Monoanionic bidentate phenoxy-ether [O(-),O] and phenoxy-thioether [O(-),S] ligands were synthesized and used to prepare octahedral group 4 metal complexes with general formula [O(-),E](2)ML(2) (complex 1: E = S, M = Hf, L = Bn; complex 2: E = O, M = Ti, L = NMe(2); complex 3: E = S, M = Ti, L = NMe(2); complex 4: E = S, M = Ti, L = Cl). Variable-temperature 1H NMR studies were performed on all the complexes 1-4 in the range of +20 to -70 degrees C. For the hafnium complex 1 a dynamic interchange of the Delta and Lambda enantiomers for a C2-symmetric isomer was revealed. Eyring analysis indicated that rearrangement of the ligands occurs via a non-dissociative mechanism. In the range of the explored temperatures, a broadening of the signals was observed for complexes 2-3. Finally, a reversible fluxional process between several isomers was revealed for complex 4. Complexes 1-4 were tested as pre-catalysts for ethylene and propene polymerization in combination with different activators and under variable conditions. Complex 1 was found to be inactive whereas complexes 2-4 showed moderate activities in the polymerization of ethylene and propene. A mixture of polymers with different microstructure was obtained in all cases, coherently with the fluxional behaviour of the pre-catalysts observed in solution.