Abstract As its P2W2(CO)10 dicomplex, the ring of 1,2,3,4-tetraphenyl-1,2-dihydro-1,2-diphosphete undergoes a reversible conrotatoryopening to give the corresponding 1,4-diphosphadiene dicomplex which can be trapped as [4+2] cycloadducts with N-phenylmaleimide, maleic anhydride, acrylonitrile and benzaldehyde. X-ray crystal structure analysis indicates a weakening of the PP bond of the 1,2-dihydro-1