The 2-methyleneimidazolone complexes I (M = Mo, W) were prepd. by the reaction of M(CO)6 with 1,3,4,5-tetramethyl-2-methyleneimidazoline. Spectroscopic data reveal the strong donor character of the ylidic ligand in I. The end-on coordination mode of the olefin is demonstrated by an x-ray structure anal. of I (M = Mo).