The present invention relates to the use of a transition metal catalyst TMC1, which comprises a transition metal M selected from metals of groups 7, 8, 9 and 10 of the periodic table of elements according to IUPAC and a tetradentate ligand of formula I wherein R1 are identical or different and are each an organic radical having from 1 to 40 carbon atoms, and R2 are identical or different and are each an organic radical having from 1 to 40 carbon atoms, as catalyst in processes for formation of compounds comprising at least one carboxylic acid ester functional group —O—C(═O)— starting from at least one primary alcohol and/or hydrogenation of compounds comprising at least one carboxylic acid ester functional group —O—C(═O)—. The present invention further relates to a process for hydrogenation of a compound comprising at least one carboxylic acid ester functional group —O—C(═O)—, to a process for the formation of a compound comprising at least one carboxylic acid ester functional group —O—C(═O)— by dehydrogenase coupling of at least one primary alcohol with a second alcoholic OH-group, to a transition metal complex comprising the tetradentate ligand of formula I and to a process for preparing said transition metal complex.