Technetium tetracarbonyl complexes [TcBr(CO)₄]₂ and [TcXan(CO)₄] (Xan is methylxanthate), being fairly stable in an inert solvent, undergo rapid decarbonylation in a donor solvent (acetonitrile). The kinetic characteristics of the reaction were determined. The experimental data and results of quantum-chemical calculations allow a conclusion that the reaction, as in the case of pentacarbonyl halides, occurs via dissociative pathway with a lower energy barrier as compared to pentacarbonyl halides. The results obtained were interpreted assuming the formation of unstable complexes of intermediate five-coordinate technetium species with an "inert" solvent molecule and the effect of this phenomenon on the relative stability of the technetium penta- and tetracarbonyl complexes. The technetium complex [TcXan(CO)₄] is considerably less stable than its rhenium analog.