Binding of a calix[5] arene containing a single phosphorus ligand and three hydroxyl groups, calix[5] PNMe2(OH) 3, 1, toward titanium and zirconium is investigated to yield insight into the factors that determine the strength of the phosphorus/metal interaction within the constraint of the calix[5] arene. Treatment of 1 with tetrakis(dimethylamino)titanium yields three complexes, 4a, 4b, and 4c, each of which shows the loss of 3 mol of dimethylamine in the reaction with the titanium bound to three oxygens. Treatment of 1 with tetrakis(diethylamino) zirconium proceeds similarly, although only two products, 5a and 5b, were isolated. X-ray structures of the products were obtained. Complexes 4a and 5a show similar geometries, with the calix[5] arene in an approximate cone conformation and the phosphorus lone pair directed toward the metal. The P center dot center dot center dot M distances are, however, markedly different: 3.69 angstrom in 4a and 3.18 angstrom in 5a, the former indicative of no interaction and the latter a weak one. Complexes 4b and 5b each are dimers, featuring a planar four-membered M-O-M-O ring; however, the titanium is five-coordinate in 4b with no phosphorus/metal bond, while the zirconium in 5b is six-coordinate with a P-Zr distance of 2.95 angstrom. Complex 4c is monomeric with the calix[5] arene in an approximate 1,2-alternate conformation with a P center dot center dot center dot Ti distance of 2.90 angstrom. The two most significant aspects controlling the phosphorus/ metal contact are the metal, with the larger zirconium showing stronger interaction, and the calix[5] arene conformation, with the cone conformation showing the weaker interaction.