In order to elucidate the preferred mode of coordination-eta(1)(sigma) or eta(6)(pi)-we have reacted phosphinine (C5H5P) with the open ferrocene (2,4-Me(2)-eta(5)-C5H5)(2)Fe. The product, pentakis(eta(1)-phosphinine)iron (1), was subjected to X-ray structural analysis : 1 crystallizes in the space group P2/n with the cell parameters a = 16.010(3) A, b = 9.727(2) Angstrom, c = 16.193(3) Angstrom, alpha = 90 degrees, beta = 102.11(3)(o), gamma = 90 degrees, Z = 4, R = 0.056, R(w) = 0.049. The structure of 1 is trigonal bipyramidal with a slight distortion toward square pyramidal. The bond distance Fe-P (2.155 Angstrom) is probably influenced by interligand repulsion. Cyclic voltammetry reveals that oxidation and reduction of 1 are irreversible even at-45 degrees C. The likely intermediate in the formation of 1, (2,4,7,9-tetramethyl-eta(4):eta(2)-decatetraene)bis(eta(1)-phosphinine)-iron (5) was also isolated and characterized by H-1, C-13, and P-31 NMR spectroscopy.