Sulfur heavy” donor atom

Based on donor atom type, macrocyclic ligands can be considered to span two extreme types. First there are those systems which chiefly contain nitrogen, sulfur, phosphorus, and/or arsenic donors. These macrocycles tend to have considerable affinity for transition and other heavy metal ions they usually show much less tendency to form stable complexes with ions of the alkali and alkaline earth metals. The present discussion will be restricted to a consideration of a selection of such ligands and their complexes. 

Heavy metal ions such as Hg(II), Pb(II), or Ag(I) hydrolyze thiol esters quite easily with the formation of a metal mercaptide as a reaction product (139, 174, 192). Although in many cases metal chelates can be postulated as intermediates in these hydrolysis reactions, there is some evidence to show that the bond formed by the coordination of the sulfur atom with a heavy metal ion is sufficiently strong to initiate the cleavage of the thiol esters. For example, simple thiol esters having no other donor atoms and therefore incapable of forming metal chelates will readily undergo hydrolysis in the presence of Hg(II) (24). 

Since sulfur is known to bond as a donor atom to vanadium as well as to nickel,it should be anticipated that dehydrosulfurization proceeds proportional to metal removal. It has been demonstrated that in reality this turns out to be so. This is an important aspect in the processing of heavy oils, but a detailed discussion is not in the scope of this paper. 

Three types of interaction between ligand and protein heavy atoms arising from different atom types. Primary and secondary amines are defined to be donors while oxygen and nitrogen atoms with no bound hydrogens are defined to be acceptors. Crystallographic water molecules and hydroxyl groups are defined to be both donor and acceptor. Carbon and sulfur atoms are defined to be nonpolar... 

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