The mechanism by which copper(I) influences the decarboxylation of cyanoacetic acid has been studied comprehensively by means of structural and kinetic investigations. The copper(I) complexes, [(R(3)P)(2)CuO2CCH2CN](1,2), have been synthesized from the reaction of copper(I) n-butyrate with 1 equiv of cyanoacetic acid and 2 equiv of phosphine. In the case of R = Ph, the complex is shown to be a dimer, both in solution and in the solid state, consisting of two copper(I) centers bridged by two cyanoacetate groups that are bound to copper through both the carboxylate functionality and the nitrogen. On the other hand, for the sterically encumbered phosphine (R = Cy), the complex (3) is found by X-ray crystallography to be monomeric and to contain a monodentate carboxylate group. The monodentate nature of the cyanoacetate binding was demonstrated to be a function of the electron-withdrawing ability of the cyanoacetate ligand as revealed by an examination of the solid-state structure of the (Cy(3)P)(2)Cu(butyrate) (4) analog, where the more basic butyrate ligand was shown to be bound in a bidentate manner. Both phosphine derivatives of copper(I) cyanoacetate were observed to readily undergo reversible decarboxylation/carboxylation processes as evidenced by their exchange reactions with (CO2)-C-13. A similar, much slower, exchange reaction with C-13-labeled CO2 was noted for the [PPN][O2CCH2CN] and eta(3)-HB(3-PhPz)(3)Zn(O2CCH2CN) (5) salts. These (CO2)-C-13 exchange processes were found to be first-order in the respective substrate, with the Cy(3)P derivative undergoing more rapid exchange than the Ph(3)P complex. Furthermore, the phosphine derivatives of copper(I) cyanoacetate were efficient catalysts for the decarboxylation of cyanoacetic acid to afford CH3CN and CO2 at rates quite similar to the CO2 exchange process. These reactions were first-order in copper(I) complexes and zero-order in cyanoacetic acid concentrations below 0.05 M. At higher acid concentrations the reaction was inhibited by cyanoacetic acid due to its complexation with copper(I).