Sulfide, coordination modes

The closo-tetraphosphorus sulfides, with their various stoichiometries and unique structures (Scheme 2), exhibit multiple and often distinct potential coordination sites. These sites often involve several distinct sets of formally non-bonding electron pairs at both phosphorus and sulfur atoms, and the tetraphosphorus sulfides could therefore be expected to exhibit a wide range of coordination modes and original behavior towards Lewis acids. But so far this has not proved so. 

Complexes of sulfide anions are widely found their coordination modes, adapted from Ref. [1], are as follows ... 

Polysulfides of several metals can be prepared by reaction of the metals with excess sulfur in liquid NH3 (group IA metals) or by heating sulfur with the molten metal sulfide. The polysulfide ion binds to metals to form coordination compounds in which it is attached to the metal by both sulfur atoms (as a so-called bidentate ligand). One example is an unusual titanium complex containing the S52-ion that is produced by the following reaction (the use of h to denote the bonding mode of the cyclo-pentadienyl ion is explained in Chapter 16) ... 

Ligands that can coordinate to an active center in an enzyme and prevent coordination by the substrate will tend to inhibit the action of that enzyme. 1 We have seen that azide can occupy the pocket tailored to fit the carbon dioxide molecule. This prevents the latter from approaching the active site. Furthermore, the infrared evidence indicates that the azide ion actually does bind the zinc atom The asymmetric stretching mode of the azide ion is strongly shifted with respect to the free ion absorption. Thus the zinc is inhibited from acting as a Lewis acid towards water with the formation of a coordinated hydroxide ion. Other inhibitors also bind to the metal atom. As little as 4 x I0-6 M cyanide or hydrogen sulfide inhibits the enzymatic activity by 85%. 

In the 7] -(S)-coordinated thiophenes, the sulfur appears to be sp -hybridized the metal does not lie in the plane of the thiophene. They are all weakly coordinating compared to THT (a dialkyl sulfide type of molecule) consequently, such complexes can be isolated only in some special cases. The tendency to coordinate to metal ions in the 77-(S) mode generally increases in the order T < BT < DBT. (As per the normal convention, the unstarred symbols T, BT, and DBT refer to the unsubstituted heterocycles thiophene, benzo[. ]thiophene, and diben-zo[i, . thiophene, respectively, while a star indicates the inclusion of substituted thiophenes as well. Cp and Cp refer to the 77 - bound CsHs and CsMcs, respectively.)... 

From the reaction of Au with alkali metal polysulfide liquids, LiAuS and NaAuS were discovered. Both compounds contain infinite one-dimensional (AuS) chains, featuring alternating sulfide anions and linear coordinated Au centers. In LiAu8, the chains are zigzag and fully extended and they pack in mutually perpendicular sets. In NaAuS, the same chains coil in an unusual fashion so that they become interwoven to form layers reminiscent of chicken-wire (Figure 17). This novel coiling mode allows Au Au contacts to form, which help to stabilize the structure. 

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