(η5-C4H4S)Cr(CO)3 and (η5-C4H4Se)Cr(CO)3: A DFT Investigation of the Ground-State Singlet and Triplet Surfaces. New Insights into the Mechanism of C-S or C-Se Insertion Reactions

The singlet and triplet surfaces for the interaction of thiophene or selenophene with a chromium tricarbonyl unit were calculated using the B3LYP/LanL2DZ+p model chemistry. The singlet surfaces confirm that the (eta(5)-C4H4E)Cr(CO)(3)(E = S or Se) are the lowest energy species. The (eta(5)-C4H4E)Cr(CO)(3)and (kappa(2)(E,C)-C4H4E)Cr(CO)(3) Species were also located in shallow energy minima. Reaction path modeling on the singlet surfaces provided activation energies for the endothermic insertion process of 172 and 160 kJ mol(-1) for E = S or Se, respectively. The activation energy for the insertion process on the triplet surfaces is on the order of 70 kJ mol(-1). A novel eta(3)(E.C.C)-eta(1)(C)-coordmated species was located as a transition state between the insertion species and (eta(5)-C4H4E)Cr(CO)(3)on the singlet surfaces and an intermediate on the triplet surface for E = S.