Metals metal-sulfur bond

In spite of much theoretical work, we still do not have a complete picture of why the Co and Ni-promoted M0S2 catalyst is so successful. Interpretations range from the promoter-induced weakening of the metal-to-sulfur bond strength to the presence of unique sulfur species bound between molybdenum and the promoter. 

For catalysts poisoned by sulfur, the metal-sulfur bond is usually broken in the presence of steam, as shown for nickel ... 

The importance of ionization of the metal-sulfur bond in the electron-transfer process accounts for inhibitory properties being observed only for compounds of types I and II. Support for this mechanism was also obtained by the isolation of the disulfide (IV) from a reaction between peroxy radicals and zinc diisopropyl dithiophosphate. 

The tetradentate ligand forms monomeric square planar complexes. Synthetic and kinetic studies reveal that the coordinated mercapto group may be converted into the coordinated thioether function without breaking the metal-sulfur bond. The nucleophilic power of the coordinated mercapto group exceeds that of RSH, but depends on the metal atom. Bridging protects the sulfur atom from alkylation. In the case of nickel(ll), alkylation is accompanied by expansion of the coordination number of the nickel from 4 to 6. Ligand reactions have led to the synthesis of planar ligands completely cydized about the metal ion. 

The simple second-order nature of the kinetics in this system leads to immediate conclusions of some consequence. The rate-determining step is clearly not the heterolytic breaking of a metal-sulfur bond to produce the free R-S group, which then might undergo reaction. Further, the fact that there is no evidence suggesting consecutive processes eliminated the possibility that any such scheme could enter into the total rate except essentially as a pre-equilibrium—e.g., Equations 14 and 15. 

A theoretical foundation for understanding these correlations is found in the calculated bulk electronic structures of the first- and second-row TMS. The electronic environment of the metal surrounded by six sulfur atoms in an octahedral configuration was calculated, using the hypotheses that all the sulfides could be represented by this symmetry as an approximation. There are several electronic factors that appear to be related to catalytic activity the orbital occupation of the HOMO (Highest Occupied Molecular Orbital), the degree of covalency of the metal-sulfur bond, and the metal-sulfur bond strength. These factors were incorporated into an activity parameter (A2), which correlates well with the periodic trends (Fig. 16) (74, 75). This parameter is equal to the product of the number of electrons contained in the... 

HOMO of the molecular sulfide by a factor B, which measures the nature (symmetry and strength) of the metal-sulfur bond. It is now possible to understand why the heat of formation was an insufficient parameter, because it did not contain the same quantities as this B parameter. In the case of manganese, the agreement between the measured and the calculated is excellent. To be a good catalyst, the sulfide should have a HOMO filled with as many electrons as possible and be of l2f symmetry instead of eg. 

Some Aspects of the Reactivity of Metal Ion-Sulfur Bonds 1658... 

Very beautiful complexes are obtainable using sulfur-containing ligands. The reactivity of the metal ion-sulfur bond has been reviewed.81611... 

Although formally metal-sulfur bonding is not treated in this section, some observations will be reported as they are pertinent to the subject at hand. 

A Classification of Metal Sulfides and Their Types of Metal-Sulfur Bonds 525... 

The reactivity of metal-sulfur bonds has been discussed in a review by Kuehn and Isied,2 but other reviews also include chapters on this topic.M... 

A straightforward explanation for the different reactivities or the solubility properties is found in the character of the particular metal-sulfur bond, i.e. whether it is covalent or ionic. This determines the charge on the sulfur and metal atoms and, implicitly, which kind of attack, by H+ or S2, will take place. Decreased electron density on the sulfur atoms of group S(a) and S(b) metal-sulfur bonds caused by strong covalent contributions stabilizes discrete thioanions like M0S4- but also HgSl-. 

In contrast to the transition elements, the main group element dithiocarbamates often have asymmetrical metal-sulfur bonds due to the lack of pn-dnMOs. In theses compounds the metal-sulfur interaction. High oxidation states for these dithiocarbamates are only found when high electron density is brought upon the metal by er-donating groups. For instance, (54) exists whereas (55) does not.73... 

---