The synthesised N-(phenyl)-, N-(pyrazin-2-yl)-, N-(pyridin-3-yl)-, N-(2,5-dimethoxycarbonylphenyl)-, N-(2-methoxycarbonylphenyl)- and the newly obtained so far unknown N-(2-carboxylphenyl)- and N-(pyridin-4-yl)-α-diphenylphosphinoglycines, as well as N-(pyrazin-2-yl)- and N-(2-methoxycarbonylphenyl)-α-mesityl(phenyl)phosphinoglycines were tested as ligands in nickel-catalysed homogeneous ethylene oligomerization. The resulting catalytic systems showed diverse selectivities. Quantum-chemical calculations were performed to rationalize the influence of substituents at nitrogen and phosphorus atoms in N-aryl-phosphinoglycines (L) on the molecular weight distribution of α-olefin products. It was revealed that the difference in the catalytic performance of L/Ni systems is mainly due to the steric effects influencing the relative thermodynamic stability of various conformations of the catalyst as well as the thermodynamic and kinetic parameters of various competing catalytic transformations.