Nowadays several metal compounds are widely used in medicine as therapeutic and diagnostic agents. Due to their versatile biochemical properties, ruthenium-based compounds have shown to be promising anti-cancer agents as alternative to cisplatin and its derivatives. [1] The aim of our work is to investigate the effects of new ruthenium(II) complexes of general formula [RuX(CO)(dppb)(phen)]X [X = acetate, pivalate, thioacetate; dppb = 1,4- bis(diphenylphosphino)butane; phen = 1,10-phenanthroline] [2] on two different types of anaplastic thyroid cancer cells, focusing our attention on the nature of the X group. Interestingly, these compounds are able to reach very low IC50 values after 72 h of incubation. In addition, the introduction of a chiral diphosphine, such as (R,R)- and (S,S)-Skewphos, leads to single enantiomers which show a marked difference in in vitro biological activity.