Modulation of Stacking Interactions by Transition-Metal Coordination: Ab Initio Benchmark Studies

A series of ab initio calculations are used to determine the C-H center dot center dot center dot pi and pi center dot center dot center dot pi-stacking interactions of aromatic rings coordinated to transition-metal centres. Two model complexes have been employed, namely, ferrocene and chromium benzene tricarbonyl. Benchmark data obtained from extrapolation of MP2 energies to the basis set limit, coupled with CCSD(T) correction, indicate that coordinated aromatic rings are slightly weaker hydrogen-bond acceptors but are significantly stronger hydrogen-bond donors than uncomplexed rings. It is found that pi center dot center dot center dot pi stacking to a second benzene is stronger than in the free benzene dimer, especially in the chromium case. This is assigned, by use of energy partitioning in the local correlation method, to dispersion interactions between metal d and benzene pi orbitals. The benchmark data is also used to test the performance of more efficient theoretical methods, indicating that spin-component scaling of MP2 energies performs well in all cases. whereas various density functionals describe some complexes well. but others with errors of more than 1 kcal mol(-1).