A series of neutral phosphinosulfonamide complexes of nickel(II) were synthesized that catalyzed the oligomerization of ethylene to branched oligomers with average degrees of polymerization between 10 and 35. Branching numbers varied from 17 to 80 branches per 1000 carbons, depending on the catalyst structure and reaction conditions. The catalysts were active in a variety of solvents, including toluene, CH2Cl2, tetrahydrofuran, ethyl acetate, and methanol, but showed decreasing activity at temperatures higher than 40 degreesC. Electron-rich phosphinosulfonamides produced the highest catalyst activities in a series of structure-reactivity studies. The mechanism of oligomer formation was investigated with H-1 NMR spectroscopy, which indicated that branching arose from the isomerization of the nickel alkyl species during propagation rather than the reincorporation of alpha -olefin products.