The mechanism by which Cu(I) and Zn(II) influence the decarboxylation of malonic acid derivatives has been investigated comprehensively by means of structural and kinetic studies. The air-sensitive bis(phosphine)copper(I) complexes of phenylmalonic acid and its benzyl hemiester have been synthesized from 1 equiv of the acid and cuprous butyrate in the presence of phosphine ligands. As revealed by structural studies the copper(I) center is three-coordinate containing two phosphine ligands and a monodentately bound carboxylate group. On the other hand, soluble zinc(II) phenylmalonic acid and ester derivatives were prepared from (HB(3-Phpz)(3))ZnO2CCH3, HB(3-Phpz)(3) = tris(3-phenylpyrazolyl)hydroborate, and the corresponding malonic acid or hemiester. The Zn(II) complexes were all shown by X-ray crystallography to be four-coordinate with monodentately bound carboxylates. Kinetic evidence is presented herein which demonstrates that decarboxylation of malonic acid or hemiester derivatives in the presence of bis(phosphine)copper(I) or eta(3)-HB(3-Phpz)(3)Zn-II carboxylate salts occurs via a predissociation step involving metal-carboxylate bond rupture. Consistent with this mechanistic proposal, the rates of decarboxylation are greatly enhanced upon sequestering the metal cations with chelating nitrogen bases or upon replacing the Cu(I) or Zn(II) cations with a noninteracting counterion. Furthermore, because of the stronger Zn-O carboxylate bonds as compared with their Cu(I)-O analogs, the zinc carboxylates are more stable toward decarboxylation or (CO2)-C-13 exchange processes. Indeed, the rates of these processes parallel the Zn-O bond lengths with the pyrazolylhydroborato complex, which has a shorter Zn-O bond distance than its triazacyclododecane analog, undergoing decarboxylation at a much slower rate. Crystal data for (Ph(3)P)(3)CuO2CC(C2H5)(C6H5)CO2H (1) : triclinic space group , a = 11.631(5) Angstrom, b = 12.206(4) Angstrom, c = 20.000(6) Angstrom, alpha = 85.91(3)degrees, beta = 89.61(3)degrees, gamma = 71.24(3)degrees, Z = 2, R = 4.20%. Crystal data for eta(3)-HB(3-Phpz)(3)ZnO2CCH2COOH (2) : triclinic space group , a = 11.935(3) Angstrom, b = 12.227(4) Angstrom, c = 12.643(5) Angstrom, alpha = 77.56(3)degrees, beta = 72.18(3)degrees, gamma = 73.21(2)degrees, Z = 2, R = 7.00%. Crystal data for eta(3)-HB(3-Phpz)(3)ZnO(2)CCH(2)Ph : triclinic space group , a = 9.884((2) Angstrom, b = 12.189(3) Angstrom, c = 15.482(2) Angstrom, alpha = 105.80(1)degrees, beta = 92.46(1)degrees, gamma = 90.94(2)degrees, Z = 2, R = 8.21%.