Sulfur selenium—oxygen bonds

The platinum metal chalcogenides in general are easier to prepare than the corresponding oxides. Whereas special conditions of temperature and pressure are required to prepare many of the oxides, the platinum metals react most readily with S, Se, and Te. A number of additional differences concerning the chemistry of the chalcogenides and the oxides are summarized as follows The metal—sulfur (selenium, tellurium) bond has considerably more covalent character than the metal-oxygen bond and, therefore, there are important differences in the structure types of the compounds formed. Whereas there may be considerable similarity between oxides and fluorides, the structural chemistry of the sulfides tends to be more closely related to that of the chlorides. The latter compounds... 

As expected, when comparing vibrational spectra of sulfur and selenium compounds, the absorptions due to bonds to selenium occur at lower frequencies than the corresponding sulfur modes. This frequency shift is normally 50-150 cm" but may be more for selenium-oxygen compounds when compared to sulfur-oxygen compounds ... 

One-Atom Insertions. Siliranes also undergo one-atom insertion reactions with elemental sulfur, oxygen, or selenium. Similarly, the insertion of aryl or alkyl isocyanates provides iminosi-lacyclobutanes in high yield as single stereoisomers (eq 7). The reaction proceeds with the retention of silacyclopropane configuration and the insertion occurs at the more substituted C-Si bond when unsymmetrical siliranes are employed, similar to the regios-electivity observed for aldehydes and formamides. 

Most polymerizations in this section can be categorized as stable (Tree) radical-mediated polymerizations (sometimes abbreviated as SFRMP). In the following discussion systems have been classed according to the type of stable radical involved, which usually correlates with the type of bond homolyzed in the activation process. Those described include systems where the stable radical is a sulfur-ccntered radical (Section 9.3.2), a selenium-centered radical (Section 9.3.3), a carbon-centered radical (Sections 9.3.4 and 9.3.5), an oxygen-centered radical (Sections 9.3.6, 9.3.7), or a nitrogcn-ccntcrcd radical (Section 9.3.8). Wc also consider polymerization mediated by cobalt complexes (Section 9.3.9) and certain monomers (Section 9.3.5). 

---