The radical-initiated hydrophosphination and hydroarsination of diethylamino-dialkynylboranes with C 6H 5EH 2 (E = P, As) give the compounds 1-phospha- and 1-arsa-4-boracyclohexadienes-2,5 (IV and V, respectively). V is further reduced by C 6H 5AsH 2 to its dihydro derivative. Rather narrow boundaries exist for a substitution chemistry at the BN bond in IVa/IVb: methanolysis yields the B-methoxy derivative which in turn can be converted into the t-butyl compound. Alkali metals cleave the exocyclic PC bond. The anion formed reacts with RX yielding P-alkyl derivatives. Chalcogens convert the phosphorus(III) derivatives into phosphorus(V) compounds.