| Popis: |
The relative stabilities of the exo- and endo-isomers of eight cationic transition metal half-sandwich η3–allyl d4 and d6 complexes ([(1,3,5-Me3C6H3)M(CO)2(C3H5)]+ (d4; M = Cr, Mo, W), [CpM(CO)2(C3H5)]+ (d4; M = Mn, Re), and [CpM(CO)(C3H5)]+ (d6; M = Co, Rh, Ir)) were systematically investigated using computational chemistry approaches. Two different mechanisms of the exo–endo interconversion were evaluated: a rotation of the allyl fragment about the metal-ligand axis (η3–η3–η3) and hapticity change of the allyl ligand (η3–η1–η3). The results were compared with the available experimental data. It was demonstrated that the considered d4 complexes usually exist as a mixture of both isomers (the endo-form becomes more stable down the group), which can interconvert only via the η3–η3–η3 mechanism. On the contrary, the exo-form of the d6 complexes is always much more stable than the endo-form. However, the exo–endo interconversion is always hindered, so the observed exo–endo ratio establishes during the synthesis. The trends and factors of the exo–endo relative stability and the possibility of the exo–endo interconversion were discussed. |
Full Text from ScienceDirect