Se and

The trifluoromethylating order of reactivity of a series of 5-, Se-, and Te-(trifluoromethyl)dibenzothiophenium, -selenophenium, and -telluro-phenium triflates was determined by relative reaction rates with aniline. 

Table 12 Selected <5( Se) and <5 Te) chemical shifts (ppm) for organoselenium and organotellurium compounds ...

Adel E. Saroflm, Se.D., Lammot DuPont Professor of Chemical Engineering and Assistant Director, Fuels Research Laboratory, Massachusetts Institute of Technology Member, American Institute of Chemical Engineers, American Chemical Society, Combustion Institute (Section 5, Heat and Mass Transfer)... 

Separation of Se and Te can also be achieved by neutralizing the alkaline selenite and tellurite leach with H2SO4 this precipitates the tellurium as a hydrous dioxide and leaves the more acidic selenous acid, H2Se03, in solution from which 99.5% pure Se can be precipitated by S02 ... 

The a- and 5-forms of red crystalline Scg are obtained respectively by the slow and rapid evaporation of CS2 or benzene solutions of black vitreous Se and more recently a third (y) form of red crystalline Scg was obtained from the reaction... 

Organoselenium compounds in particular, once ingested, are slowly released over prolonged periods and result in foul-smelling breath and perspiration. The element is also highly toxic towards grazing sheep, cattle and other animals, and, at concentrations above about 5 ppm, causes severe disorders. Despite this, Se was found (in 1957) to play an essential dietary role in animals and also in humans — it is required in the formation of the enzyme glutathione peroxidase which is involved in fat metabolism. It has also been found that the Incidence of kwashiorkor (severe protein malnutrition) in children is associated with inadequate uptake of Se, and it may well be involved in protection... 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

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