The phosphavinylidene complexes (h5-C5H5)(CO)2M:P:C(SiMe3)2 (I, M = Mo, W) react with equimolar amts. of sulfur ylide Me2S(O):CH2 to afford the red metallophosphine-substituted sulfur ylides (h5-C5H5)(CO)2M:P[CH(SiMe3)2][CH:S(O)Me2]. I and II are transformed to the metallaheterocycles III by excess of ylide. In addn., the mol. structure of III (M = Mo) was elucidated by single crystal x-ray diffraction anal.