Starting from [MO 4] − (M = Tc, Re) the low-pressure (1 atm.) synthesis of the important starting material (NEt 4) 2[MCl 3(CO) 3] could be improved in terms of yield and time. These M I complexes were used for the preparation of compounds containing the “ fac-M(CO) 3” moiety under ambient conditions. The substitution with the isocyanide ligand CN-Bu t is described. In the case of rhenium, IR spectroscopical investigations in the CO stretching region allowed the observation of the stepwise formation of [Re(CN-Bu t) 3(CO) 3] +. It could be demonstrated that 99Tc NMR spectroscopy allows similar investigations, although not all types of intermediates could be detected. The structure of the complex [Tc(CN-Bu t) 3(CO) 3](NO 3) was elucidated by X-ray diffraction analysis. Substitution with the protic ligand 2-mercaptoethanol HSCH 2CH 2OH revealed similar behaviour. Two intermediates were observed with IR spectroscopical methods. The final product was the dimer (NEt 4)[Tc 2(μ-SCH 2CH 2OH) 3(CO) 6], the structure of which was determined by X-ray diffraction.