Coordinated Sulfur or Selenium

The number and variety of coordination complexes of palladium(II) with sulfur-containing bidentate or multidentate ligands is legion, and it is possible here only to summarize the different kinds of complexes formed and to guide the reader into the literature. Table 2 contains a selection of typical palladium(II) complexes of bi- or multi-dentate sulfur or selenium donor ligands, and comments on pertinent aspects of particular complexes. Chelate... 

Among these ligand systems, the pseudohalide ions NCX (X = S, Se) are of permanent interest [1,5,7,11,19,21,25-41]. To explain coordination modes in the complexes of these ligands, the HSAB principle (details in Sec. 1.2.2) was first applied [42,43] the hard Pearson acids mostly bind the hard nitrogen atom and the soft Pearson acids bind the soft sulfur or selenium atoms. This approach, related with hard-soft interactions, was also used later on [1,2,6,7,11]. 

The preceding sections have shown that the behavior of the thiocyanate and selenocyanate groups in homogeneous complex anions is identical in both cases the nitrogen atom coordinates to the class a metal, and the sulfur or selenium atoms coordinate to the class b metal. Although fewer comparable cyanate complexes are known, it is quite clear that the coordination of the cyanate group is not governed by the same factors. 

It may be concluded, therefore, that attempts to provide too detailed a comparison of the coordination behavior of the chalcogeno-cyanate ions are not useful until a better understanding is obtained of the differing effects of the cyano group on the atom X in NCX . Such an interaction may, and probably does, modify the comparative chemistry that might be expected for the oxygen, sulfur, or selenium... 

A silacyclopropene intermediate 56 was trapped by a reaction with 2-propanol during the thermal degradation of pentacoordinate disilanes 55 bearing an 8-thiona- or 8-seleno-l-naphthyl group and a methoxy group on the same silicon center. This reaction generates intramolecular sulfur- or selenium-coordinated silylenes 57 (Scheme 10) <2003OM3299>. 

In fact, hardly any donor atoms other than nitrogen have been used to create multiporphyrin assemblies. Using hard Lewis acids as central metal opens the possibility to bind two axial ligands on either side of the porphyrins. With ditopic Ugands such as diols this can lead to polymerization. However, a few examples can be found in the Uterature, where the central metal of the porphyrin is axially coordinated by oxygen, sulfur or selenium to assemble porphyrins. The use of (non-porphyrinic) phosphine ligands apparently has so far not been successful in the construction of supramolecular multiporphyrin arrays. 

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