[RuCl(SiMe3)(CO)(PPh3)(2)] (1) was found to catalyze dimerization of aldehydes RC(O)H yielding esters RC(O)-OCH2R(3) in a Tishchenko type reaction. Aldehydes with alpha-hydrogen atoms (R = Me, Et, iPr, iBu) reacted quantitatively within 2 days (5 mol% catalyst, 70degreesC in C6D6) whereas those without alpha-hydrogen atoms (R = tBu, Ph) did not react at all. In the case of benzaldehyde (R = Ph), well shaped crystals of [RuCl(O2CPh)(CO)(PPh3)(2)] (4) precipitated from the reaction mixture. Molecular structures of catalyst 1 and benzoato complex 4 were established by single-crystal X-ray diffraction analyses exhibiting square pyramidal (1) and octahedral (4) complex, respectively. Using 1:1 mixture of acetaldehyde and propanal a crossed esterification reaction took place forming esters RC(O)-OCH2R' (3, R/R' = Me/Me, Et/Et; 5. R/R' = Me/Et, Et/Me). In the case of tBuC(O)H a crossed esterification reaction with RC(O)H (R = Me, Et) took place yielding only two products. namely the symmetrical esters RC(O)-OCH2R (3, R = Me, Et) and the crossed products of the type RC(O)-OCH(2)tBu (6, R = Me, Et). As shown with propanal, ester formation is completely suppressed by water. Because ruthenium catalyst I was prepared by reaction of [RuCl(H)(CO)(PPh3)(3)] (2) with H2C-CHSiMe3, complex 2 was also used as catalyst in the reaction with aldehydes. It proved to be non-active, but in the presence of H2C-CHSiMe3 (H2C-CHSiMe3:RC(O)H = 1:1) catalytic formation of cyclotrimers of aldehydes (RCHO)(3) (7) and aldol condensation products RCH-CR'-C(O)H (8, R = Et, iBu; R' = R -CH2) was observed. (C) 2004 Elsevier B.V. All rights reserved.