Novel eta(3)-silapropargyl/alkynylsilyl complexes Cp-star(CO)(2)Mo(eta(3)-Ph2SiCCR) (3a, R = Bu-t; 3b, R = Pr-t) were synthesized by the reactions of Cp-star(CO)(2)(MeCN)MoMe (1) with alkynylsilanes HPh2SiC CR. The structures of 3a and 3b were fully characterized by NMR spectroscopy and X-ray crystallography. The reaction of 3a with MeOH at room temperature gave the stable four-membered metallacycle Cp-star(CO)(2)MoC(=(CHBu)-Bu-t)SiPh2OMe (5a), while the corresponding reaction of 3b led to the formation of the eta(3)-allyl complex Cp-star(CO)(2)Mo{eta(3)-(MeOPh2Si)HCCHCMe2} (7) via Cp-star(CO)(2)MoC(=CH'Pr)SiPh2OMe (5b).