The synthesis, isolation, spectroscopic characterization (IR, multi-nuclear NMR) and single-crystal x-ray diffraction anal. of FSi(PH2)3 (1a), the 1st isolable fluorophosphinylsilane, is reported along with the gas phase decompn. of 1a, MeSi(PH2)3 (1b) and EtSi(PH2)3 (1c) under flash vacuum or pulsed pyrolysis conditions and matrix isolation of the products. The title compd. is formed quant. by PH2/F-ligand exchange reaction of tetraphosphinylsilane Si(PH2)4 with the difluorodiarylstannane Is2SnF2 (Is = 2,4,6-triisopropylphenyl) in the molar ratio of 1:1 in benzene as solvent. Since 1a cannot be sepd. from the solvent by fractional condensation its isolation was achieved by preparative GC. A single crystal of 1a (triclinic, P.hivin.1) suitable for x-ray diffraction anal. was grown by in situ crystn. on a diffractometer at 175 K through miniaturized zone melting with focused IR radiation. The Si atom is remarkably distorted tetrahedrally coordinated with F-Si-P angles of 120.4(7), 110.4(7), 106.3(1) Deg and normal Si-F (1.60(2) .ANG.) and Si-P distances (av. 2.241(2) .ANG.). According to ab initio (MP2/6-31G(d,p); MP2/6-311G(2d,p)) and DFT calcns. (BLYP, B3LYP, B3PW91 functionals), the distortion is not an intrinsic property of the mol. but due to crystal packing forces. The best agreement between the exptl. vs. calcd. geometrical and vibrational data is achieved at the B3PW91/6-311G(2d,p) level of theory. Since 1a-c appeared as potential precursors for the resp. silylidyne-phosphines ('silaphospha-acetylene') RSiP through stepwise extrusion of PH3, some thermodn. data for the decompn. and the relative energies of linear RSiP vs. bent :SiPR isomers (R = H, Me, Et, Pr, Ph, CF3, OMe, halogen and SiH3) were also calcd. The latter revealed that electroneg. substituents R favor the Si-P triple bond in RSiP (except for R = CF3 which stabilizes the :SiPR form) while strong s-donating substituents R (H, SiH3) favor the :SiPR isomer with Si:P double bond. Although elimination of PH3 and other fragmentation products could be detected by controlled thermal decompn. and matrix isolation, neither flash vacuum expts. of 1a, 1b and 1c (400-600 Deg) nor pulsed pyrolyzes of 1a at (1100 Deg) provided any direct evidence for the formation of the desired species with Si-P multiple bonds.