A process for polymerizing an olefin monomer, the process comprising a step of contacting together a catalytic amount of a catalyst comprising a mixture or reaction product of ingredients (a) and (b), wherein ingredient (a) comprises a metal-ligand complex and ingredient (b) comprises an activating co-catalyst; and an olefin monomer as ingredient (c); the ingredient (a) being one or more metal-ligand complexes (also referred to herein as precatalysts) of formula: wherein: M is a hafnium in a formal oxidation state of +3 or +4; n is an integer of from 1 to 5; Each X independently is a monodentate ligand that is neutral, monoanionic, or dianionic; or two X are taken together to form a bidentate ligand that is neutral, monoanionic, or dianionic; X and n are chosen in such a way that the metal-ligand complex of formula is, in aggregate, neutral; R 1 is (C 1 -C 40)hydrocarbyl, (C 1 -C 40)heterohydrocarbyl, (C 1 -C 40)hydrocarbyl-O-, or halogen atom Each of R 2 to R 6 independently is a hydrogen atom, (C 1 -C 40)hydrocarbyl, (C 1 -C 40)heterohydrocarbyl, (C 1 -C 40)hydrocarbyl-O-, or halogen atom; R 7 independently is a (C 1 -C 40)hydrocarbyl or (C 1 -C 40)heterohydrocarbyl; Or one X, when a monodentate ligand, is taken together with any one of R 1 to R 7 in such a way so that the metal-ligand complex of formula contains a tridentate ligand; Each of the aforementioned hydrocarbyl and heterohydrocarbyl independently is unsubstituted or substituted with one or more substituents R s; and Each R s independently is a halogen atom, polyfluoro, perfluoro, unsubstituted (C 1 -C 18)alkyl, F 3 C-, FCH 2 O-, F 2 HCO-, F 3 CO-, R 3 Si-, RO-, RS-, RS -, RS 2 -, R 2 P-, R 2 N-, R 2 C=N-, NC-, RC O-, ROC -, RC N -, or R 2 NC -, wherein each R independently is an unsubstituted (C 1 -C 18)alkyl; and the ingredient (b) being one or more activating co-catalysts, or a reaction product thereof, wherein the ratio of total number of moles of the one or more metal-ligand complexes of formula to total number of moles of the one or more activating co-catalysts is from 1:10,000 to 100:1; wherein the contacting step is performed under olefin polymerizing conditions and prepares a polyolefin. M is a hafnium in a formal oxidation state of +3 or +4; n is an integer of from 1 to 5; Each X independently is a monodentate ligand that is neutral, monoanionic, or dianionic; or two X are taken together to form a bidentate ligand that is neutral, monoanionic, or dianionic; X and n are chosen in such a way that the metal-ligand complex of formula is, in aggregate, neutral; R 1 is (C 1 -C 40)hydrocarbyl, (C 1 -C 40)heterohydrocarbyl, (C 1 -C 40)hydrocarbyl-O-, or halogen atom Each of R 2 to R 6 independently is a hydrogen atom, (C 1 -C 40)hydrocarbyl, (C 1 -C 40)heterohydrocarbyl, (C 1 -C 40)hydrocarbyl-O-, or halogen atom; R 7 independently is a (C 1 -C 40)hydrocarbyl or (C 1 -C 40)heterohydrocarbyl; Or one X, when a monodentate ligand, is taken together with any one of R 1 to R 7 in such a way so that the metal-ligand complex of formula contains a tridentate ligand; Each of the aforementioned hydrocarbyl and heterohydrocarbyl independently is unsubstituted or substituted with one or more substituents R s; and Each R s independently is a halogen atom, polyfluoro, perfluoro, unsubstituted (C 1 -C 18)alkyl, F 3 C-, FCH 2 O-, F 2 HCO-, F 3 CO-, R 3 Si-, RO-, RS-, RS -, RS 2 -, R 2 P-, R 2 N-, R 2 C=N-, NC-, RC O-, ROC -, RC N -, or R 2 NC -, wherein each R independently is an unsubstituted (C 1 -C 18)alkyl; and the ingredient (b) being one or more activating co-catalysts, or a reaction product thereof, wherein the ratio of total number of moles of the one or more metal-ligand complexes of formula to total number of moles of the one or more activating co-catalysts is from 1:10,000 to 100:1; wherein the contacting step is performed under olefin polymerizing conditions and prepares a polyolefin.