Selenium complexes four-coordinate

Selenium forms a series of catena dianions Se similar to those described for sulfur, but the chain length is limited to six atoms (x = 6) in simple salts. However, the catenaSQ dianion has been structurally characterised in the black complex [Na(12-crown-4) ]2[Se8 ] (cyclo-Se6,cyclo-Se7). In addition, selenium forms several higher polyselenides (x > 8), in which one or more of the selenium atoms is three- or four-coordinate. The structures of the hyper-valent species Scio , Sen , and Sei6" are especially interesting. The structure of the Seio dianion in the black PPN salt is bicyclic with both six-membered rings in chair conformations (Figure 12.24a). The three-coordinate... 

Several four-coordinate selenium(U) complexes with bidentate ligands, all of which involve highly distorted square planar SeS4 or SeSe4 coordination spheres, and some tellurium analogues have been described.11... 

Although numbering even fewer than the structurally characterized selenium zinc complexes there are examples of zinc tellurides and tellurates with typical coordination numbers of three or four and species from monomeric up to polymeric. 

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