Reaction of dimethylselenourea and selenourea with dibromine to produce selenourea-dibromine, the 'T'-shaped 1 : 1 molecular adduct N,N-dimethyl-2-selenourea-dibromine, its solvent of crystallisation-containing analogue and the unusual ionic compound 5[(H2N)(Me2N)CBr](+)[SeBr6](2-)[Se2Br9](-)2[Br-3](-). A low temperature crystallographic reinvestigation of N-methylbenzothiazole-2-selone-dibromine

The reactions of the selenoamides N,N-dimethyl-2-selenourea, dmsu, selenour ea, su and N-methylbenzothiazole-2-selone, mbts, with dibromine has been st udied. In all reactions the bulk product formed is the 1:1 T-shaped additio n compound, selenoamide . Br-2. The crystal structure of dmsu . Br-2 1 has been determined and compared to its solvated analogue, dmsu . Br-2. CH2Cl2. Despite the different crystal packing in these two adducts, an asymmetry i n d(Se-Br) is exhibited by both, ruling out the possibility that difference s in Se-Br bond lengths are attributed solely to crystal packing forces. Bo th structures are essentially zwitterionic, a negative charge resides on th e SeBr2 moiety and the positive charge is supported by the two nitrogen ato ms. The recently reported mbts . Br-2, previously described as a carbene in teracting with SeBr2, was reinvestigated. Considering findings from a low t emperature X-ray study and the C-13 NMR spectrum, this compound is in fact also best described as zwitterionic, analogous to 1 and previously describe d structures. An interesting minor product from the reaction of two equival ents of dibromine with dmsu has also been characterised crystallographicall y. This complicated ionic structure of formula 5[(H2N)(Me2N)CBr](+)[SeBr6]( 2-)[Se2Br9](-)2[Br-3](-) illustrates the susceptibility of certain selenoam ides to carbon-selenium bond cleavage when a dihalogen that is more strongl y oxidising than I-2 or IBr is treated with them in a stoichiometric ratio greater than 1:1. This curious (and disordered) structure represents the fi rst report of the anion [Se2Br9](-).