The title complexes were synthesized by reaction of (η -C~5H~4R)(CO) ~3) Mo^-Li^+ (R: H, SiMe~3, Si~2Me~5) with X(CH~2)~3X(X: Br, I) in THF. Their structures were characterized by elemental analyses, IR, ^1H NMR and ^1^3C NMR spectra. The crystal structure of η -C~5H~5(CO) ~2MoBrCO(CH~2)~CH~2 was determined by X-ray diffraction. The crystals are orthorhombic, space group P2~12~12~1, with a=0.8600(2), b=1.4775 (3), c=1.9900(7)nm; V=2.529nm^3, Z=8, D~x=1.93g.cm^-^3; μ=41.3cm^-^1, F(000)=1424. The final deviation factor R=0.055, R~w=0.060.

Key words: MOLYBDENUM, SYNTHESIS, CYCLOPENTADIENYL GROUP, BROMINE, ANIONS, CRYSTAL STRUCTURE, CARBONYL GROUP, X- RAY DIFFRACTION ANALYSIS