Phosphirano [1,2-c][1,2,3]diazaphospholes 5, which feature a strongly pyramidalized phosphorus atom at a ring fusion position, were identified as a new type of phosphane ligands for transition metals. The derivatives 5a-c smoothly form W(CO) 5 complexes, which are thermally quite stable -in contrast to the known lability of various carbonylmetal complexes of simple phosphiranes. In the temperature range 120-150°C, they undergo a clean decomplexation in toluene solution. Bicyclic phosphiranes 5a and 5b, but not the acceptor-substituted derivative 5c, readily react with the [(E,E)-

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