Nitrogen/sulfur ligands

Nitrogen-Sulfur Ligands. Many types of these are known for transition metals. Two examples are the heterocyclic dianion214 Ph4P2N4S2 and the sulfenamido anion RNSR the latter forms compounds such as Zr(i72-Bu NSPh)4 by interaction of ZrCl4 with Bu N(Li)SPh.215... 

Chemical Properties. Although the chemical properties of the trivalent lanthanides are quite similar, some differences occur as a consequence of the lanthanide contraction (see Table 3). The chemical properties of yttrium are very similar too, on account of its external electronic stmcture and ionic radius. Yttrium and the lanthanides are typical hard acids, and bind preferably with hard bases such as oxygen-based ligands. Nevertheless they also bind with soft bases, typicaUy sulfur and nitrogen-based ligands in the absence of hard base ligands. 

Although a large number of such compounds have been prepared, many of the mixed nitrogen-sulfur macrocycles are included in other tables, most notably in Chap. 4. Some have also been recorded in Chap. 8 as monocyclic precursors to bi- or polycyclic ligands (i.e., cryptands). The reader is directed to the tables of other chapters where the desired compound, if known, may be reported. 

There are hundreds of iron-containing enzymes. In general, the iron can exist as (a) a mononuclear site, in which it is coordinated by a tetrapyrrole structure (hemes) or strictly by amino acid residues that donate oxo, nitrogen, or sulfur ligands (b) a dinuclear site in which the irons are bridged by oxo, nitrogen, or sulfur coordination (c) a trinuclear site as in the 3Fe-4S clusters or (d) a tetranuclear site as in the [4Fe-4S] clusters. 

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