Ease of generation, stablity in solution at ambient temperature, high enantioselectivity in Diels-Alder reactions, efficient catalyst recovery, and large rate differences on variation of the anion are all characteristics of the new Ru Lewis acid [CpRu((S,S)-biphop-F)]+ (biphop-F=(C6F5)2POCH2(Ph)CH2(Ph)OP(C6F5)2). The structure of complex 1 (L=methacrolein, Y=SbF6) provides evidence for a cooperative binding of the dienophile by both the Lewis acid and the anion.