Abstract: The crystal structure of guanidinlum pentamolybdobis(n-ainylphcsphonate), which contains the longest carbon chain hitherto known, was determined by single-crystal X-ray diffraction analysis. It belongs to monoclinic, space group C2/c, Z=4, 0=18.857(2), b=12.170(1), c=18.294(2)λ.β=102.73(2), V=4095.2λ³.u= 140.34cm^-1. The intensity data were collected on an Enraf-Nonius CAD4 diffractometer with MoKa radiation. The positions of all molybdenum and phosphorus atoms were determined by the direct method. The other non-hydrogen atoms were revealed by difference Fourier synthesis. The structure was refined by full-matrix least-squares procedure to a final Rvalue of 0.068. The structure of the anion is similar to [(CH₂P)₂Mo₅O₂₁]^4-, [(NH₃C₂H₄P)₂Mo₆O₂₁]^2-, [(n-C₃H₇As)₂Mo₅O₂₁]^4-, and [(C₃H₅As)₂Mo₆O₂₁]^4-. It consists of a ring of five distorted MoO, octahedra joined by edge-sharing, except for one pair which is joined by corner- sharing. The size of the Mo₆O₁₆ ring is close to those of [(CH₃P)₂Mo₅O₂₁]^4- and [(NH₂C₂H₄P)₂Mo₅O₂₁]^2- and much smaller than those of [(n-C₂H₇As)₂Mo₅O₂₁]^4-and [(C₂H₆As)₂Mo₆O₂₁]^4-. The symmetry of the anion, however, to the same at those of its arsenic con-seners [(n- C₂H₇As )₂Mo₅O₂₁]^4- and [(C₃H₅- As )₂Mo₅O₂₁]^4-, but different from those of [(CH₃P)₂Mo₅O₂₁]^4-and [(NH₃C₂H₄P)₂Mo₅O₂₁]^2-. The average bond angles O-P-Owere estimated on the basis of assuiming that anion [(RP)₂Mo₅O₂₄₄-could be formed.

Key words: Crystal structure, Guanidinium pentamolybdobis(z-amylphosphonate), molybdenum