The synthesis and characterization of a zirconium complex, having two alkoxide funtionalized N-heterocyclic carbene ligands, and its behaviour as catalyst in the polymerization of ethylene and propylene, have been reported. NMR analysis showed that more than one species was obtained during synthesis. These data were confirmed by ethylene polymerization, that gave rise to a linear polyethylene having a high Mw and a polydispersity index (MDI) < 2 and often bimodal. The same catalytic system was able to produce highly isotactic polypropylene together with an atactic fraction. DFT studies on the complex stereoisomer stability gave indications on the species possibly involved in the polymerizations.