The reaction of high-valent fac-{(99m)TcO3}(+) complexes with alkenes ((3 + 2)-cycloaddition) has been used to develop a novel labelling procedure for silica based particles. This strategy can help to gain more detailed in vivo data of mesoporous silica by non-invasive radioimaging in the future. By applying norbornene as a coupling unit the (3 + 2)-cycloaddition reaction proceeds stereoselectively and with high reaction rates.