The use of transition metal complexes of non-heterocyclic N-donor ligands as olefin oligo/polymerization precatalysts is reviewed. These metal complexes with definite steric and electronic variations offer high catalytic activity resulting in the desired microstructure of the polymer products currently attracting significant attention by the catalysis community. This review primarily discusses the fundamental principles in the design of mononuclear, homodinuclear, heterodinuclear, homomultinuclear and heteromultinuclear transition metal complexes having non-heterocyclic N-donor ligands and their reactivity towards olefins in oligo/polymerization reactions. Numerous non-heterocyclic N-donor ligands of the donor types N-2, NO, NP, NC, N2P, N2O, N2C, N2S, N-3, NCP, NCp, NOCp, N3Cp, N2O2 for mononuclear, N-4, N2O2, N4O4, N3C, NCp2, N2Cp2, N(2)lndene(2) and N(2)S(2)Indene (Cp: cyclopentadiene) for bimetallic and N-8, N8O4, N8O8 and N12O4 for multinuclear complex formations have been efficiently employed, and the resulting complexes display promising reactivity patterns with a variety of olefins. The majority of these catalytic systems are tethered with appropriate alkyl/aryl/alkyl-aryl substituents to afford the required steric hindrance to the metal center(s), and with functional groups for electronic variations. Such systems produce a linear polymer product in most of the cases, any deviation from this behavior resulting in the formation of oligomers. The major focus of this review is on reports published over the last 25 years in the research and development of non-heterocyclic, N-donor coordinated transition metal complexes with a variety of spacers. This review article is organized into main sections on the basis of the nuclearity of the transition metal complex; each subsection deals with complexes starting from late transition metals on the basis of ligand type. (C) 2017 Elsevier B.V. All rights reserved.