Abstract The [Cp* Ru (CO) P 2]+ fragment (Cp*= η 5-C 5 Me 5) binds α, β-unsaturated ketones in an η 2-C C fashion. The resulting complex is subject to attack by H− at the 4- position, leaving the ruthenium bound to the substrate at the 3-position. The Ru C bond, formed in this way, can be cleaved to give a variety of 4, 3-addition products.