Sulfur fluxionality

Monomeric sulfur diimides have an extensive coordination chemistry, as might be anticipated from the availability of three potential donor sites and two re-bonds.131 Under mild conditions with suitable coordinatively unsaturated metal complexes, sulfur diimides may coordinate without rupture of the -N=S=N- unit. Four modes of coordination have been identified or invoked as intermediates in fluxional processes (Scheme 8). 

The high nucleophilicity of heterosubstituted allyllithium compounds makes them attractive reagents in synthetic organic chemistry. Structural studies of these compounds give a fundamental understanding about the control of the regioselectivity. Often, these studies are difficult due to the tendency of the compounds to form complex fluxional aggregates in solution. Piffl and coworkers have studied the dependency of the oxidation state of sulfur on the structure and electronic properties of the heterosubstituted... 

NMR spectroscopy ( Se, /= 1/2, 7%) is a powerful technique for identifying cyclic selenium molecules, especially the heteroatomic ring systems that contain sulfur or tellurium in addition to selenium, for which several isomers are possible for most compositions (Section 12.1.2). Solutions of monoclinic selenium in CS2 have been shown by high-performance liquid chromatography to form an equilibrium mixture of cyclo-Seg, cyclo-Sey and cyclo-Se6. The Se NMR spectra of such solutions show two singlets that are attributable to cyclo-Seg and cyclo-Se with relative intensities that correspond to a molar ratio of ca 6 No resonance is observed for cyclo-Sey presumably as a result of the fluxional behaviour (pseudorotation) of the seven-membered ring (Section 12.1.2). 

The already voluminous review literature on clusters will be considered as a basis for this review. The topics treated so far are clusters in general (109, 241) and in connection with metal-metal bonding (30, 338, 380), special types of clusters like those with TT-acceptor ligands (231), hydrides (233), carbonyls (85, 86) or methinyl tricobalt enneacarbonyls (313, 317) properties of clusters like structures (56, 316), fluxionality (110), mass spectra (226), vibrational spectra (365), and redox behavior (292). Clusters have been treated in the context of metal carbonyls (3, 4), metal sulfur complexes (2, 381), and in relation to coordination polyhedra (297). Reviews... 

Thus, pentacarbonyl(thioketone) complexes [M(CO)5L] were prepared by irradiation of solutions of [M(CO)6] (M = Cr, Mo, W) in the presence of thioketones L (L = thiobenzophenone, adamantanethione)119 [Eq. (4)] and by refluxing solutions of [Mo(CO)6] in tetrahydrofuran (THF) in the presence of L = p,p -disubstituted thiobenzophenones, thiofenchnone, and thiocamphor, respectively.120 The formation of two diastereomers of the thiocamphor complex was observed, probably due to a high inversion barrier at sulfur. Only one isomer of the thiofenchnone complex could be detected. However, fluxional behavior could not be ruled out.120... 

Inversion at the sulfur is also observed with a series of dinuclear Pt complexes with the ligand MeS—CHR—SMe (R = Me, H). However, at higher temperatures a fluxional behavior is encountered where the sulfur atoms obviously swap between the Pt centers.41... 

The structural chemistry associated with polysulfide molecules and materials reflects the stereochemical freedom of the sulfur atom as mentioned above, disulfide ligands can coordinate in a number of ways. Similarly, larger rings may adopt a variety of geometries (from X-ray studies) and the evidence that is available suggests that many of the rings are fluxional in solution. Muller has classified and discussed some aspects of this problem for molecular species, but the rationalization of the structures adopted in solid-state materials is not yet complete. ... 

The fluxional behavior of the osmium equivalents of the ruthenium(II) complexes mentioned in the final citation of the previous section has been described/ Time scales have been established for proton exchange at sulfur and at phosphorus in (26), R = Me or Et, and for deprotonation of (27) by the base DBU (1,8-diazabicyclo[5.4.0]undec-7-ene)/ ... 

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