Mn²⁺-, Fe²⁺-, Co²⁺-, Ni²⁺-, Cu²⁺-, and Zn²⁺-Binding Chalcogen−Chalcogen Bridges: A Compared MP2 and B3LYP Study

Abstract

We investigated the binding of late first row transition metals with chalcogen-chalcogen bridges represented by minimal models (H(2)O(2), H(2)S(2), and H(2)Se(2)). The use of such small models allows us to employ a large atomic basis set and compare DFT and MP2 results with CCSD(T) reference data. All methods agree in finding Cu(2+) complexes the most stable ones, and for each given metal, H(2)Se(2) complexes are more stable than H(2)S(2) ones and the latter more stable than the corresponding H(2)O(2) ones. Despite this qualitative agreement between all the considered methods, quantitatively we found a big difference between MP2 and B3LYP, in structural and energetic properties. In particular, DFT largely overestimates the binding energies, while MP2 slightly underestimates them with respect to CCSD(T) calculations. Note that also other popular functionals (MPW1PW91, M05, TPSS, BLYP, and SVWN) overestimate the binding energy, such that it seems to be an intrinsic DFT failure. The main discrepancy was found for Cu(2+). The comparative analysis of B3LYP and MP2 wave functions explains the differences found between two methods and why the Cu(2+) complexes show the bigger one. Finally, CCSD(T) calculations, slightly modifying MP2 insights, found that all three complexes present the same metal binding energy order, and notably Cu(2+) > Ni(2+) > Zn(2+) > Co(2+) > Fe(2+) > Mn(2+).