Carbonyl halides of the Group VI transition metals. XXIII. An N.M.R. study of a series of Bis(diphenylarsino)methane derivatives of molybdenum(II) and tungsten(II)

Abstract

A variable temperature nuclear magnetic resonance study of a series of molybdenum(II) and tungsten(II) complexes of the general formula M(CO)2(dam)₂X₂ (M = Mo, W; X = Cl, Br, I; dam = bis(diphenylarsino)methane) has been carried out. These compounds have previously been shown to be seven-coordinate non-electrolytes containing one chelated and one monodentate dam ligand. The detailed study of their N.M.R. spectra reported here shows that, near room temperature, exchange occurs between the non-equivalent dam molecules. At lower temperatures this exchange ceases, but further fine structure appears in the spectra due to the freezing-in of particular conformation of the molecules. On warming, this fine structure disappears as a result of the inherent non-rigid character of the ligand.     The experimental spectra have been compared with a set of computed spectra based on the model of an exchanging system to determine the various exchange rate constants, and the observed spectra have been interpreted m terms of the molecular stereochemistry about the metal.