Synthesis, bonding and dynamic behavior of fac-[Mo(II)(CO)2(η3-allyl)] derivatives

Cationic complexes [Mo(η3-allyl)(CO)2(L–L)L′]PF6, (L–L=C6H5SCH2CH2SC6H5, L′=NCCH3 (1); bipy, NCCH3 (2); py, (NCCH3)2 (3); (NCCH3)3 (4); dppe, NCCH3 (5) and the neutral analogues [Mo(η3-allyl)(CO)2(L–L)X] (L–L=phen (6); bipy (7); X=Br) were synthesized. Complexes 2, 5, 6 and 7 were characterized by single crystal X-ray diffraction. Depending on the chelating ligand, these pseudo-octahedral complexes undergo different dynamic processes in solution and NMR spectroscopic evidence was provided for those studies. The structural trends of the limiting structures depicted by these complexes as well as the pathways to their inter-conversion were analyzed by ab initio theoretical calculations. Both NMR data and the calculations showed that for complex 2 the equatorial species predominates at room temperature but that two forms differing only by the conformation of the allyl coexist. Lowering the temperature leads to the appearance of the equatorial–axial isomer.