The reaction of (acetonitrile)-undeca(carbonyl)-tri-osmium(0) and bis(tert-butyl)fluorophosphine led to the formation of (bis(tert-butyl)fluorophosphine) undeca(carbonyl)-tri-osmium(0) (1). However, the reaction of bis(acetonitrile)-deca(carbonyl)-tri-osmium(0) and bis(tert-butyl)fluorophosphine yielded bis(bis(tert-butyl)fluorophosphine)-deca(carbonyl)-tri-osmium(0) (2) and (tert-butyl-difluorophosphine)(bis(tert-butyl)fluorophosphine)-deca(carbonyl)-tri-osmium(0) (3). Compounds 1 and 2 are the first aliphatic mono-fluorophosphine osmium clusters and were characterized by an X-ray diffraction study. Both compounds show a regular three-membered osmium cycle with equatorially oriented phosphorous atoms. Compound is 3 is the first example of a mixed mono-fluorophosphine-di-fluorophosphine compound and has been characterized by NMR spectra, a mass spectrum and an IR spectrum.