A new P4C4 eight-membered macrocycle has been obtained in one step from 3,4-dimethyl-1-phenylphosphole. A comparison of the X-ray crystal structure of this P4C4 macrocycle with that of its decacarbonyldimolybdenum complex shows that the P4C4 crown gains a very high flexibility through an easy rotation of the adjacent phospholyl units round the Câ C and Pâ P bonds of the central eight-membered ring. This means that this P4C4 macrocycle can adapt its shape to the stereochemical requirements of the complexed metals. The Pâ P and Câ C bonds of the crown [average 2.191(3) and 1.47(1)Ã respectively] are significantly shorter than normal Pâ P and Câ C single bonds. Thus some conjugative interactions probably exist between the adjacent phospholyl rings. These interactions are suppressed in the decacarbonyldimolybdenum complex.