Abstract: W irradiation of mixtures of 3, 4-dimethylphospholes (L) with M (C0) 6 (M= Cr, Mo, W) leads mainly to (LL) M (CO), complexes derived from Diels-Alder [4+ 21 phosphole dimers acting as chelating ligands. An X-ray structural study of one of these complexes shows that, contrary to normal endo phosphole dimers, these compounds have the exo configuration and that the phosphorus bridge is very strained: LCPC= 79.4'. With 1- ...