Bromotelluronium salts

The second, slower reaction represents the ionic dissociation of the 171-association complex to form bromoselenonium and bromotelluronium intermediates. These intermediates should be similar in structure to iodotelluronium salt 18 and bromotelluronium salt 19 for which X-ray crystallographic structures have been determined. Values of the rate constant, k, are comparable in all three molecules with values of (4.4 0.1) X 10 s for diphenylselenide, (2.28 0.03) X... 

On the other hand. Mechanism 2 proposes the displacement of a Br by nucleophilic attack of the neighboring bromine atom (neighboring group participation), and formation of a bromonium ion intermediate 4. In a second step, the teiiuride acts as a scavenger of the Br in the bromonium intermediate to yield the olefin (Scheme 29.4). In both cases, the bromotelluronium salts 5 formed at first instance, give the isolated neutral Te(IV) by-products 6 after bromide addition. 

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