21 46 Pd The Use of Palladium Complexes in Medicine Achilleas Garoufis, Sotiris K. Hadjikakou and Nick Hadjiliadis 21.1 Introduction Palladium with atomic number 46 is a member of the Platinum-group metals lying above platinum and below nickel and belongs to the 2nd transition metal series. It has a very similar chemistry to that of platinum. The most stable oxidation states for both metals are the þ2 and þ4 states. In their most common oxidation state (þ2), both metals have a d 8 configuration and con- stantly form square planar diamagnetic complexes with various ligands of dsp 2 hybridization. More rarely, five coordinate complexes of trigonal bipyramidal structures are also formed. Both metals are characterized as soft acids, forming stronger bonds with nitrogen or sulfur donors than oxygen donors. Pd(II) complexes are generally less stable than Pt(II) both thermodynamically and kinetically. The use of palladium and its complexes in medicine is limited, compared to other metals. 103 Pd has been used as a radioactive isotope in the treatment of rapidly growing, high-grade prostate cancer. 1,2 The discovery of the anti- tumor properties of cis-platin (cis-Pt(NH 3 ) 2 Cl 2 ) and related complexes raised Metallotherapeutic Drugs and Metal-Based Diagnostic Agents: The Use of Metals in Medicine Edited by Gielen and Tiekink. Copyright  2005 John Wiley & Sons, Ltd. ISBN: 0-470-86403-6