Palladium-catalyzed coordination-insertion copolymerization of ethylene with acrylonitrile (AN) proceeded only by using phosphine-sulfonate (P-SO3) as a ligand among the neutral and anionic ligands we examined, those are phosphine-sulfonate (P-SO3), diphosphine (P-P), and imine-phenolate (N-O). In order to answer a question that is unique for P-SO3, theoretical and experimental studies were carried out for the three catalyst systems. By comparing P-SO3 and P-P, it was elucidated that (i) the pi-acrylonitrile complex [(L-L')PdPr(pi-AN)] is less stable than the corresponding a-complex [(L-L')PdPr(sigma-AN)] in both the phosphine-sulfonato complex (L-L' = P-SO3) and the diphosphine complex (L-L' = P-P) and (ii) the energetic difference between the pi-complex and the a-complex is smaller in the P-SO3 complexes than in the P-P complexes. Thus, the energies of the transition states for both AN insertion and its subsequent ethylene insertion relative to the most stable species [(L-L')PdPr(sigma-AN)] are lower for P-SO3 than for P-P. The results nicely explain the difference between these two types of ligands. That is, ethylene insertion subsequent to AN insertion was detected for P-SO3, while aggregate formation was reported for cationic [(L-L)Pd(CHCNCH2CH3)] complex. Aggregate formation with the cationic complex can be considered as a result of the retarded ethylene insertion to [(L-L)Pd(CHCNCH2CH3)]. In contrast, theoretical comparison between P-SO3 and N-O did not show a significant energetic difference in both AN insertion and its subsequent ethylene insertion, implying that ethylene/AN copolymerization might be possible. However, our experiment using [(N-O)PdMe(lutidine)] complex revealed that beta-hydride elimination terminated the ethylene oligomerization and, more importantly, that the resulting Pd-H species lead to formation of free N-OH and Pd(0) particles. The beta-hydride elimination process was further studied theoretically to clarify the difference between the two anionic ligands, P-SO3 and N-O.