the chelate ring-closure reaction of Cr(CO)(5) DPPM is of zero-order. Activation parameters were determined by evaluating the kinetic data. The large negative value of the activation entropy impl ies that the mechanism of the chelate ring-closure reaction is of asso ciative nature in the transition states. The rate determining step mig ht involve both Cr-CO bond breaking and Cr-P bond making. This will ge nerate a seven coordinate transient complex from which the complete de tachment of a CO Ligand will lead to the formation of Cr(CO)(4) (DPPM) .