Though metal complexes of dialkyl diselenophosphate ligands [Se 2 P(OR) 2 ] À (dsep) have been reported in several occasions, the solid-state structure of isolated dsep ligands is still unknown. Herein the ammonium salt of the dsep ligand, NH 4 Se 2 P(O i Pr) 2 , is structurally characterized and N-HÁ Á ÁSe type of H-bonding interactions are revealed. In addition, the NH 4 Se 2 P(OR) 2 ligands serve as an ammonia source in the Fe powder catalyzed condensation of acetone with ammonia to form 2,2,6,6-tetramethyl-4-oxopiperidinium salts of the dsep ligand, [{H 2 N(CH 2 ) 2 (CMe 2 ) 2 CO}{Se 2 P(OR) 2 }] (1a = i Pr, 1b = Et) of which 1b is structurally characterized. Novel syntheses of Se-esters of O,O 0 -dialkylphosporodiselenoic acid, 2, 3, 4, 5, 6, 7, directly from the ammonium salts have been achieved. The reactions, where one of the Se atoms of the dsep unit acts as the nucleophilic center, include Michael addition, epoxide ring opening addition, acylation and a-alkynation.