Dimethyl maleate adds to the Pt-OH bonds of cis-Pt(OH)(Me)(PPh(3))(2) (2), Pt(OH)(Me)(dppe) (3), and Pr(OH)(Me)(pdmp) (5) [dppe = 1,2-bis(diphenylphosphino)ethane, Ph(2)PCH(2)CH(2)PPh(2); pdmp = o-phenylenebis-(dimethylphosphine), o-C6H4(PMe(2))(2)] to give cis-PtMe{CH(CO(2)Me)CH(OH)(CO(2)Me)}L(2) [L(2) = 2PPh(3) (6), dppe (7), pdmp (8)]. Crystals of 6 are monoclinic, space group P2(1)/n, with a = 12.858(2) Angstrom, b = 21.688(4) Angstrom, c = 14.340(2) Angstrom, beta = 97.25(1)degrees, and Z = 4. The structure was solved by the heavy atom method and refined by least-squares analysis to R = 0.027 and R(w) = 0.032 based on 4508 unique observed reflections. The platinum atom is planar-coordinated by two cis-PPh(3) ligands, a methyl group [r(Pt-C) = 2.111(6) Angstrom], and a CH(CO(2)Me)CH(OH)(CO(2)Me) group [r(Pt-C) = 2.152(7) Angstrom]. The threo-(R*,R*)-configuration of the PtCH(CO(2)Me)CH(OH)(CO(2)Me) moiety indicates that dimethyl maleate undergoes syn addition to the Pt-OH bond of 2, probably by cis-migratory insertion into the Pt-OH bond of a five-coordinate intermediate Pt(OH)(Me)(PPh(3))(2)(eta(2)-(Z)-MeO(2)CCH=CHCO(2)Me). Dimethyl maleate does not insert into the Pt-OH bonds of trans-Pt(OH)(Ph)(PEt(3))(2) (1) or Pt(OH)(CF3)(dppe) (4), and dimethyl fumarate is inert toward 1, 2, and 4, but it reacts with 5 to give the monomethyl fumarato complex PtMe{(E)-OC(O)CH=CHCO(2)Me}(pdmp) (9). Treatment of 6 or 7 with aqueous acids cleaves their Pt-CH(CO(2)Me)CH(OH)(CO(2)Me) bonds forming [PtMe(H2O)L(2)](+) and dimethyl malate, MeO(2)CCH(2)CH(OH)CO(2)Me, thus demonstrating two steps necessary for catalysis of addition of water to dimethyl maleate.