The title complex (I) has been isolated after running the hydro-methoxycarbonylation (HMC) of ethene (4.5 MPa Of CO/C2H4 = 1/1, 343 K) in MeOH, catalyzed by [Pd(TSO)(2)(PPh3)(2)]. It has been characterized by IR, H-1 and P-31 NMR spectroscopy. Complex (I) reacts with MeOH, saturated with CO, even at r.t., yielding methylpropanoate (NIP) in stoichiometric amount and Pd(0) complexes and/or trans- [Pd(COOCH3)(TsO)(PPh3)(2)] (II), the latter forms in the presence of PPh3 and of p-toluenesulfonic acid (TsOH); complex (I) catalyses the HMC of ethene to NIP; it catalyses also the HNIC of a different olefin yielding also a stoichiometric amount of MP. After catalysis. complex (I) is recovered as such or as [Pd(TSO)(2)(PPh3)(2)] (III), the latter forming when an excess of TsOH is used (Pd/TsOH = 1/8). Complex (II), a potential catalytic intermediate, has been prepared under conditions similar to those employed to synthesize complex (I), except for the presence of ethene. This complex, dissolved in MeOH saturated with C2H4, does not yield MP at r.t., whilst at 353 K, it becomes a catalyst precursor for the HMC of ethene, however, it is recovered as complex (I). The conversion of (II) to (I) occurs with CO2 evolution. For the conversion of (I) to (II), it is proposed that: (i) (I) reacts with MeOH yielding NIP and a Pd(II)-hydride; (ii) this reacts with TsOH with hydrogen evolution and yielding complex (III); (iii) this reacts with CO and MeOH yielding (II). For the conversion of (II) to (I) it is proposed that: (i) (II) reacts with H2O yielding MeOH and a Pd-COOH species; (ii) this evolves CO2 with formation of a Pd(II)-hydride; (iii) sequential addition of ethene and CO gives (I). In addition, it has also been found that catalysis is accompanied by formation of CO, also when using complex (I) as catalyst and that the catalytic activity passes through a maximum with increasing the concentration of water (TOF = 420 h(-1) at 353 K, 4.5 MPa CO/C2H4 = 1/1, (1)/PPh3/TsOH = 1/6/8t H2O = 800 ppm). It is proposed that: (i) catalysis occurs through initial formation of a Pd(II)-H species (which form after CO, evolution from a Pd-(COOH) species formed via interaction of H2O with CO), followed by the. insertion of the olefin into the Pd(H)-H bond to form a Pd(II)-(alkyl) intermediate, which in turn inserts CO with formation of an acyl complex of type (I), which reacts with MeOH yielding the ester and Pd(II)-H back to the catalytic cycle; (ii) a carbomethoxy complex of type (II) does not play a major direct role in the catalytic cycle; (iii) during the catalysis Pd(H)-H consuming side reactions occur with formation of Pd(II) species of type (II) and/or (III); these species are reintroduced, as hydrides, back to the catalytic cycle via interaction with H2O and CO. (C) 2004 Elsevier B.V. All rights reserved.