Reaction of tertiary phosphine selenides, R3PSe (R = Me2N, Et2N or C6H11),with dibromine. The first reported examples of 1 : 1 addition

The R3PSeBr2 compounds (R = Me2N, Et2N or C6H11) have been prepared and cha racterised by P-31-{H} and infrared spectroscopy. The compounds R3PSeBr2 (R = Me2N or C6H11) have also been crystallographically characterised. In con trast to the analogous diiodo compounds R3PSeI2 (which have a molecular Psi -tetrahedral charge-transfer structure, R3PSeI-I), the R3PSeBr2 compounds a dopt Psi-trigonal bipyramids at the selenium centre (taking account of the stereochemically active lone pairs). The crystal structure of(Me2N)(3)PSeBr 2 exhibits very different d(Se-Br), 2.602(2) and 2.544(2) Angstrom. This ph enomenon is reasoned to be due to the fact that both staggered and eclipsed Se-Br bonds are observed in the structure. The crystal structure of(C6H11) (3)PSeBr2 shows two crystallographically independent molecules in the asymm etric unit, d(Se-Br) being identical in one molecule, 2.568(3) and 2.566(3) Angstrom, but significantly different in the second molecule, 2.591(3) and 2.556(3) Angstrom. A possible explanation for this is the presence of a cl ose non-bonded Br ... Br contact in this second (C6H11)(3)PSeBr2 molecule. The compounds R3PSeBr2 (R = Me2N or C6H11) both exhibit P-Se bonds typical of those expected for single bonds, 2.262(2) and 2.263(2) average, respecti vely, again in contrast to the analogous diiodo compounds, R3PSeI2, in whic h significant P-Se double bond character was retained. The P-31-{H} NMR and infrared spectroscopic data for R3PSeBr2 (R = Me2N, Et2N or C6H11) are dis cussed with respect to those of the parent tertiary phosphine selenide, R3P Se.