Mononuclear η2 (4e)-Bonded Phosphaalkyne Complexes; Selective Formation of a 1,2-Diphosphacyclobutadiene Tantalum Complex.

Only the 1,2-diphosphacyclobutadiene and none of its 1,3 isomer was obtained in the reaction of 1 with tBuC≡P to give 2. In complex 1, which was prepared from [TaCl2 (η5 -C5 Me5 )(CO)2 (thf)] and tBuC≡P, the phosphaalkyne adopts an η2 (4e) bonding mode according to NMR data, crystal structure analysis, and theoretical calculations.