Analysis of magnesium-carbon bonding in magnesium anthracene systems

Ab initio calculations at the MP2/3-21G(*) level of theory have been carried out for the magnesium-anthracene complexes 9,10-magnesiumanthracene·3H₂O (1) and the 9-methyl (2), dimethyl (3), and 9,10-bis(methylsilyl) (4) substituted derivatives. The theoretically predicted geometries of the anthracene ligands are also reported. The calculated geometries of 1-4 are in very good agreement with experimental values for the corresponding THF complexes. The Mg-C_9,10 bonds of the bridged structures are rather long and the anthracene ligands are folded by ∼40° along the C₉-C₁₀ line in the complexes. Analysis of the electronic structure shows clearly that the Mg-C_9,10 bonds should be considered as purely ionic. This is revealed by topological analysis of the electron density distribution and its associated Laplacian. The electron density at the Mg-C_9,10 bond critical points ρ(r_b) is very low and the Laplacian ∇²ρ(r_b) and the energy density H_b have positive values. The ionic nature of the Mg-C_9,10 bond is also indicated by the natural bond order (NBO) analysis, which gives a Lewis structure with two lone pairs at C₉ and C₁₀ but no Mg-C_9,10 bonds. The NBO method gives a charge donation from Mg to the anthracene ligand of nearly two. The theoretically predicted NMR chemical shifts using the GIAO method give ¹³C resonances for the complex 1 and for anthracene and anthracene dianion that are in good agreement with experimental values.