CHELATION OF GROUP-6 METAL TETRACARBONYL FRAGMENTS BY DICOBALT THIOALKYNE COMPLEXES

Reactions of [Co2(mu-RSCH2C=CCH2SR)(mu-dppm)(CO)4] (dppm = bis(dipheny lphosphino)methane) with [M(CO)4nbd] (nbd = norbornadiene) afford [Co2 {mu-C2(CH2SR)2M(CO)4}(muA-dPPM)(CO)4] (M = Mo, W; R = Me, Et, (n)Bu, ( t)Bu, Bz and M = Cr; R = Me, Et). The molecular structure of [Co2{mu-C 2(CH2SMe)2Mo(CO)4}(mu-dppm)(CO)4] has been established by X-ray diffra ction, and comprises a Co2(mu-dppm)(CO)4 unit transversely bridged by 2,7-dithio-4-octyne co-ordinated through the thioether sulphur atoms t o a molybdenum tetracarbonyl moiety. The chelation of [Co2(mu-MeSCH2C= CCH2SMe)(mu-dppm)(CO)4] to the molybdenum centre produces a seven-memb ered ring which adopts a chair conformation, and the thiomethyl groups adopt a cis configuration with respect to the plane defined by the mo lybdenum and the two sulphur atoms. The mass spectra and NMR spectra o f the new compounds are reported. The compounds display dynamic NMR be haviour which can be interpreted in terms of alkyne rocking and sulphu r inversion.