X-Ray Photoelectron Spectra of Inorganic Molecules. 18. Observations on Sulfur 2p Binding Energies in Transition Metal Complexes of Sulfur-Containing Ligands

The S 2p binding energies of a series of nickel(II), palladium(II), and platinum(II) complexes of 1,2-ethanedithiol and benzenethiol occur between 163.5 and 161.5 eV. A comparison between data for the square-planar sulfur-bridged polymers [Ni(SCH₂CH₂S)]_n and [M(SPh)₂]_n, where M = Ni, Pd, or Pt, and square-planar monomers of the type Ni(SCH₂CH₂S)L₂ and M(SPh)₂L₂, where L = C₆H₁₁NC, PMe₂Ph, or 1/2 dppe, shows that the bridging sulfur atoms usually have S 2p binding energies which are higher than those of sulfur atoms in terminal thiol ligands (i.e., R-S_b^- ≳ R-S_t^-). For the ligand methionine, which contains a thioether moiety, coordination of the sulfur atom to platinum(II) causes a S 2p chemical shift of +1.2eV. However, in methionine complexes of cobalt(II), nickel(II), copper(II), and zinc(II), in which the sulfur atoms are not complexed, the S 2p binding energies (S 2p_3/2 = 163.0 eV) are unchanged from that of the free ligand. Related data for molecules such as [Fe(SCH₃)(CO)₃]₂, [Fe(SC₂H₅)(NO)₂]₂ and a variety of dithiocarbamate- and dithiene-metal complexes have also been obtained. These results are discussed in the light of some recent XPS results for several sulfur-containing metalloproteins.