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The S/Se-containing bidentate ligands LH of the type R2P(E)NHP(E)R'2, E = S, Se and R, R' = Ph or iPr have been employed to synthesize ML2 (M = Mn, Co) complexes which contain the biologically important MS4 core. Theoretical calculations on the LH and L- forms of the ligands probe the geometric and electronic changes induced by the deprotonation of the LH form, which are correlated with structural data from X-ray crystallography. These results reflect the flexibility of the ligands, which enables them to be rather versatile with respect to the formation of ML2 complexes with varied geometries and MEPNPE metallacycle conformations. A series of old and new ML2 complexes have been synthesized and their structural, spectroscopic and magnetic properties characterized in detail. The nephelauxetic ratio beta of the CoL2 complexes provides evidence of covalent interactions, whereas the EPR properties of the MnL2 complexes are interpreted on the basis of predominant ionic interactions, between the metal center and the ligands, respectively. Additional evidence for the existence of covalent interactions in the CoL2 complexes (R = Ph, iPr, or mixed Ph/iPr), is offered by comparisons between their 31P NMR. The aforementioned notations are supported by extensive theoretical calculations on the ML2 (E = S, R = Me) modelled structures, which probe the covalent and ionic character of the M-S bonds when M = Co or Mn. Wider implications of the findings of the present study on the M-S covalency and its importance in the active sites of various metalloenzymes are also discussed.

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The reactions of Pd(II) with inosine (InoH) and guanosine (GuoH) have been studied in aqueous solutions. The complexes isolated from these reactions have the following formulas: cisand trans-[Pd(NuclH)2Cl2], cisand Zrons-[Pd(Nucl)2], cisand trozis-[Pd(InoH)2(NuclH)2]Cl2, cisand irons-[Pd(GuoH) 2(NuclH)2]Cl2, cis-[Pd(H2G)2(GuoH)2]Cl2, cis-[Pd(GuoH)?(Guo)]Cl, ds-[Pd(GuoH)2(Ino)]Cl, [Pd(GuoH)3Cl]Cl, and [Pd(GuoH)2(InoH)Cl]Cl, where NuclH = inosine and guanosine. The compounds have been identified by elemental analyses, conductivity measurements, and IR and NMR spectra. In this case, i.e., with palladium, both cis and trans isomers have been isolated and characterized, in contrast to the platinum analogues. The diaquo cis-[Pd(H20)2(GuoH)2]Cl2 complex and the trinucleosides, [Pd(GuoH)3Cl]Cl and [Pd(GuoH)2(InoH)Cl] Cl, are ionic and novel in this series of complexes. The trans effect of nucleosides follows the order Nucl < Cl™, Br". N706 chelation has been found here, comparable to that proposed for the analogous platinum complexes. This N706 chelation is discussed with relation to the mode of action and the antitumor activity of ris-[Pt(NH3)2Cl2].