Heterodimerization of olefins is a reaction of enormous synthetic potential, since it has been demonstrated that excellent yields and selectivities can be achieved under exceptionally mild conditions in many cases. 1,2 For example, we have recently shown that using catalysts derived from [(allyl)Ni(Br)] 2 and a tertiary phosphine in combination with a silver salt, a nearly quantitative hydrovinylation of vinylarenes can be achieved (eq 1). In our initial efforts to find a broadly applicable asymmetric version of this reaction, 2 we considered the requirement of a coordination site for ethylene on the putative cationic Ni intermediates, and chose (R)-2-diphenylphosphino-2′-alkoxy-1,1′-binaphthyl (MOP), 3 in which the 2′-methoxy group would play the role of a 'hemilabile′ ligand 4 (eq 2). With the fortuitous choice of this ligand, we also realized that counterions played a key role in the success of this reaction. For example, we found that AgOTf, which gave in many cases isolated yields >95% with Ph 3 P as a ligand, gave low yields in the hydrovinylation reactions using the MOP ligands. In sharp contrast, the use of Na + B[(3,5-(CF 3 ) 2 C 6 H 3 )] 4 -(NaBARF) in conjunction with MOP fully restored the activity of the catalyst (eq 2). 5 Since this original discovery, we have sought to clarify the role of hemilabile chelation and of the effect of various counterions on the efficiency and selectivity of the Ni-and Pd-catalysts for this reaction. With this goal, variation of the 2′ substituent of the MOP ligand was studied. In addition, we synthesized a new class of "tunable" hemilabile 1-aryl-2,5-dialkylphospholanes which were found to be exceptionally good ligands for the exacting hydrovinylation reaction, if and only if the appropriate counterions are used. The results of these studies are reported in this paper.