Selenium pseudohalides

Oxohalides and pseudohalides Table 16.7 Some physical properties of selenium oxohalides... 

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]. 

There are no examples of xenon bonded to sulfur or selenium and few for the halogens. The reported compounds involve the thiocyanate and selenocyanate ligands acting as pseudohalides, in the anions I(SCN)i and I(SeCN)2, for which IR and Raman spectra show the thiocyanate to have a linear S—I—S arrangement but give no definitive results for the selenocyanate, and two examples with more complex thio ligands for which crystal structures have been determined. 

The quadrupole splitting observed in the bis pyridine complexes also indicates the nature of the coordination environment in these linear chain bridging ligand complexes. This is observed in terms of the large A values found for Fe(pyridine)2(NCS)2 and Fe(pyridine)2(NCSe)2 as compared with the other bis complexes. These large values arise because in these two compounds the coordinated atoms are two trans pyridine nitrogen atoms, two pseudohalide nitrogen atoms, and two sulphur or selenium atoms from... 

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