This chapter reviews recent development of cationic α-diimine nickel(II) and palladium(II) complexes for olefin polymerization. The contributions of ligand structure to the catalytic polymerization properties of late metal complexes are particularly emphasized in this review. Unique among transition metal polymerization systems, these late metal complexes also catalyze the formation of branched polymers. The mechanisms of elemental reaction steps for these catalytic systems are discussed. The Pd(II) system also permits the copolymerization of ethylene and α-olefins with various polar comonomers, especially acrylates. Finally, a discussion of new research in ligand design is provided. A new class of cyclophane-based ligands has exhibited high thermal stability. The mechanism of copolymerizations is significantly altered, leading to elevated incorporations of polar monomers. An axial-donation hemilabile cyclic ligand is also discussed, the Ni(II) and Pd(II) complexes of which afford high molecular weight polyethylene despite their reduced steric bulk.