Dicarbonylbis(1-oxa-1,3-diene) complexes of molybdenum and tungsten can be prepared from ethyl and methyl methacrylate by employing either (η 6 -C 6 H 6 )Mo(CO) 3 or (CH 3 CH 2 CN) 3 W(CO) 3 in modified standard complexation reactions. The crystalline, air-sensitive compounds can be obtained in 61%-94% yield. Both NMR spectroscopic studies and the X-ray crystal structure of the tungsten methyl methacrylate complex [H 2 C=C(CH 3 )C(O)OCH 3 ] 2 W(CO) 2 suggest that the bonding mode of the unsaturated ester ligands in these compounds differs from η 4 to some extent, i.e. the coordination of the carbonyl fragment is an intermediate between η 2 -π and η 1 at the carbonyl oxygen. The unsaturated ester ligands are labile with respect to exchange reactions, which allows other 1-oxa-1,3-dienes to replace them under very mild conditions. Most interestingly, in the case of the tungsten compounds, even complexes of temperature-sensitive α,β-unsaturated ketones can thus be easily obtained in excellent yield at room temperature.