Reaction of the highly basic, sterically hindered tertiary phosphine ligand tris(2,4,6-trimethoxyphenyl)phosphine (TMPP) with copper(I) iodide in acetonitrile yields 1:1 complexes whose structures depend on the mole ratio of the reactants. The crystals which first deposit from the reaction of TMPP with CuI in a 1:2 ratio are the 1:1 molecular adduct [(TMPP)CuI], whose structure was determined by room-temperature single-crystal X-ray studies. This complex is isomorphous with the corresponding Cl and Br compounds and their Ag analogues, crystallizing in the tetragonal space group P4(3), with a = 15.292(7) angstrom, c = 12.748(5) angstrom, Z = 4, and R = 0.044 for N(o) = 2027 "observed" (I > 3 sigma(I) reflections. The complex contains mononuclear [(TMPP)CuI] molecules with approximately linear two-coordination about the copper atom; Cu-P = 2.188(4) angstrom, Cu-I = 2.417(2) angstrom, P-Cu-I = 171.0(1)-degree. The slight deviation from linearity presumably arises because of contacts with the methoxy oxygen atoms; the Cu---O contacts involving the nearest ortho-methoxy oxygen atoms on the three phenyl groups in the ligand are 3.07(1), 2.858(7), and 2.674(9) angstrom. This represents the first structural characterization of a compound with a two-coordinate P-Cu-I grouping, and the first example of a two-coordinate complex of CuI with a neutral ligand. The far-infrared spectrum shows a nu(Cu-I) band at 239 cm-1. Reaction of TMPP with CuI in a 1:1 mole ratio in acetonitrile yields a different product of 1:1 stoichiometry which is shown by means of solid state P-31 NMR and IR spectroscopy to be an ionic complex of the type [(TMPP)2Cu]n[Cu(n)I2n].