Fourier transform infrared study of dichlorodioxochromium(VI), dichlorodioxomolybdenum(VI) and dichlorodioxotungsten(VI) complexes with 2,2 '-bipyridine

The Fourier transform infrared spectra of the 2,2'-bipyridine complexes of dichlorodioxochromium(VI), dichlorodioxomolybdenum(VI) and dichlorodioxotun gsten(VI) were studied in the 4000 to 400 cm-l region. The assignment of th e bands originating from vibrations mainly localized in the bipyridine liga nd is relatively straightforward. The essentially pure MO2 stretching vibra tions give rise to very strong bands, the separation of which increases in the order Cr < Mo < W as a consequence of the greater sensitivity of the an tisymmetric stretching O-M-O vibration to the changes in the mass of the AI I atoms. For this to be true, the symmetric stretching vibration should hav e a higher frequency than the antisymmetric one. The results of the prelimi nary ab initio HF SCF calculations on simple model structures are in line ( at least qualitatively) with such a conclusion.