The main aim of this project was to develop new reagents capable of exchanging selenium for oxygen under mild conditions, to obtain new carboselenaldehyde (1) and selone (2) compounds. Phenylphosphonoselenoic dichloride (3) was prepared as a solution in xylene. It was reacted with (1,2-dithiol-3-ylidene)carbaldehydes and indolizine-3-carbaldehydes to afford 1,6aλ4-dithia-6-selena-pentalenes (5) and indolizine-3-carboselenaldehydes (6), respectively. The indolizine-3-carbaldehydes had previously been prepared from the corresponding indolizines, in turn prepared from the appropriate pyridinium bromide salts. CHSe(5)(6)Phenylphosphonoselenoic dichloride (3) was also reacted with several other carbonyl compounds. Reactions with N,N.-dimethyl- fonnamide, 5-phenyl-3H-1,2-dithiol-3-one, 2,6-dimethyl-4H.-pyran-4-one, 4-hydroxypyridine, 1-methylpyrrolidin-2-one, hexeihydro-2H-azepin-2-one, and 2,4,6-cycloheptatrien-1-one met with varied success, and only N,N-dimethylselenoformamide (7), 5-phenyl-3H-1,2-dithiole-3-selone (8), and 2,6-dimethy1-4H-pyraxi-4-selone (9) were obtained.(8)(9)The presence of a stabilising substituent was therefore required, and was introduced as a tungsten pentacarboiyl species. Pentacarbon-yl(indolizine-3-carboselenaldehyde-Se) tungsten(0) (10) and penta-carbonyl(2,4,6-cycloheptatriene-1-selone-Se)tungsten(0) (11) were obtained from the reaction of the corresponding oarboryl compounds with phenylphosphonoselenoic dichloride (3) in the presence of tetra- ethylanunonium iodopentacarbonyltungstate(0).The reagent (4) was prepared from the reaction of chlorodlphenyl- phosphine and tetramethylammonium selenocyanate, and was reacted in situ with indolizine-3-carbaldehydes to produce not only indolizine-3-carboselenaldehydes (6), but also compounds which were proposed on the strength of spectral and analytical evidence as being members of the novel 3-(indolizin-3-yl)-2,5-dihydro-2-selenoformyl-1,2,4-selena- diazole-5-selone (12) system.