Synthesis of a mononuclear η²(4e)-bonded phosphaalkyne complex, transformation into an η⁴-1,3-diphosphacyclobutadiene complex

The synthesis of the first mononuclear η²(4e)-bonded phosphaalkyne complex 1 is described, the ³¹P and ¹³C NMR spectra showing the expected low field contact resonances; reaction of 1 with a molecule of Bu^tC≡P leads to the formation of an η⁴-1,3-diphosphacyclobutadiene complex, thus demonstrating the stepwise nature of the formation of the P₂C₂ ring.