Chelate ring-closure kinetics of Cr(CO)(5) (DPPM) studied by P-31-NMR spectroscopy


Ozkar S. , Kayran C. , Tekkaya A.

TURKISH JOURNAL OF CHEMISTRY, vol.20, no.1, pp.74-79, 1996 (Journal Indexed in SCI) identifier

  • Publication Type: Article / Article
  • Volume: 20 Issue: 1
  • Publication Date: 1996
  • Title of Journal : TURKISH JOURNAL OF CHEMISTRY
  • Page Numbers: pp.74-79

Abstract

P-31-NMR spectroscopy was used to study the kinetics of the chelate ring-closure reaction in Cr(CO)(5) (DPPM) where DPPM is bis (diphenylphosphino)methane. DPPM is potentially a bidentate ligand, but in this complex it is coordinated to the transition metal in a monodentate fashion and therefore can undergo chelation. The thermal chelate ring-closure reaction was carried out in an NMR sample tube at a constant temperature and the course of the reaction was quantitatively followed by taking P-31-NMR spectra in appropriate time intervals. Kinetic results have 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 implies that the mechanism of the chelate ring-closure reaction is of associative nature in the transition states. The rate determining step might involve both Cr-CO bond breaking and Cr-P bond making. This will generate a seven coordinate transient complex from which the complete detachment of a CO Ligand will lead to the formation of Cr(CO)(4) (DPPM).