The reaction between CpNbCl(4) and the phosphine ligands PMe(x)Ph(3-x) (x = 0, 1, 2, 3) has been investigated. While no reaction occurs with PPh(3), stable adducts are formed with all the other ligands in the above series. These have been isolated and characterized by elemental analysis, H-1 and P-31-NMR spectroscopy, and an X-ray structural determination for the PMePh(2) adduct. The three phosphines PMe(3), PMe(2)Ph, and PMePh(2), however, also engage in a subsequent reduction process with formation of Cl radicals and Nb(IV) complexes. CpNbCl(3)(PMe(3))(2) has been independently synthesized from CpNbCl(4), Na, and PMe(3) in a 1:1:2 ratio and fully characterized. The relative rate of reduction depends on the nature of the phosphine, increasing in the order PMePh(2) < PMe(2)Ph < PMe(3). X-ray data for CpNbCl(4)(PMePh(2)) : monoclinic, C2/c, a = 23.818(2) Angstrom, b = 12.4719(10) Angstrom, c = 15.8137(12) Angstrom, beta = 106.365(7)degrees, V = 4507.2(6) Angstrom(3), Z = 8, D-x = 1.609 g . cm(-3), mu(Mo K alpha) = 0.603 mm(-1), R = 0.0349 for 329 parameters and 3989 data with I > 2 sigma(I). The structure shows a distorted pseudooctahedral geometry with the Cp and PMePh(2) ligands occupying two axial coordination sites and the four equatorial Nb-Cl bonds being bent away from the Cp ring.