The reactions of the
potentially bidentate ditertiary arsine ligand bis-(diphenylarsino)methane,
dam, with the hexacarbonyls of the Group VI transition metals have been
investigated. N.m.r, spectroscopy has been used to determine the mode of
coordination of the ligand. In the compounds M(CO)5(dam), M(CO)4(dam)2,
and M(CO)3(dam)3 [M = Cr, Mo, W], dam is acting as a
monodentate ligand, but in M(CO)4(dam) it is acting as a normal
chelate coordinating to the metal through both arsenic atoms. In the cases of
molybdenum and chromium, only, a further type of complex M(CO)2(dam)
has been isolated. The results of N.M.R., infrared, and mass spectrometry,
together with other physical measurements, can only be explained on the
assumption that the ligand was coordinated to the metal by an arene π-linkage
and by one arsenic atom. This novel structure has been confirmed by a single
crystal X-ray diffraction study on the chromium compound.