[en] Tc(S₂CNEt₂)₃CO has been prepared by the reduction of NH₄TcO₄ with formamidinesulphinic acid in the presence of NaS₂CNEt₂. It is suggested that the co-ordinated carbon monoxide is formed after co-ordination of formamidinesulphinic acid, or some decomposition product, with technetium. The crystal structure of Tc(S₂CNEt₂) ₃CO has been determined by single-crystal X-ray diffraction methods at 17 deg. C. Diffractometry has provided significant Bragg intensities for 2045 independent reflections and the structure has been refined by full-matrix least-squares methods to R 0.049. The compound is isostructural with the rhenium analogue and consists of discrete Tc(S₂CNEt₂)₃CO molecules, each containing a terminal linear CO group. The technetium atom has a seven co-ordinate environment which is best described as a distorted pentagonal bipyramid