The Ni-II literature complex cis-[Ni(C6F5)(2)(THF)(2)] is a synthon of cis-Ni(C6F5)(2) that allows us to establish a protocol to measure and compare the ligand effect on the Ni-II -> Ni-0 reductive elimination step (coupling), often critical in catalytic processes. Several ligands of different types were submitted to this Ni-meter comparison: bipyridines, chelating diphosphines, monodentate phosphines, PR2(biaryl) phosphines, and PEWO ligands (phosphines with one potentially chelate electron-withdrawing olefin). Extremely different C6F5-C6F5 coupling rates, ranging from totally inactive (producing stable complexes at room temperature) to those inducing almost instantaneous coupling at 25 degrees C, were found for the different ligands tested. The PR2(biaryl) ligands, very efficient for coupling in Pd, are slow and inefficient in Ni, and the reason for this difference is examined. In contrast, PEWO type ligands are amazingly efficient and provide the lowest coupling barriers ever observed for Ni-II complexes; they yield up to 96% C6F5-C6F5 coupling in 5 min at 25 degrees C (the rest is C6F5H) and 100% coupling with no hydrolysis in 8 h at -22 to -53 degrees C.